SMSC COM20020

COM20020 3.3V
COM20020 3.3V ULANC
Universal Local Area Network Controller
with 2K x 8 On-Board RAM
FEATURES
!" New Features
!" Eight, 256 Byte Pages Allow Four Pages TX
and RX Plus Scratch-Pad Memory
-
Data Rates up to 5 Mbps
-
Programmable Reconfiguration Times
!" Next ID Readable
!" 24 Pin DIP, 28 Pin PLCC Package
!" Internal Clock Scaler and Clock Multiplier for
Adjusting Network Speed
!" Ideal for Industrial/Factory/Building
Automation and Transportation
Applications
!" Operating Temperature Range of -40 C to
o
+85 C
o
!" Deterministic, (ANSI 878.1), Token Passing
ARCNET Protocol
!" Self-Reconfiguration Protocol
!" Supports up to 255 Nodes
!" Minimal Microcontroller and Media
Interface Logic Required
!" Supports Various Network Topologies (Star,
Tree, Bus...)
!" Flexible Interface For Use With All
Microcontrollers or Microprocessors
!" CMOS, Single +3.3V Supply
!" Duplicate Node ID Detection
!" Automatically Detects Type of
Microcontroller Interface
!" Powerful Diagnostics
!" Receive All Packets Mode
!" 2Kx8 On-Chip Dual Port RAM
!" Command Chaining for Packet Queuing
!" Sequential Access to Internal RAM
!" Software Programmable Node ID
!" Flexible Media Interface:
-
Traditional Hybrid Interface For Long
Distances up to Four Miles at 2.5Mbps.
-
RS485 Differential Driver Interface For
Low Cost, Low Power, High Reliability
ORDERING INFORMATION
Order Numbers:
COM20020ILJP3V
28 PLCC Package
COM20020IP3V
24 DIP Package
GENERAL DESCRIPTION
SMSC's COM20020 is a member of the family of
Embedded ARCNET Controllers from Standard
Microsystems Corporation. The device is a
general purpose communications controller for
networking microcontrollers and intelligent
peripherals in industrial, automotive, and
embedded control
environments
using an
ARCNET protocol engine. The small 24 pin
package, flexible microcontroller and media
interfaces, eight- page message support, and
extended temperature range of the COM20020
make it the only true network controller optimized
for use in industrial, embedded, and automotive
applications. Using an ARCNET protocol engine
is the ideal solution for embedded control
applications because it provides a deterministic
token-passing protocol, a highly reliable and
proven networking scheme, and a data rate of up
to 5 Mbps when using the COM20020.
A token-passing protocol provides predictable
response times because each network event
occurs within a predetermined time interval, based
upon the number of nodes on the network. The
deterministic nature of ARCNET is essential in
real time applications. The integration of the 2Kx8
RAM buffer on-chip, the Command Chaining
feature, the 5 Mbps maximum data rate, and the
internal diagnostics make the COM20020 the
highest performance embedded communications
device available. With only one COM20020 and
one microcontroller, a complete communications
node may be implemented.
2
TABLE OF CONTENTS
FEATURES.............................................................................................................................................. 1
GENERAL DESCRIPTION ...................................................................................................................... 1
GENERAL DESCRIPTION ...................................................................................................................... 2
PIN CONFIGURATION ............................................................................................................................ 4
DESCRIPTION OF PIN FUNCTIONS ...................................................................................................... 5
PROTOCOL DESCRIPTION ................................................................................................................... 8
NETWORK PROTOCOL ..................................................................................................................... 8
DATA RATES ...................................................................................................................................... 8
NETWORK RECONFIGURATION ...................................................................................................... 9
BROADCAST MESSAGES ............................................................................................................... 10
EXTENDED TIMEOUT FUNCTION .................................................................................................. 10
LINE PROTOCOL ............................................................................................................................. 10
SYSTEM DESCRIPTION ....................................................................................................................... 13
MICROCONTROLLER INTERFACE................................................................................................. 13
TRANSMISSION MEDIA INTERFACE.............................................................................................. 17
FUNCTIONAL DESCRIPTION .............................................................................................................. 23
MICROSEQUENCER........................................................................................................................ 23
INTERNAL REGISTERS ................................................................................................................... 26
INTERNAL RAM................................................................................................................................ 39
COMMAND CHAINING ..................................................................................................................... 44
INITIALIZATION SEQUENCE ........................................................................................................... 46
IMPROVED DIAGNOSTICS.............................................................................................................. 47
OPERATIONAL DESCRIPTION............................................................................................................ 49
MAXIMUM GUARANTEED RATINGS* ............................................................................................. 49
DC ELECTRICAL CHARACTERISTICS ........................................................................................... 49
TIMING DIAGRAMS .............................................................................................................................. 52
APPENDIX A ......................................................................................................................................... 67
APPENDIX B ......................................................................................................................................... 71
SOFTWARE IDENTIFICATION OF THE COM20020 REV B, REV C AND REV D .............................. 71
COM20020 REVISIONS ........................................................................................................................ 72
For more details on the ARCNET protocol engine and traditional dipulse signaling schemes,
please refer to the ARCNET Local Area Network Standard, available from Standard Microsystems
Corporation or the ARCNET Designer's Handbook, available from Datapoint Corporation.
For more detailed information on cabling options including RS485, transformer-coupled RS-485
and Fiber Optic interfaces, please refer to the following technical note which is available from
Standard Microsystems Corporation: Technical Note 7-5 - Cabling Guidelines for the COM20020
ULANC.
3
PIN CONFIGURATION
A0/nMUX
1
24
VDD
A1
2
23
nRD/nDS
A2/ALE
3
22
nWR/DIR
AD0
4
21
nCS
AD1
5
20
nINTR
AD2
6
19
nRESET IN
D3
7
18
nTXEN
D4
8
17
RXIN
D5
9
16
nPULSE2
D6
10
15
nPULSE1
D7
11
14
XTAL2
VSS
12
13
XTAL1
nWR/DIR
nRD/nD
n
nI n
V nT R n
C 24
N 23
R 22
S 21
X 20
XI 19
P
25
E
N U 18
T E
26
S
L
E
17
27
XTAL2
28
16
XTAL1
1
15
VDD
A1
2
14
VSS
A2/ALE
3
13
N/C
AD0
4
12
D7
VDD
A0/nMU
Packages: 24-Pin DIP or 28-Pin
5
6
7
A
D
A
D
V
S
8
9
D
3
D
4
10 11
D
5
D
6
Ordering
COM2002
nPULSE
I P
PACKAGE TYPE: P = Plastic, LJP =
TEMP RANGE:
I = Industrial: -40°C to
DEVICE TYPE: 20020 = Universal Local Area Network
(with 2K x 8
4
DESCRIPTION OF PIN FUNCTIONS
DIP PIN
NO.
PLCC PIN
NO.
NAME
SYMBOL
MICROCONTROLLER INTERFACE
DESCRIPTION
1-3
1-3
Address
0-2
A0/nMUX,
A1,A2/ALE
Input. On a non-multiplexed mode, A0-A2
are address input bits. (A0 is the LSB) On a
multiplexed address/data bus, nMUX tied
Low, A1 is left open, and ALE is tied to the
Address Latch Enable signal. A1 is
connected to an internal pull-up resistor.
4-11
4-6,8-12
Data 0-7
AD0-AD2,
D3-D7
Input/Output.
On a non-multiplexed bus,
these signals are used as the data lines for
the device. On a multiplexed address/data
bus, AD0-AD2 act as the address lines
(latched by ALE) and as the low data lines for
the device. D3-D7 are always used for data
only. These signals are connected to internal
pull-up resistors.
23
27
nRead/nData nRD/nDS
Strobe
Input. On a 68XX-like bus, nDS is an active
low signal issued by the microcontroller as the
data strobe signal to strobe the data onto the
bus. On a 80XX-like bus, nRD is an active
low signal issued by the microcontroller to
indicate a read operation.
22
26
nWrite/
Direction
nWR/DIR
Input. On a 68XX-like bus, DIR is issued by
the microcontroller as the Read/nWrite signal
to determine the direction of data transfer. In
this case, a logic "1" selects a read operation,
while a logic "0" selects a write operation. In
this case, data is actually strobed by the nDS
signal. On an 80XX-like bus, nWR is an
active low signal issued by the microcontroller
to indicate a write operation. In this case, a
logic "0" on this pin, when the COM20020 is
accessed, enables data from the data bus to
be written to the device.
19
23
nReset in
nRESET
Input. This active low signal executes a
hardware reset.
20
24
nInterrupt
nINTR
Output. This active low signal is generated by
the COM20020 when an enabled interrupt
condition occurs.
21
25
nChip Select
nCS
Input. This active low signal selects the
COM20020 for an access.
TRANSMISSION MEDIA INTERFACE
5
DIP PIN
NO.
16,15
PLCC PIN
NO.
NAME
19,18
nPulse 2,
nPulse 1
SYMBOL
nPULSE2,
nPULSE1
DESCRIPTION
Output (nPULSE1), Input/Output (nPULSE2).
In Normal Mode, these active low signals
carry the transmit data information, encoded
in pulse format, as DIPULSE waveform.
When the device is in Backplane Mode, the
nPULSE1 signal driver is programmable
(push/pull or open-drain), while the nPULSE2
signal provides a clock with frequency of
double the data rate. nPULSE1 is connected
to a weak internal pull-up resistor on the
open/drain driver in backplane mode.
17
20
Receive In
RXIN
Input. This signal carries the receive data
information from the line tranceiver.
18
21
nTransmit
nEnable
nTXEN
Output. This signal is used prior to the Powerup to enable the line drivers for transmission.
The polarity of the signal is programmable
through the nPULSE2 pin.
nPULSE2 floating before Power-up: nTXEN
active low (Default option)
nPULSE2 grounded before Power-up:
nTXEN active high (This option is only
available in Backplane Mode)
13,14
16,17
Crystal
Oscillator
XTAL1,
XTAL2
An external crystal should be connected to
these pins. Oscillation frequency range is
from 10 to 20 MHz. If an external TTL clock is
used instead, it must be connected to XTAL1
with a 390! pull-up resistor, and XTAL2
should be left floating.
24
15,28
Power
Supply
VDD
+3.3 Volt Power Supply pin.
12
7,14,22
Ground
VSS
Ground pin.
6
Power On
Reconfigure
Timer has
Timed Out
Send
Reconfigure
Burst
Read Node ID
Write ID to
RAM Buffer
1
Set NID=ID
Y
Start
Reconfiguration
Timer (420 mS)*
N
Y
Y
TA?
Transmit
NAK
Y
RI?
Y
N
Y
ACK?
Y
N
No
Activity
for 37.4
us?
Y
N
Y
Y
N
Broadcast
Enabled?
DID
=ID?
N
Y
N
Set NID=ID
Start Timer:
T=(255-ID)
x 73 us
Y
Write Buffer
with Packet
Activity
On Line?
Pass the
Token
Set TA
Increment
NID
Y
Set TMA
No
Activity
for 37.4
us?
N
N
N
N
CRC
OK?
T=0?
Y
Y
LENGTH N
OK?
Y
DID
=0?
- ID refers to the identification number of the ID assigned to this node.
- NID refers to the next identification number that receives the token
after this ID passes it.
- SID refers to the source identification.
- DID refers to the destination identification.
- SOH refers to the start of header character; preceeds all data packets.
Y
Set RI
N
DID
=ID?
N
Y
* Reconfig timer is programmable via setup2 register bits 1, 0.
SEND ACK
Note - All time values are valid for 5 Mbps.
FIGURE 1 - COM20020 OPERATION
7
N
No Activity
for 41
uS?
Y
DID
=0?
Y
1
N
ACK?
N
RI?
Set TA
NAK?
N
No
Activity Y
for 37.4
us?
N
SOH?
Write SID
to Buffer
Send
Packet
N
Y
N
Transmit
Free Buffer
Enquiry
N
Was Packet
Broadcast?
N
Free Buffer
Enquiry to
this ID?
Transmit
ACK
Broadcast?
Y
N
Invitation
to Transmit to
this ID?
Y
PROTOCOL DESCRIPTION
NETWORK PROTOCOL
message (or nothing if it is not received
successfully) allowing the transmitter to set the
appropriate status bits to indicate successful or
unsuccessful delivery of the packet. An interrupt
mask permits the COM20020 to generate an
interrupt to the processor when selected status
bits become true. Figure 1 is a flow chart
illustrating the internal operation of the
COM20020 connected to a 20 MHz crystal
oscillator.
Communication on the network is based on a
token passing protocol. Establishment of the
network configuration and management of the
network protocol are handled entirely by the
COM20020's internal microcoded sequencer. A
processor or intelligent peripheral transmits data
by simply loading a data packet and its destination
ID into the COM20020's internal RAM buffer, and
issuing a command to enable the transmitter.
When the COM20020 next receives the token, it
verifies that the receiving node is ready by first
transmitting a FREE BUFFER ENQUIRY
message. If the receiving node transmits an
ACKnowledge message, the data packet is
transmitted followed by a 16-bit CRC. If the
receiving node cannot accept the packet (typically
its receiver is inhibited), it transmits a Negative
AcKnowledge message and the transmitter
passes the token. Once it has been established
that the receiving node can accept the packet and
transmission is complete, the receiving node
verifies the packet. If the packet is received
successfully, the receiving node transmits an
ACKnowledge
INTERNAL
CLOCK
FREQUENCY
40 MHz
20 MHz
CLOCK
PRESCALER
Div. by 8
Div. by 8
Div. by 16
Div. by 32
Div. by 64
Div. by 128
DATA RATES
The COM20020 is capable of supporting data
rates from 156.25 Kbps to 5 Mbps. The following
protocol description assumes a 5 Mbps data rate.
To attain the faster data rates, the clock frequency
may be doubled by the internal clock multiplier
(see next section). For slower data rates, an
internal clock divider scales down the clock
frequency. Thus all timeout values are scaled as
shown in the following table:
Example: IDLE LINE Timeout @ 5 Mbps = 41 "s.
IDLE LINE Timeout for 156.2 Kbps is 41 "s * 32 =
1.3 ms
DATA RATE
5 Mbps
2.5 Mbps
1.25 Mbps
625 Kbps
312.5 Kbps
156.25 Kbps
8
TIMEOUT SCALING
FACTOR (MULTIPLY BY)
1
2
4
8
16
32
Selecting Clock Frequencies Above 2.5 Mbps
Therefore, higher frequency clocks are
generated from the 20 MHz crystal as selected
through two bits in the Setup2 register,
CKUP[1,0] as shown below. The selected clock
is supplied to the ARCNET controller.
To realize a 5 Mbps network, an external 40 MHz
clock must be input. However, since 40 MHz is
near the frequency of FM radio band, it is not
practical for use for noise emission reasons.
CKUP1
0
0
1
1
CKUP0
0
1
0
1
CLOCK FREQUENCY (DATA RATE)
20 MHz (Up to 2.5Mbps) Default (Bypass)
40 MHz (Up to 5Mbps)
Reserved
Reserved
This clock multiplier is powered-down (bypassed)
on default. After changing the CKUP1 and
CKUP0 bits, the ARCNET core operation is
stopped and the internal PLL in the clock
generator is awakened and it starts to generate
the 40 MHz. The lock out time of the internal PLL
is 8uSec typically. After more than 8 "sec (this
wait time is defined as 1 msec in this data sheet),
it is necessary to write command data '18H' to
the command register to re-start the ARCNET
core operation. This clock generator is called
“clock multiplier”.
TRANSMIT, destroy the token and keep any other
node from assuming control of the line.
When any COM20020 senses an idle line for
greater than 41"S, which occurs only when the
token Is lost, each COM20020 starts an internal
timeout equal to 73"s times the quantity 255
minus its own ID. The COM20020 starts network
reconfiguration by sending an invitation to transmit
first to itself and then to all other nodes by
decrementing the destination Node ID. If the
timeout expires with no line activity, the
COM20020 starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal to
the currently stored NID. Within a given network,
only one COM20020 will timeout (the one with the
highest ID number). After sending the
INVITATION TO TRANSMIT, the COM20020
waits for activity on the line. If there is no activity
for 37.4"S, the COM20020 increments the NID
value and transmits another INVITATION TO
TRANSMIT using the NID equal to the DID. If
activity appears before the 37.4"S timeout
expires, the COM20020 releases control of the
line. During NETWORK RECONFIGURATION,
INVITATIONS TO TRANSMIT are sent to all NIDs
(1-255).
Changing the CKUP1 and CKUP0 bits must be
one time or less after releasing hardware reset.
The EF bit in the SETUP2 register must be set
when the data rate is over 5 Mbps.
NETWORK RECONFIGURATION
A significant advantage of the COM20020 is its
ability to adapt to changes on the network.
Whenever a new node is activated or deactivated,
a NETWORK RECONFIGURATION is performed.
When a new COM20020 is turned on (creating a
new active node on the network), or if the
COM20020 has not received an INVITATION TO
TRANSMIT for 420mS, or if a software reset
occurs, the COM20020 causes a NETWORK
RECONFIGURATION
by
sending
a
RECONFIGURE BURST consisting of eight
marks and one space repeated 765 times. The
purpose of this burst is to terminate all activity on
the network. Since this burst is longer than any
other type of transmission, the burst will interfere
with the next INVITATION TO
Each COM20020 on the network will finally have
saved a NID value equal to the ID of the
COM20020 that it released control to. At this
point, control is passed directly from one node to
the next with no wasted INVITATIONS TO
TRANSMIT being sent to ID's not on the network,
until the next NETWORK RECONFIGURATION
occurs. When a node is powered off, the previous
node attempts to pass the token to it by issuing an
9
INVITATION TO TRANSMIT. Since this node
does not respond, the previous node times out
and
transmits
another
INVITATION TO
TRANSMIT to an incremented ID and eventually a
response will be received.
network topology. For a typical system using
RG62 coax in a baseband system, a one way
cable propagation delay of 15.5 "S translates to a
distance of about 2 miles. The flow chart in Figure
1 uses a value of 37.4 "S (15.5 + 15.5 + 6.4) to
determine if any node will respond.
The NETWORK RECONFIGURATION time
depends on the number of nodes in the network,
the propagation delay between nodes, and the
highest ID number on the network, but is typically
within the range of 12 to 30.5 mS.
Idle Time
The Idle Time is associated with a NETWORK
RECONFIGURATION. Figure 1 illustrates that
during a NETWORK RECONFIGURATION one
node will continually transmit INVITATIONS TO
TRANSMIT until it encounters an active node. All
other nodes on the network must distinguish
between this operation and an entirely idle line.
During NETWORK RECONFIGURATION, activity
will appear on the line every 41 "S. This 41 "S is
equal to the Response Time of 37.4 "S plus the
time it takes the COM20020 to start retransmitting
another message (usually another INVITATION
TO TRANSMIT).
BROADCAST MESSAGES
Broadcasting gives a particular node the ability to
transmit a data packet to all nodes on the network
simultaneously. ID zero is reserved for this
feature and no node on the network can be
assigned ID zero. To broadcast a message, the
transmitting node's processor simply loads the
RAM buffer with the data packet and sets the DID
equal to zero. Figure 4 illustrates the position of
each byte in the packet with the DID residing at
address 0X01 or 1 Hex of the current page
selected in the "Enable Transmit from Page fnn"
command. Each individual node has the ability to
ignore broadcast messages by setting the most
significant bit of the "Enable Receive to Page fnn"
command to a logic "0".
Reconfiguration Time
There are three timeouts associated with the
COM20020 operation.
The values of these
timeouts are controlled by bits 3 and 4 of the
Configuration Register and bit 5 of the Setup 1
Register.
If any node does not receive the token within the
Reconfiguration Time, the node will initiate a
NETWORK RECONFIGURATION. The ET2 and
ET1 bits of the Configuration Register allow the
network to operate over longer distances than the
2 miles stated earlier. The logic levels on these
bits control the maximum distances over which the
COM20020 can operate by controlling the three
timeout values described above. For proper
network operation, all COM20020's connected to
the same network must have the same Response
Time, Idle Time, and Reconfiguration Time.
Response Time
LINE PROTOCOL
The Response Time determines the maximum
propagation delay allowed between any two
nodes, and should be chosen to be larger than
the round trip propagation delay between the two
furthest nodes on the network plus the maximum
turn around time (the time it takes a particular
COM20020 to start sending a message in
response to a received message) which is
approximately 6.4 "S. The round trip propagation
delay is a function of the transmission media and
The ARCNET line protocol is considered
isochronous because each byte is preceded by a
start interval and ended with a stop interval. Unlike
asynchronous protocols, there is a constant
amount of time separating each data byte. On a 5
Mbps network, each byte takes exactly 11 clock
intervals of 200ns each. As a result, one byte is
transmitted every 2.2 "S and the time to transmit
a message can be precisely determined. The line
idles in a spacing (logic "0") condition. A logic "0"
EXTENDED TIMEOUT FUNCTION
10
Free Buffer Enquiries
is defined as no line activity and a logic "1" is
defined as a negative pulse of 100nS duration. A
transmission starts with an ALERT BURST
consisting of 6 unit intervals of mark (logic "1").
Eight bit data characters are then sent, with each
character preceded by 2 unit intervals of mark and
one unit interval of space.
Five types of
transmission can be performed as described
below:
A Free Buffer Enquiry is used to ask another node
if it is able to accept a packet of data. It is sent by
the following sequence:
# An ALERT BURST
# An ENQ (ENQuiry: ASCII code 85H)
#
Two (repeated) DID (Destination ID)
characters
Invitations To Transmit
ALERT
BURST
An Invitation To Transmit is used to pass the
token from one node to another and is sent by the
following sequence:
# An ALERT BURST
# An EOT (End Of Transmission: ASCII code
04H)
#
Two (repeated) DID (Destination ID)
characters
ALERT
BURST
EOT
DID
#$
#$
#$
#$
DID
#$
SID
DID
DID
DID
A Data Packet consists of the actual data being
sent to another node. It is sent by the following
sequence:
#$
SOH
DID
Data Packets
#$
ALERT
BURST
ENQ
COUNT
11
An ALERT BURST
An SOH (Start Of Header--ASCII code 01H)
An SID (Source ID) character
Two (repeated) DID (Destination ID)
characters
A single COUNT character which is the 2's
complement of the number of data bytes to
follow if a short packet is sent, or 00H
followed by a COUNT character if a long
packet is sent.
N data bytes where COUNT = 256-N (or 512N for a long packet)
Two CRC (Cyclic Redundancy Check)
characters. The CRC polynomial used is:
16
15
2
X + X + X + 1.
data
data
CRC
CRC
Acknowledgements
Negative Acknowledgements
An Acknowledgement is used to acknowledge
reception of a packet or as an affirmative
response to FREE BUFFER ENQUIRIES and is
sent by the following sequence:
#$ An ALERT BURST
#$ An ACK (ACKnowledgement--ASCII code
86H) character
A Negative Acknowledgement is used as a
negative
response
to
FREE
BUFFER
ENQUIRIES and is sent by the following
sequence:
#$ An ALERT BURST
#$ A NAK (Negative Acknowledgement--ASCII
code 15H) character
ALERT BURST
ACK
ALERT BURST
12
NAK
SYSTEM DESCRIPTION
MICROCONTROLLER INTERFACE
Whenever nCS and nRD are activated, the preset
determinations are assumed as final and will not
be changed until hardware reset. Refer to
Description of Pin Functions section for details on
the related signals. All accesses to the internal
RAM and the internal registers are controlled by
the COM20020. The internal RAM is accessed via
a pointer-based scheme (refer to the Sequential
Access Memory section), and the internal
registers are accessed via direct addressing.
Many peripherals are not fast enough to take
advantage of high-speed microcontrollers. Since
microcontrollers do not typically have READY
inputs, standard peripherals cannot extend cycles
to extend the access time. The access time of the
COM20020, on the other hand, is so fast that it
does not need to limit the speed of the
microcontroller. The COM20020 is designed to
be flexible so that it is independent of the
microcontroller speed.
The top halves of Figures 2 and 3 illustrate typical
COM20020 interfaces to the microcontrollers.
The interfaces consist of a 8-bit data bus, an
address bus and a control bus. In order to support
a wide range of microcontrollers without requiring
glue logic and without increasing the number of
pins, the COM20020 automatically detects and
adapts to the type of microcontroller being used.
Upon hardware reset, the COM20020 first
determines whether the read and write control
signals are separate READ and WRITE signals
(like the 80XX) or DIRECTION and DATA
STROBE (like the 68XX). To determine the type of
control signals, the device requires the software to
execute at least one write access to external
memory before attempting to access the
COM20020. The device defaults to 80XX-like
signals. Once the type of control signals are
determined, the COM20020 remains in this
interface mode until the next hardware reset
occurs. The second determination the COM20020
makes is whether the bus is multiplexed or nonmultiplexed. To determine the type of bus, the
device requires the software to write to an odd
memory location followed by a read from an odd
location before attempting to access the
COM20020. The signal on the A0 pin during the
odd location access tells the COM20020 the type
of bus. Since multiplexed operation requires A0 to
be active low, activity on the A0 line tells the
COM20020 that the bus is non-multiplexed. The
device defaults to multiplexed operation. Both
determinations may be made simultaneously by
performing a WRITE followed by a READ
operation to an odd location within the COM20020
Address space 20020D registers. Once the type
of bus is determined, the COM20020 remains in
this interface mode until hardware reset occurs.
The COM20020 provides for no wait state
arbitration via direct addressing to its internal
registers and a pointer based addressing scheme
to access its internal RAM. The pointer may be
used in auto-increment mode for typical
sequential buffer emptying or loading, or it can be
taken out of auto-increment mode to perform
random accesses to the RAM. The data within
the RAM is accessed through the data register.
Data being read is prefetched from memory and
placed into the data register for the microcontroller
to read. It is important to notice that only by
writing a new address pointer (writing to an
address pointer low), one obtains the contents of
COM20020 internal RAM. Performing only read
from the Data Register does not load new data
from the internal RAM. During a write operation,
the data is stored in the data register and then
written into memory. Whenever the pointer is
loaded for reads with a new value, data is
immediately prefetched to prepare for the first
read operation.
13
XTAL1
XTAL2
AD0-
COM20020
AD0-AD2, D3-
ALE
A2/BAL
A15
nCS
RESET
RXIN
nRESET
LTC1480 or
Equiv.
nTXEN
nRD
nRD/nD
nPULSE
nPULSE
nWR
nINT1
nWR/DI
nINTR
GND
Differential
Configuratio
8051
A0/nMU
XTAL1
27 pF
* Media
may be
with Figure A, B or
XTAL2
27 pF
20 MHz
XTAL
+5V
RXIN
2
+3.3V
Receive
6 HFD3212-
RXIN
3.3V-5V Converter
100
7
TXEN
Transmitte
3 HFE4211-
nPULSE
nPULSE
nPULSE
+5V
GND
2
6
7
2 Fiber
(ST
BACKPLANE
NOTE: COM20020 must be in backplane mode
FIGURE
FIGURE
FIGURE 2 - MULTIPLEXED, 8051-LIKE BUS INTERFACE WITH RS-485 INTERFACE
14
XTAL1
COM2002
XTAL2
D0-D7
D0-D7
A0
A0/nMUX
A1
A1
A2
A2/BALE
A7
nCS
RXIN
nPULSE1
nRES
nRESET
nIOS
nRD/nDS
R/nW
nWR/nDIR
nIRQ1
LTC1480 or
Equiv.
TXEN
nPULSE2
GND
nINTR
Differential Driver
Configuration
6801
XTAL1
27 pF
* Media Interface
may be replaced
with Figure A, B or C.
XTAL2
27 pF
20MHz
XTAL
+5V
3.3V-5V Converter
RXIN
HYC9068 or
HYC9088
10
uF
+
0.47
uF
6
RXIN
12
nTXEN
N/C
11
nPULSE1
nPULSE1
nPULSE2
nPULSE2
GND
17, 19,
4, 13, 14
5.6K
1/2W
5.6K
1/2W
0.01 uF
1KV
3
0.47
uF
+ 10
uF
-5V
Traditional Hybrid
Configuration
*Valid for 2.5 Mbps only.
FIGURE C
FIGURE 3 - NON-MULTIPLEXED, 6801-LIKE BUS INTERFACE WITH RS-485 INTERFACE
15
High Speed CPU Bus Timing Support
generate a noise spike at the above tight timing.
The DIAG register is cleared by the spike signal
without reading itself. This is unexpected
operation. Reading the internal RAM and Next
Id Register have the same mechanism as
reading the DIAG register.
High speed CPU bus support was added to the
COM20020. The reasoning behind this is as
follows: With the Host interface in Nonmultiplexed Bus mode, I/O address and Chip
Select signals must be stable before the read
signal is active and remain after the read signal
is inactive. But the High Speed CPU bus timing
doesn't adhere to these timings. For example, a
RISC type single chip microcontroller (like the
HITACHI SuperH series) changes I/O address at
the same time as the read signal. Therefore,
several external logic ICs would be required to
connect to this microcontroller.
Therefore, the address decode and host
interface mode blocks were modified to fit the
above CPU interface to support high speed CPU
bus timing. In Intel CPU mode (nRD, nWR
mode), 3 bit I/O address (A2-A0) and Chip Select
(nCS) are sampled internally by Flip-Flops on the
falling edge of the internal delayed nRD signal.
The internal real read signal is the more delayed
nRD signal. But the rising edge of nRD doesn't
delay. By this modification, the internal real
address and Chip Select are stable while the
internal real read signal is active. Refer to figure
4 below.
In addition, the Diagnostic Status (DIAG) register
is cleared automatically by reading itself. The
internal DIAG register read signal is generated
by decoding the Address (A2-A0), Chip Select
(nCS) and Read (nRD) signals. The decoder will
VALID
A2-A0, nCS
nRD
Delayed nRD
(nRD1)
Sampled A2-A0, nCS
VALID
More delayed nRD
(nRD2)
FIGURE 4 - HIGH SPEED CPU BUS TIMING - INTEL CPU MODE
The I/O address and Chip Select signals, which
are supplied to the data output logic, are not
sampled. Also, the nRD signal is not delayed,
because the above sampling and delaying paths
decrease the data access time of the read cycle.
Diagnostic register and generates the starting
pulse of the RAM Arbitration. Typical delay time
between nRD and nRD1 is around 15nS and
between nRD1 and nRD2 is around 10nS.
Longer pulse widths are needed due to these
delays on nRD signal. However, the CPU can
insert some wait cycles to extend the width
without any impact on performance.
The above sampling and delaying signals are
supplied to the Read Pulse Generation logic
which generates the clearing pulse for the
16
The RBUSTMG bit was added to Disable/Enable
the High Speed CPU Read function. It is defined
as:
RBUSTMG=0,
Disabled
(Default);
RBUSTMG BIT
0
1
RBUSTMG=1, Enabled.
In the MOTOROLA CPU mode (DIR, nDS
mode), the same modifications apply.
BUS TIMING MODE
Normal Speed CPU Read and Write
High Speed CPU Read and Normal Speed CPU Write
A logic "0" is transmitted by the absence of the
dipulse. During reception, the 200nS dipulse
appearing on the media is coupled through the RF
transformer of the LAN Driver, which produces a
positive pulse at the RXIN pin of the COM20020.
The pulse on the RXIN pin represents a logic "1".
Lack of pulse represents a logic "0". Typically,
RXIN pulses occur at multiples of 400nS. The
COM20020 can tolerate distortion of plus or minus
100nS and still correctly capture and convert the
RXIN pulses to NRZ format. Figure 5 illustrates
the events which occur in transmission or
reception of data consisting of 1, 1, 0.
TRANSMISSION MEDIA INTERFACE
The bottom halves of Figures 2 and 3 illustrate the
COM20020 interface to the transmission media
used to connect the node to the network. Table 1
lists different types of cable which are suitable for
1
ARCNET applications. The user may interface to
the cable of choice in one of three ways:
Traditional Hybrid Interface
The Traditional Hybrid Interface is that which is
used with previous ARCNET devices. The Hybrid
Interface is recommended if the node is to be
placed in a network with other Hybrid-Interfaced
nodes. The Traditional Hybrid Interface is for use
with nodes operating at 2.5 Mbps only. The
transformer coupling of the Hybrid offers isolation
for the safety of the system and offers high
Common Mode Rejection. The Traditional Hybrid
Interface uses circuits like SMSC's HYC9068 or
HYC9088 to transfer the pulse-encoded data
between the cable and the COM20020. The
COM20020 transmits a logic "1" by generating two
100nS
non-overlapping
negative
pulses,
nPULSE1 and nPULSE2.
Lack of pulses
indicates a logic "0". The nPULSE1 and nPULSE2
signals are sent to the Hybrid, which creates a
200nS dipulse signal on the media.
1
Please refer to TN7-5 – Cabling Guidelines for
the COM20020 ULANC, available from SMSC, for
recommended cabling distance, termination, and
node count for ARCNET nodes.
17
Backplane Configuration
push/pull or open drain driver can be selected by
programming the P1MODE bit of the Setup 1
Register (see register descriptions for details).
The COM20020 defaults to an open drain output.
The Backplane Open Drain Configuration is
recommended for cost-sensitive, short-distance
applications like backplanes and instrumentation.
This mode is advantageous because it saves
components, cost, and power.
The Backplane Configuration provides for direct
connection between the COM20020 and the
media. Only one pull-up resistor (in open drain
configuration of the output driver) is required
somewhere on the media (not on each individual
node). The nPULSE1 signal, in this mode, is an
open drain or push/pull driver and is used to
directly drive the media. It issues a 200nS
negative pulse to transmit a logic "1". Note that
when used in the open-drain mode, the
COM20020 does not have a fail/safe input on the
RXIN pin. The nPULSE1 signal actually contains
a weak pull-up resistor. This pull-up should not
take the place of the resistor required on the
media for open drain mode.
Since the Backplane Configuration encodes data
differently
than
the
traditional
Hybrid
Configuration, nodes utilizing the Backplane
Configuration cannot communicate directly with
nodes
utilizing
the
Traditional
Hybrid
Configuration. The Backplane Configuration does
not isolate the node from the media nor protects it
from Common Mode noise, but Common Mode
Noise is less of a problem in short distances.
The COM20020 supplies a programmable output
driver for Backplane Mode operation.
A
18
RT
RT
+3.3V
+3.3V
RBIAS
RBIAS
LTC1480 or
Equiv.
COM2002
+3.3V
COM2002
RBIAS
COM2002
FIGURE 5 - COM20020 NETWORK USING RS-485 DIFFERENTIAL TRANSCEIVERS
19
ONLY)
1
1
20MHZ
CLOCK
(FOR REF.
0
100ns
nPULSE1
100ns
nPULSE2
200ns
DIPULSE
400ns
RXIN
FIGURE 6 - DIPULSE WAVEFORM FOR DATA OF 1-1-0
In typical applications, the serial backplane is
terminated at both ends and a bias is provided by
the external pull-up resistor.
When the device is in Backplane Mode, the clock
provided by the nPULSE2 signal may be used for
encoding the data into a different encoding
scheme or other synchronous operations needed
on the serial data stream.
The RXIN signal is directly connected to the cable
via an internal Schmitt trigger. A negative pulse
on this input indicates a logic "1". Lack of pulse
indicates a logic "0". For typical single-ended
backplane applications, RXIN is connected to
nPULSE1 to make the serial backplane data line.
A ground line (from the coax or twisted pair)
should run in parallel with the signal.
For
applications requiring different treatment of the
receive signal (like filtering or squelching),
nPULSE1 and RXIN remain as independent pins.
External differential drivers/receivers for increased
range and common mode noise rejection, for
example, would require the signals to be
independent of one another.
Differential Driver Configuration
The Differential Driver Configuration is a special
case of the Backplane Mode. It is a dc coupled
configuration recommended for applications like
car-area networks or other cost-sensitive
applications which do not require direct
compatibility with existing ARCNET nodes and do
not require isolation.
20
Programmable TXEN Polarity
The Differential Driver Configuration cannot
communicate directly with nodes utilizing the
Traditional Hybrid Configuration.
Like the
Backplane Configuration, the Differential Driver
Configuration does not isolate the node from the
media.
To accommodate transceivers with active high
ENABLE pins, the COM20020 contains a
programmable TXEN output. To program the
TXEN pin for an active high pulse, the nPULSE2
pin should be connected to ground. To retain the
normal active low polarity, nPULSE2 should be
left open. The polarity determination is made at
power on reset and is valid only for Backplane
Mode operation.
The nPULSE2 pin should
remain grounded at all times if an active high
polarity is desired.
The Differential Driver interface includes a RS485
Driver/Receiver to transfer the data between the
cable and the COM20020. The nPULSE1 signal
transmits the data, provided the Transmit Enable
signal is active. The nPULSE1 signal issues a
200nS (at 2.5Mbps) negative pulse to transmit a
logic "1". Lack of pulse indicates a logic "0". The
RXIN signal receives the data, the transmitter
portion of the COM20020 is disabled during reset
and the nPULSE1, nPULSE2 and nTXEN pins are
inactive.
21
A0/nMUX
A1
A2/BALE
ADDRESS
DECODING
CIRCUITRY
2K x 8
RAM
ADDITIONAL
REGISTERS
AD0-AD2,
D3-D7
nINTR
nRESET
STATUS/
COMMAND
REGISTER
RESET
LOGIC
TX/RX
LOGIC
MICROSEQUENCER
AND
WORKING
REGISTERS
OSCILLATOR
nRD/nDS
nWR/DIR
nCS
BUS
ARBITRATION
CIRCUITRY
RECONFIGURATION
TIMER
NODE ID
LOGIC
FIGURE 7 - INTERNAL BLOCK DIAGRAM
22
nPULSE1
nPULSE2
nTXEN
RXIN
XTAL1
XTAL2
CABLE TYPE
RG-62 Belden #86262
Table 1 - Typical Media
NOMINAL
ATTENUATION PER 1000 FT.
IMPEDANCE
AT 5 MHz
5.5dB
93!
RG-59/U Belden #89108
75!
7.0dB
RG-11/U Belden #89108
75!
5.5dB
IBM Type 1* Belden #89688
150!
7.0dB
IBM Type 3* Telephone Twisted
Pair Belden #1155A
100!
17.9dB
COMCODE 26 AWG Twisted
Pair Part #105-064-703
105!
16.0dB
*Non-plenum-rated cables of this type are also available.
Note: For more detailed information on Cabling options including RS-485, transformer-coupled RS-485
and Fiber Optic interfaces, please refer to TN7-5 – Cabling Guidelines for the COM20020 ULANC,
available from Standard Microsystems Corporation.
FUNCTIONAL DESCRIPTION
MICROSEQUENCER
is stored in the ROM and the instructions are
fetched and then placed into the instruction
registers. One register holds the opcode, while
the other holds the immediate data. Once the
instruction is fetched, it is decoded by the internal
instruction decoder, at which point the COM20020
proceeds to execute the instruction. When a noop instruction is encountered, the microsequencer
enters a timed loop and the program counter is
temporarily stopped until the loop is complete.
When a jump instruction is encountered, the
program counter is loaded with the jump address
from the ROM. The COM20020 contains an
internal reconfiguration timer which interrupts the
microsequencer if it has timed out. At this point
the program counter is cleared and the
MYRECON bit of the Diagnostic Status Register is
set.
The
COM20020
contains
an
internal
microsequencer which performs all of the control
operations necessary to carry out the ARCNET
protocol. It consists of a clock generator, a 544 x
8 ROM, a program counter, two instruction
registers, an instruction decoder, a no-op
generator, jump logic, and reconfiguration logic.
The COM20020 derives a 10 MHz and a 5 MHz
clock from the output clock of the Clock Multiplier.
These clocks provide the rate at which the
instructions are executed within the COM20020.
The 10 MHz clock is the rate at which the program
counter operates, while the 5 MHz clock is the rate
at which the instructions are executed. The
microprogram
23
Table 2 - Read Register Summary
READ
REGISTER
STATUS
MSB
RI/TRI
X/RI
X/TA
POR
TEST
RECON
TMA
DIAG.
STATUS
MY-RECON
DUPID
RCVACT
TOKEN
EXCNAK
TENTID
ADDRESS
PTR HIGH
ADDRESS
PTR LOW
DATA
SUB ADR
RD-DATA
X
X
X
A10
A7
AUTOINC
A6
NEW
NEXT
ID
A9
A5
A4
A3
A2
D7
(R/W)*
D6
0
D5
0
D4
0
D3
(R/W)*
CONFIGURATION
TENTID
NODE ID
SETUP1
RESET
TXEN
ET1
ET2
TID7
NID7
P1 MODE
TID5
NID5
X
NXT ID7
SETUP2
RBUS-TMG
TID4
NID4
RCVALL
NXT
ID4
CKUP0
TID3
NID3
CKP3
NEXT ID
CCHE
N
TID6
NID6
FOUR
NAKS
NXT
ID6
X
Note*:
NXT
ID5
CKU
P1
NXT
ID3
EF
LSB
TA/
TTA
X
ADDR
00
A8
02
A1
A0
03
D2
SUBAD2
BACKPLANE
TID2
NID2
CKP2
D1
SUBAD1
SUBAD1
TID1
NID1
CKP1
04
05
NXT
ID2
NOSYNC
NXT
ID1
RCNTM1
D0
SUBAD0
SUBAD0
TID0
NID0
SLOWARB
NXT
ID0
RCMTM2
(R/W) This bit can be Written or Read. For more information see Appendix C.
24
01
06
07-0
07-1
07-2
07-3
07-4
Table 3 - Write Register Summary
WRITE
ADDR
00
MSB
RI/TR1
0
0
0
EXCNAK
01
02
C6
AUTOINC
A6
C5
0
C4
0
03
C7
RDDATA
A7
A5
04
05
D7
(R/W)*
D6
0
06
RESE
T
TID7
NID7
P1MODE
0
RBUSTMG
07-0
07-1
07-2
07-3
07-4
C3
0
RECO
N
C2
A10
NEW
NEXTID
C1
A9
LSB
TA/
TTA
C0
A8
A4
A3
A2
A1
A0
D5
0
D4
0
D3
(R/W)*
CCHEN
TXEN
ET1
ET2
TID6
NID6
FOUR
NAKS
0
0
TID5
NID5
0
TID4
NID4
RCVALL
0
CKUP0
TID3
NID3
CKP3
D2
SUBAD2
BACKPLANE
TID2
NID2
CKP2
D1
SUBAD1
SUBAD1
TID1
NID1
CKP1
0
NOSYNC
0
RCNTM1
D0
SUBAD0
SUBAD0
TID0
NID0
SLOWARB
0
RCNTM0
0
CKUP
1
0
EF
Note*: (R/W) This bit can be Written or Read. For more information see Appendix C.
25
REGISTER
INTERRUPT
MASK
COMMAND
ADDRESS
PTR HIGH
ADDRESS
PTR LOW
DATA
SUBADR
CONFIGURATION
TENTID
NODEID
SETUP1
TEST
SETUP2
INTERNAL REGISTERS
Tentative ID Register
The COM20020 contains 14 internal registers.
Tables 2 and 3 illustrate the COM20020 register
map. All undefined bits are read as undefined and
must be written as logic "0".
The Tentative ID Register is a read/write 8-bit
register accessed when the Sub Address Bits are
set up accordingly (please refer to the
Configuration Register and SUB ADR Register).
The Tentative ID Register can be used while the
node is on-line to build a network map of those
nodes existing on the network. It minimizes the
need for operator interaction with the network.
The node determines the existence of other nodes
by placing a Node ID value in the Tentative ID
Register and waiting to see if the Tentative ID bit
of the Diagnostic Status Register gets set. The
network map developed by this method is only
valid for a short period of time, since nodes may
join or depart from the network at any time. When
using the Tentative ID feature, a node cannot
detect the existence of the next logical node to
which it passes the token. The Next ID Register
will hold the ID value of that node. The Tentative
ID Register defaults to the value 0000 0000 upon
hardware reset only.
Interrupt Mask Register (IMR)
The COM20020 is capable of generating an
interrupt signal when certain status bits become
true. A write to the IMR specifies which status bits
will be enabled to generate an interrupt. The bit
positions in the IMR are in the same position as
their corresponding status bits in the Status
Register and Diagnostic Status Register. A logic
"1" in a particular position enables the
corresponding interrupt. The Status bits capable of
generating an interrupt include the Receiver
Inhibited bit, New Next ID bit, Excessive NAK bit,
Reconfiguration Timer bit, and Transmitter
Available bit. No other Status or Diagnostic Status
bits can generate an interrupt.
Node ID Register
The six maskable status bits are ANDed with their
respective mask bits, and the results are ORed to
produce the interrupt signal. An RI or TA
interrupt is masked when the corresponding mask
bit is reset to logic "0", but will reappear when the
corresponding mask bit is set to logic "1" again,
unless the interrupt status condition has been
cleared by this time. A RECON interrupt is
cleared when the "Clear Flags" command is
issued. An EXCNAK interrupt is cleared when the
"POR Clear Flags" command is issued. A New
Next ID interrupt is cleared by reading the Next ID
Register. The Interrupt Mask Register defaults to
the value 0000 0000 upon hardware reset.
The Node ID Register is a read/write 8-bit register
accessed when the Sub Address Bits are set up
accordingly (please refer to the Configuration
Register and SUB ADR Register). The Node ID
Register contains the unique value which
identifies this particular node. Each node on the
network must have a unique Node ID value at all
times. The Duplicate ID bit of the Diagnostic
Status Register helps the user find a unique Node
ID. Refer to the Initialization Sequence section for
further detail on the use of the DUPID bit. The
core of the COM20020 does not wake up until a
Node ID other than zero is written into the Node
ID Register. During this time, no microcode is
executed, no tokens are passed by this node, and
no reconfigurations are caused by this node.
Once a non-zero NodeID is placed into the Node
ID Register, the core wakes up but will not join the
network until the TXEN bit of the Configuration
Register is set. While the Transmitter is disabled,
the Receiver portion of the device is still functional
and will provide the user with useful information
about the network. The Node ID Register defaults
to the value 0000 0000 upon hardware reset only.
Data Register
This read/write 8-bit register is used as the
channel through which the data to and from the
RAM passes. The data is placed in or retrieved
from the address location presently specified by
the address pointer. The contents of the Data
Register are undefined upon hardware reset. In
case of READ operation, the Data Register is
loaded with the contents of COM20020 Internal
Memory upon writing Address Pointer low only
once.
26
Next ID Register
Register represent different situations. All of
these bits, except the Excessive NAcK bit and the
New Next ID bit, are reset to logic "0" upon
reading the Diagnostic Status Register or upon
software or hardware reset. The EXCNAK bit is
reset by the "POR Clear Flags" command or upon
software or hardware reset. The Diagnostic
Status Register defaults to the value 0000 000X
upon either hardware or software reset.
The Next ID Register is an 8-bit, read-only
register, accessed when the sub-address bits are
set up accordingly (please refer to the
Configuration Register and SUB ADR Register).
The Next ID Register holds the value of the Node
ID to which the COM20020 will pass the token.
When used in conjunction with the Tentative ID
Register, the Next ID Register can provide a
complete network map. The Next ID Register is
updated each time a node enters/leaves the
network or when a network reconfiguration occurs.
Each time the microsequencer updates the Next
ID Register, a New Next ID interrupt is generated.
This bit is cleared by reading the Next ID Register.
Default value is 0000 0000 upon hardware or
software reset.
Command Register
Execution of commands are initiated by
performing microcontroller writes to this register.
Any combinations of written data other than
those listed in Table 5 are not permitted and may
result in incorrect chip and/or network operation.
Address Pointer Registers
Status Register
These read/write registers are each 8-bits wide
and are used for addressing the internal RAM.
New pointer addresses should be written by first
writing to the High Register and then writing to the
Low Register because writing to the Low Register
loads the address. The contents of the Address
Pointer High and Low Registers are undefined
upon hardware reset. Writing to Address Pointer
low loads the address.
The COM20020 Status Register is an 8-bit readonly register. All of the bits, except for bits 5 and
6, are software compatible with previous SMSC
ARCNET devices. In previous SMSC ARCNET
devices the Extended Timeout status was
provided in bits 5 and 6 of the Status Register. In
the COM20020, the COM20020, the COM90C66,
and the COM90C165, COM20020-5, COM20051
and COM20051+ these bits exist in and are
controlled by the Configuration Register. The
Status Register contents are defined as in Table
4, but are defined differently during the Command
Chaining operation. Please refer to the Command
Chaining section for the definition of the Status
Register during Command Chaining operation.
The Status Register defaults to the value 1XX1
0001 upon either hardware or software reset.
Configuration Register
The Configuration Register is a read/write register
which is used to configure the different modes of
the COM20020.
The Configuration Register
defaults to the value 0001 1000 upon hardware
reset only. SUBAD0 and SUBAD1 point to the
selection in Register 7.
Diagnostic Status Register
The Diagnostic Status Register contains seven
read-only bits which help the user troubleshoot the
network or node operation. Various combinations
of these bits and the TXEN bit of the Configuration
27
Sub-Address Register
Setup 2 Register
The sub-address register is new to the
COM20020, previously a reserved register. Bits 2,
1 and 0 are used to select one of the registers
assigned to address 7h. SUBAD1 and SUBAD0
already exist in the Configuration register on the
COM20020B. They are exactly same as those in
the Sub-Address register. If the SUBAD1 and
SUBAD0 bits in the Configuration register are
changed, the SUBAD1and SUBAD0 in the SubAddress register are also changed. SUBAD2 is a
new sub-address bit. It Is used to access the 1
new Set Up register, SETUP2. This register is
selected by setting SUBAD2=1. The SUBAD2 bit
is cleared automatically by writing the
Configuration register.
The Setup 2 Register is new to the COM20020.
It is an 8-bit read/write register accessed when
the Sub Address Bits SUBAD[2:0] are set up
accordingly (see the bit definitions of the Sub
Address Register). This register contains bits for
various functions. The CKUP1,0 bits select the
clock to be generated from the 20 MHz crystal.
The RBUSTMG bit is used to Disable/Enable
Fast Read function for High Speed CPU bus
support. The EF bit is used to enable the new
timing for certain functions in the COM20020 (if
EF = 0, the timing is the same as in the
COM20020 Rev. B). See Appendix “A”. The
NOSYNC bit is used to enable the NOSYNC
function during initialization. If this bit is reset,
the line has to be idle for the RAM initialization
sequence to be written. If set, the line does not
have to be idle for the initialization sequence to
be written. See Appendix “A”.
Setup 1 Register
The Setup 1 Register is a read/write 8-bit register
accessed when the Sub Address Bits are set up
accordingly
(see the bit definitions of the
Configuration Register). The Setup 1 Register
allows the user to change the network speed (data
rate) or the arbitration speed independently,
invoke the Receive All feature and change the
nPULSE1 driver type. The data rate may be
slowed to 156.25Kbps and/or the arbitration speed
may be slowed by a factor of two. The Setup 1
Register defaults to the value 0000 0000 upon
hardware reset only.
The RCNTM[1,0] bits are used to set the timeout period of the recon timer. Programming this
timer for shorter time periods has the benefit of
shortened network reconfiguration periods. The
time periods shown in the table on the following
page are limited by a maximum number of nodes
in the network. These time-out period values are
for 5Mbps. For other data rates, scale the timeout period time values accordingly; the maximum
node count remains the same.
TIME-OUT
MAX NODE
RCNTM1 RCNTM0
PERIOD
COUNT
0
0
420 mS
Up to 255 nodes
0
1
105 mS
Up to 64 nodes
1
0
52.5 mS
Up to 32 nodes
1
1
26.25 mS*
Up to 16 nodes*
Note*: The node ID value 255 must exist in the network for the 26.25 mS time-out to be valid.
28
Table 4 - Status Register
BIT
7
6,5
BIT NAME
Receiver
Inhibited
SYMBOL
RI
(Reserved)
DESCRIPTION
This bit, if high, indicates that the receiver is not enabled because
either an "Enable Receive to Page fnn" command was never
issued, or a packet has been deposited into the RAM buffer page
fnn as specified by the last "Enable Receive to Page fnn"
command. No messages will be received until this command is
issued, and once the message has been received, the RI bit is set,
thereby inhibiting the receiver. The RI bit is cleared by issuing an
"Enable Receive to Page fnn" command. This bit, when set, will
cause an interrupt if the corresponding bit of the Interrupt Mask
Register (IMR) is also set. When this bit is set and another station
attempts to send a packet to this station, this station will send a
NAK.
These bits are undefined.
4
Power On Reset
POR
This bit, if high, indicates that the COM20020 has been reset by
either a software reset, a hardware reset, or writing 00H to the
Node ID Register. The POR bit is cleared by the "Clear Flags"
command.
3
Test
TEST
This bit is intended for test and diagnostic purposes. It is a logic
"0" under normal operating conditions.
2
Reconfiguration
RECON
This bit, if high, indicates that the Line Idle Timer has timed out
because the RXIN pin was idle for 41"S. The RECON bit is
cleared during a "Clear Flags" command. This bit, when set, will
cause an interrupt if the corresponding bit in the IMR is also set.
The interrupt service routine should consist of examining the
MYRECON bit of the Diagnostic Status Register to determine
whether there are consecutive reconfigurations caused by this
node.
1
Transmitter
Message
Acknowledged
TMA
This bit, if high, indicates that the packet transmitted as a result of
an "Enable Transmit from Page fnn" command has been
acknowledged. This bit should only be considered valid after the
TA bit (bit 0) is set. Broadcast messages are never acknowledged.
The TMA bit is cleared by issuing the "Enable Transmit from Page
fnn" command.
0
Transmitter
Available
TA
This bit, if high, indicates that the transmitter is available for
transmitting. This bit is set when the last byte of scheduled packet
has been transmitted out, or upon execution of a "Disable
Transmitter" command. The TA bit is cleared by issuing the
"Enable Transmit from Page fnn" command after the node next
receives the token. This bit, when set, will cause an interrupt if the
corresponding bit in the IMR is also set.
29
Table 5 - Diagnostic Status Register
BIT
BIT NAME
SYMBOL
DESCRIPTION
7
My
Reconfiguration
MYRECON
This bit, if high, indicates that a past reconfiguration was caused by
this node. It is set when the Lost Token Timer times out, and
should be typically read following an interrupt caused by RECON.
Refer to the Improved Diagnostics section for further detail.
6
Duplicate ID
DUPID
This bit, if high, indicates that the value in the Node ID Register
matches both Destination ID characters of the token and a
response to this token has occurred. Trailing zero's are also
verified. A logic "1" on this bit indicates a duplicate Node ID, thus
the user should write a new value into the Node ID Register. This
bit is only useful for duplicate ID detection when the device is off
line, that is, when the transmitter is disabled. When the device is
on line this bit will be set every time the device gets the token. This
bit is reset automatically upon reading the Diagnostic Status
Register. Refer to the Improved Diagnostics section for further
detail.
5
Receive
Activity
RCVACT
This bit, if high, indicates that data activity (logic "1") was detected
on the RXIN pin of the device. Refer to the Improved Diagnostics
section for further detail.
4
Token Seen
TOKEN
This bit, if high, indicates that a token has been seen on the
network, sent by a node other than this one. Refer to the Improved
Diagnostic section for further detail.
3
Excessive NAK
EXCNAK
This bit, if high, indicates that either 128 or 4 Negative
Acknowledgements have occurred in response to the Free Buffer
Enquiry. This bit is cleared upon the "POR Clear Flags" command.
Reading the Diagnostic Status Register does not clear this bit.
This bit, when set, will cause an interrupt if the corresponding bit in
the IMR is also set. Refer to the Improved Diagnostics section for
further detail.
2
Tentative ID
TENTID
This bit, if high, indicates that a response to a token whose DID
matches the value in the Tentative ID Register has occurred. The
second DID and the trailing zero's are not checked. Since each
node sees every token passed around the network, this feature
can be used with the device on-line in order to build and update a
network map. Refer to the Improved Diagnostics section for further
detail.
1
New Next ID
NEW
NXTID
This bit, if high, indicates that the Next ID Register has been
updated and that a node has either joined or left the network.
Reading the Diagnostic Status Register does not clear this bit. This
bit, when set, will cause an interrupt if the corresponding bit in the
IMR is also set. The bit is cleared by reading the Next ID Register.
1,0
(Reserved)
These bits are undefined.
30
DATA
COMMAND
Table 6 - Command Register
DESCRIPTION
0000 0000
Clear
Transmit
Interrupt
This command is used only in the Command Chaining operation.
Please refer to the Command Chaining section for definition of
this command.
0000 0001
Disable
Transmitter
This command will cancel any pending transmit command
(transmission that has not yet started) and will set the TA
(Transmitter Available) status bit to logic "1" when the COM20020
next receives the token.
0000 0010
Disable
Receiver
This command will cancel any pending receive command. If the
COM20020 is not yet receiving a packet, the RI (Receiver
Inhibited) bit will be set to logic "1" the next time the token is
received. If packet reception is already underway, reception will
run to its normal conclusion.
b0fn n100
Enable
Receive to
Page fnn
This command allows the COM20020 to receive data packets into
RAM buffer page fnn and resets the RI status bit to logic "0". The
values placed in the "nn" bits indicate the page that the data will
be received into (page 0, 1, 2, or 3). If the value of "f" is a logic
"1", an offset of 256 bytes will be added to that page specified in
"nn", allowing a finer resolution of the buffer. Refer to the
Selecting RAM Page Size section for further detail. If the value of
"b" is logic "1", the device will also receive broadcasts
(transmissions to ID zero). The RI status bit is set to logic "1"
upon successful reception of a message.
00fn n011
Enable
Transmit from
Page fnn
This command prepares the COM20020 to begin a transmit
sequence from RAM buffer page fnn the next time it receives the
token. The values of the "nn" bits indicate which page to transmit
from (0, 1, 2, or 3). If "f" is logic "1", an offset of 256 bytes is the
start of the page specified in "nn", allowing a finer resolution of the
buffer. Refer to the Selecting RAM Page Size section for further
detail. When this command is loaded, the TA and TMA bits are
reset to logic "0". The TA bit is set to logic "1" upon completion of
the transmit sequence. The TMA bit will have been set by this
time if the device has received an ACK from the destination node.
The ACK is strictly hardware level, sent by the receiving node
before its microcontroller is even aware of message reception.
Refer to Figure 1 for details of the transmit sequence and its
relation to the TA and TMA status bits.
0000 c101
Define
Configuration
This command defines the maximum length of packets that may
be handled by the device. If "c" is a logic "1", the device handles
both long and short packets. If "c" is a logic "0", the device
handles only short packets.
000r p110
Clear Flags
This command resets certain status bits of the COM20020. A
logic "1" on "p" resets the POR status bit and the EXCNAK
Diagnostic status bit. A logic "1" on "r" resets the RECON status
bit.
31
DATA
COMMAND
DESCRIPTION
0000 1000
Clear
Receive
Interrupt
This command is used only in the Command Chaining operation.
Please refer to the Command Chaining section for definition of
this command.
0001 1000
Start Internal
Operation
This command restarts the stopped internal operation after
changing CKUP1 or CKUP0 bit.
Table 7 - Address Pointer High Register
BIT
BIT NAME
SYMBOL
DESCRIPTION
7
Read Data
RDDATA
This bit tells the COM20020 whether the following access
will be a read or write. A logic "1" prepares the device for a
read, a logic "0" prepares it for a write.
6
Auto Increment
AUTOINC
This bit controls whether the address pointer will increment
automatically. A logic "1" on this bit allows automatic
increment of the pointer after each access, while a logic "0"
disables this function. Please refer to the Sequential
Access Memory section for further detail.
5-3
(Reserved)
2-0
Address 10-8
These bits are undefined.
A10-A8
These bits hold the upper three address bits which provide
addresses to RAM.
Table 8 - Address Pointer Low Register
BIT
7-0
BIT NAME
Address 7-0
SYMBOL
A7-A0
DESCRIPTION
These bits hold the lower 8 address bits which provide the
addresses to RAM.
32
Table 9 - Sub Address Register
BIT
7-3
2,1,0
BIT NAME
SYMBOL
Reserved
Sub Address 2,1,0
DESCRIPTION
These bits are undefined.
SUBAD
2,1,0
These bits determine which register at address 07 may be
accessed. The combinations are as follows:
Register
SUBAD2 SUBAD1 SUBAD0
0
0
0 Tentative ID \ (Same
0
0
1 Node ID
\ as in
0
1
0 Setup 1
/ Config
0
1
1 Next ID
/ Register)
1
0
0 Setup 2
1
0
1 Reserved
1
1
0 Reserved
1
1
1 Reserved
SUBAD1 and SUBAD0 are exactly the same as exist in the
Configuration Register. SUBAD2 is cleared automatically by
writing the Configuration Register.
Table 10 - Configuration Register
7
Reset
RESET
A software reset of the COM20020 is executed by writing a
logic "1" to this bit. A software reset does not reset the
microcontroller interface mode, nor does it affect the
Configuration Register. The only registers that the software
reset affect are the Status Register, the Next ID Register, and
the Diagnostic Status Register. This bit must be brought
back to logic "0" to release the reset.
6
Command
Chaining Enable
CCHEN
This bit, if high, enables the Command Chaining operation of
the device. Please refer to the Command Chaining section
for further details. A low level on this bit ensures software
compatibility with previous SMSC ARCNET devices.
5
Transmit Enable
TXEN
When low, this bit disables transmissions by keeping
nPULSE1, nPULSE2 if in non-Backplane Mode, and nTXEN
pin inactive. When high, it enables the above signals to be
activated during transmissions. This bit defaults low upon
reset. This bit is typically enabled once the Node ID is
determined, and never disabled during normal operation.
Please refer to the Improved Diagnostics section for details
on evaluating network activity.
33
Table 10 - Configuration Register
4,3
Extended
Timeout 1,2
ET1, ET2
These bits allow the network to operate over longer distances
than the default maximum 2 miles by controlling the
Response, Idle, and Reconfiguration Times. All nodes should
be configured with the same timeout values for proper
network operation. For the COM20020 with a 20 MHz crystal
oscillator, the bit combinations follow:
ET2
0
0
1
1
ET1
0
1
0
1
Response
Time ("S)
596.6
298.4
149.2
37.4
Idle Time
("S)
656
328
164
41
Reconfig
Time
(mS)
840
840
840
420
Note: These values are for 5Mbps and RCNTMR[1,0]=00.
Reconfiguration time is changed by the RCNTMR1 and
RCNTMR0 bits.
2
1,0
Backplane
BACKPLANE
A logic "1" on this bit puts the device into Backplane Mode
signaling which is used for Open Drain and Differential Driver
interfaces.
Sub Address 1,0
SUBAD
1,0
These bits determine which register at address 07 may be
accessed. The combinations are as follows:
SUBAD1 SUBAD0 Register
0
0
Tentative ID
0
1
Node ID
1
0
Setup 1
1
1
Next ID
See also the Sub Address Register.
34
Table 11 - Setup 1 Register
BIT
7
BIT NAME
Pulse1 Mode
SYMBOL
P1MODE
6
Four NACKS
FOUR
NACKS
5
4
Reserved
Receive All
RCVALL
3,2,1 Clock Prescaler Bits CKP3,2,1
3,2,1
0
Slow Arbitration
Select
SLOWARB
DESCRIPTION
This bit determines the type of PULSE1 output driver used
in Backplane Mode. When high, a push/pull output is used.
When low, an open drain output is used. The default is
open drain.
This bit, when set, will cause the EXNACK bit in the
Diagnostic Status Register to set after four NACKs to Free
Buffer Enquiry are detected by the COM20020. This bit,
when reset, will set the EXNACK bit after 128 NACKs to
Free Buffer Enquiry. The default is 128.
Do not set.
This bit, when set, allows the COM20020 to receive all valid
data packets on the network, regardless of their destination
ID. This mode can be used to implement a network monitor
with the transmitter on- or off-line. Note that ACKs are only
sent for packets received with a destination ID equal to the
COM20020's programmed node ID. This feature can be
used to put the COM20020 in a 'listen-only' mode, where
the transmitter is disabled and the COM20020 is not
passing tokens. Defaults low.
These bits are used to determine the data rate of the
COM20020. The following table is for a 20 MHz crystal:
(Clock Multiplier is bypassed)
CKP3 CKP2 CKP1 DIVISOR
SPEED
0
0
0
8
2.5Mbs
0
0
1
16
1.25Mbs
0
1
0
32
625Kbs
0
1
1
64
312.5Kbs
1
0
0
128
156.25Kbs
NOTE: The lowest data rate achievable by the COM20020
is 156.25Kbs. Defaults to 000 or 2.5Mbs. For Clock
Multiplier output clock speed greater than 20 MHz, CKP3,
CKP2 and CKP1 must all be zero.
This bit, when set, will divide the arbitration clock by 2.
Memory cycle times will increase when slow arbitration is
selected.
NOTE: For clock multiplier output clock speeds greater
than 40 MHz, SLOWARB must be set. Defaults to low.
35
Table 12 - Setup 2 Register
BIT
7
BIT NAME
Read Bus Timing
Select
6
5,4
Reserved
Clock Multiplier
3
Enhanced
Functions
SYMBOL
DESCRIPTION
RBUSTMG This bit is used to Disable/Enable the High Speed CPU
Read function for High Speed CPU bus support.
RBUSTMG=0: Disable (Default), RBUSTMG=1: Enable. It
does not influence write operation. High speed CPU Read
operation is only for non-multiplexed bus.
This bit is undefined.
CKUP1, 0 Higher frequency clocks are generated from the 20 MHz
crystal through the selection of these two bits as shown.
This clock multiplier is powered-down on default. After
changing the CKUP1 and CKUP0 bits, the ARCNET core
operation is stopped and the internal PLL in the clock
multiplier is awakened and it starts to generate the 40 MHz.
The lock out time of the internal PLL is 8"Sec typically.
After 1 mS it is necessary to write command data '18H' to
command register for re-starting the ARCNET core
operation. EF bit must be ‘1’ if the data rate is over 5Mbps.
CAUTION: Changing the CKUP1 and CKUP0 bits must be
one time or less after releasing a hardware reset.
CKUP1
CKUP0
Clock Frequency (Data Rate)
0
0
20 MHz (Up to 2.5Mbps) Default
0
1
40 MHz (Up to 5Mbps)
1
0
Reserved
1
1
Reserved
Note: After changing the CKUP1 or CKUP0 bits, it is
necessary to write a command data '18H' to the command
register. Because after changing the CKUP [1, 0] bits, the
internal operation is stopped temporarily. The writing of the
command is to start the operation.
These initializing steps are shown below.
1) Hardware reset (Power ON)
2) Change CKUP[1, 0] bit
3) Wait 1mSec (wait until stable oscillation)
4) Write command '18H' (start internal operation)
5) Start initializing routine (Execute existing software)
EF
This bit is used to enable the new enhanced functions in the
COM20020. EF = 0: Disable (Default), EF = 1: Enable. If EF
= 0, the timing and function is the same as in the
COM20020, Revision B. See appendix “A”. EF bit must be
‘1’ if the data rate is over 5Mbps.
EF bit should be ‘1’ for new design customers.
EF bit should be ‘0’ for replacement customers.
36
BIT
2
BIT NAME
No Synchronous
1,0
Reconfiguration
Timer 1, 0
SYMBOL
NOSYNC
DESCRIPTION
This bit is used to enable the SYNC command during
initialization. NOSYNC= 0, Enable (Default) The line must
be idle for the RAM initialization sequence to be written.
NOSYNC= 1, Disable:) The line does not have to be idle for
the RAM initialization sequence to be written. See appendix
“A”.
RCNTM1,0 These bits are used to program the reconfiguration timer as a
function of maximum node count. These bits set the time out
period of the reconfiguration timer as shown below. The
time out periods shown are for 5 Mbps.
RCNTM1 RCNTM0
Time Out
Max Node Count
Period
0
0
420 mS
Up to 255 nodes
0
1
105 mS
Up to 64 nodes
1
0
52.5 mS
Up to 32 nodes
1
1
26.25 mS*
Up to 16 nodes
Note*: The node ID value 255 must exist in the network for
26.25 mS timeout to be valid.
37
Data Register
Memory
Data Bus
I/O Address 04H
2K x 8
INTERNAL
RAM
8
D0-D7
Address Pointer Register
I/O Address 02H
I/O Address 03H
High
Low
Memory
Address Bus
11-Bit Counter
11
FIGURE 8 – SEQUENTIAL ACCESS OPERATION
38
INTERNAL RAM
the pointer being loaded and the first read (see
timing parameters).
The integration of the 2K x 8 RAM in the
COM20020 represents significant real estate
savings. The most obvious benefit is the 48 pin
package in which the device is now placed (a
direct result of the integration of RAM). In
addition, the PC board is now free of the
cumbersome external RAM, external latch, and
multiplexed address/data bus and
control
functions which were necessary to interface to the
RAM.
The integration of RAM represents
significant cost savings because it isolates the
system designer from the changing costs of
external RAM and it minimizes reliability problems,
assembly time and costs, and layout complexity.
Access Speed
The COM20020 is able to accommodate very fast
access cycles to its registers and buffers.
Arbitration to the buffer does not slow down the
cycle because the pointer based access method
allows data to be prefetched from memory and
stored in a temporary register. Likewise, data to
be written is stored in the temporary register and
then written to memory.
For systems which do not require quick access
time, the arbitration clock may be slowed down by
setting bit 0 of the Setup1 Register equal to logic
"1". Since the Slow Arbitration feature divides the
input clock by two, the duty cycle of the input clock
may be relaxed.
Sequential Access Memory
The internal RAM is accessed via a pointer-based
scheme. Rather than interfering with system
memory, the internal RAM is indirectly accessed
through the Address High and Low Pointer
Registers. The data is channeled to and from the
microcontroller via the 8-bit data register. For
example: a packet in the internal RAM buffer is
read by the microcontroller by writing the
corresponding address into the Address Pointer
High and Low Registers (offsets 02H and 03H).
Note that the High Register should be written first,
followed by the Low Register, because writing to
the Low Register loads the address. At this point
the device accesses that location and places the
corresponding data into the data register. The
microcontroller then reads the data register (offset
04H) to obtain the data at the specified location. If
the Auto Increment bit is set to logic "1", the
device will automatically increment the address
and place the next byte of data into the data
register, again to be read by the microcontroller.
This process is continued until the entire packet
is read out of RAM. Refer to Figure 8 for an
illustration of the Sequential Access operation.
When switching between reads and writes, the
pointer must first be written with the starting
address. At least one cycle time should separate
SOFTWARE INTERFACE
The microcontroller interfaces to the COM20020
via software by accessing the various registers.
These actions are described in the Internal
Registers section.
The software flow for
accessing the data buffer is based on the
Sequential Access scheme. The basic sequence
is as follows:
#$ Disable Interrupts
#$ Write to Pointer Register High (specifying
Auto-Increment mode)
#$ Write to Pointer Register Low (this loads the
address)
#$ Enable Interrupts
#$ Read or Write the Data Register (repeat as
many times as necessary to empty or fill the
buffer)
#$ The pointer may now be read to determine
how many transfers were completed.
The software flow for controlling the Configuration,
Node ID, Tentative ID, and Next ID registers is
generally limited to the initialization sequence and
the maintenance of the network map.
39
Additionally, it is necessary to understand the
details of how the other Internal Registers are
used in the transmit and receive sequences and to
know how the internal RAM buffer is properly set
up. The sequence of events that tie these actions
together is discussed as follows.
each receive page if the device is configured to
handle long packets. The COM20020 does not
check page boundaries during reception. If the
device is configured to handle only short packets,
then both transmit and receive pages may be
allocated as 256 bytes long, freeing at least
1KByte at any given time.
Selecting RAM Page Size
Even if the Command Chaining operation is being
used, 1KByte is still guaranteed to be free
because Command Chaining only requires two
pages for transmit and two for receive (in this
case, a total of four 256 byte pages, leaving 1K
free).
During normal operation, the 2K x 8 of RAM is
divided into four pages of 512 bytes each. The
page to be used is specified in the "Enable
Transmit (Receive) from (to) Page fnn" command,
where "nn" specifies page 0, 1, 2, or 3. This
allows the user to have constant control over the
allocation of RAM.
The general rule which may be applied to
determine where in RAM a page begins is as
follows:
Address = (nn x 512) + (f x 256).
When the Offset bit "f" (bit 5 of the "Enable
Transmit (Receive) from (to) Page fnn" command
word) is set to logic "1", an offset of 256 bytes is
added to the page specified. For example: to
transmit from the second half of page 0, the
command "Enable Transmit from Page fnn"
(fnn=100 in this case) is issued by writing 0010
0011 to the Command Register. This allows a
finer resolution of the buffer pages without
affecting software compatibility. This scheme is
useful for applications which frequently use packet
sizes of 256 bytes or less, especially for
microcontroller systems with limited memory
capacity. The remaining portions of the buffer
pages which are not allocated for current transmit
or receive packets may be used as temporary
storage for previous network data, packets to be
sent later, or as extra memory for the system,
which may be indirectly accessed.
Transmit Sequence
During a transmit sequence, the microcontroller
selects a 256 or 512 byte segment of the RAM
buffer and writes into it. The appropriate buffer
size is specified in the "Define Configuration"
command. When long packets are enabled, the
COM20020 interprets the packet as either a long
or short packet, depending on whether the buffer
address 2 contains a zero or non-zero value. The
format of the buffer is shown in Figure 9. Address
0 contains the Source Identifier (SID); Address 1
contains the Destination Identifier (DID); Address
2 (COUNT) contains, for short packets, the value
256-N, where N represents the number of
information bytes in the message, or for long
packets, the value 0, indicating that it is indeed a
long packet. In the latter case, Address 3
(COUNT) would contain the value 512-N, where N
represents the number of information bytes in
If the device is configured to handle both long and
short packets (see "Define Configuration"
command), then receive pages should always be
512 bytes long because the user never knows
what the length of the receive packet will be. In
this case, the transmit pages may be made 256
bytes long, leaving at least 512 bytes free at any
given time. Even if the Command Chaining
operation is being used, 512 bytes is still
guaranteed to be free because Command
Chaining only requires two pages for transmit and
two for receive (in this case, two 256 byte pages
for transmit and two 512 byte pages for receive,
leaving 512 bytes free). Please note that it is the
responsibility of software to reserve 512 bytes for
40
ADDRESS
0
SHORT PACKET
FORMAT
LONG PACKET
FORMAT
SID
ADDRESS
0
1
DID
1
DID
2
COUNT = 256-N
2
0
3
COUNT = 512-N
NOT USED
COUNT
SID
NOT USED
DATA BYTE 1
DATA BYTE 2
COUNT
DATA BYTE 1
DATA BYTE 2
DATA BYTE N-1
255
DATA BYTE N
DATA BYTE N-1
NOT USED
511
511
DATA BYTE N
N = DATA PACKET LENGTH
SID = SOURCE ID
DID = DESTINATION ID
(DID = 0 FOR BROADCASTS)
FIGURE 9 - RAM BUFFER PACKET CONFIGURATION
41
the message. The SID in Address 0 is used by
the receiving node to reply to the transmitting
node. The COM20020 puts the local ID in this
location, therefore it is not necessary to write into
this location. Please note that a short packet may
contain between 1 and 253 data bytes, while a
long packet may contain between 257 and 508
data bytes. A minimum value of 257 exists on a
long packet so that the COUNT is expressible in
eight bits. This leaves three exception packet
lengths which do not fit into either a short or long
packet; packet lengths of 254, 255, or 256 bytes.
If packets of these lengths must be sent, the user
must add dummy bytes to the packet in order to
make the packet fit into a long packet.
with an ACK. An unsuccessful transmission
occurs when the receiving node does not respond
to the packet.
The second possibility is if the destination node
responds to the Free Buffer Enquiry with a
Negative AcKnowledgement.
A NAK occurs
when the RI bit of the destination node is a logic
"1". In this case, the token is passed on from the
transmitting node to the next node. The next time
the transmitter receives the token, it will again
transmit a FREE BUFFER ENQUIRY. If a NAK is
again received, the token is again passed onto the
next node.
The Excessive NAK bit of the
Diagnostic Status Register is used to prevent an
endless sending of FBE's and NAK's. If no limit of
FBE-NAK sequences existed, the transmitting
node would continue issuing a Free Buffer
Enquiry, even though it would continuously
receive a NAK as a response. The EXCNAK bit
generates an interrupt (if enabled) in order to tell
the microcontroller to disable the transmitter via
the "Disable Transmitter" command. This causes
the transmission to be abandoned and the TA bit
to be set to a logic "1" when the node next
receives the token, while the TMA bit remains at a
logic "0". Please refer to the Improved Diagnostics
section for further detail on the EXCNAK bit.
Once the packet is written into the buffer, the
microcontroller awaits a logic "1" on the TA bit,
indicating that a previous transmit command has
concluded and another may be issued. Each time
the message is loaded and a transmit command
issued, it will take a variable amount of time before
the message is transmitted, depending on the
traffic on the network and the location of the token
at the time the transmit command was issued.
The conclusion of the Transmit Command will
generate an interrupt if the Interrupt Mask allows
it. If the device is configured for the Command
Chaining operation, please see the Command
Chaining section for further detail on the transmit
sequence. Once the TA bit becomes a logic "1",
the microcontroller may issue the "Enable
Transmit from Page fnn" command, which resets
the TA and TMA bits to logic "0". If the message
is not a BROADCAST, the COM20020
automatically sends a FREE BUFFER ENQUIRY
to the destination node in order to send the
message. At this point, one of four possibilities
may occur.
The third possibility which may occur after a
FREE BUFFER ENQUIRY is issued is if the
destination node does not respond at all. In this
case, the TA bit is set to a logic "1", while the TMA
bit remains at a logic "0". The user should
determine whether the node should try to reissue
the transmit command.
The fourth possibility is if a non-traditional
response is received (some pattern other than
ACK or NAK, such as noise). In this case, the
token is not passed onto the next node, which
causes the Lost Token Timer of the next node to
time
out,
thus
generating
a
network
reconfiguration.
The first possibility is if a free buffer is available at
the destination node, in which case it responds
with an ACKnowledgement. At this point, the
COM20020 fetches the data from the Transmit
Buffer and performs the transmit sequence. If a
successful transmit sequence is completed, the
TMA bit and the TA bit are set to logic "1". If the
packet was not transmitted successfully, TMA will
not be set. A successful transmission occurs
when the receiving node responds to the packet
The "Disable Transmitter" command may be used
to cancel any pending transmit command when
the COM20020 next receives the token. Normally,
in an active network, this command will set the TA
status bit to a logic "1" when the token is received.
If the "Disable Transmitter" command does not
cause the TA bit to be set in the time it takes the
42
token to make a round trip through the network,
one of three situations exists. Either the node is
disconnected from the network, or there are no
other nodes on the network, or the external
receive circuitry has failed. These situations can
be determined by either using the improved
diagnostic features of the COM20020 or using
another software timeout which is greater than the
worst case time for a round trip token pass, which
occurs when all nodes transmit a maximum length
message.
Configuration" command has enabled the
reception of long packets, the COM20020
interprets the packet as either a long or short
packet, depending on whether the content of the
buffer location 2 is zero or non-zero. The format
of the buffer is shown in Figure 10. Address 0
contains the Source Identifier (SID), Address 1
contains the Destination Identifier (DID), and
Address 2 contains, for short packets, the value
256-N, where N represents the message length,
or for long packets, the value 0, indicating that it is
indeed a long packet. In the latter case, Address
3 contains the value 512-N, where N represents
the message length. Note that on reception, the
COM20020 deposits packets into the RAM buffer
in the same format that the transmitting node
arranges them, which allows for a message to be
received and then retransmitted without
rearranging any bytes in the RAM buffer other
than the SID and DID. Once the packet is
received and stored correctly in the selected
buffer, the COM20020 sets the RI bit to logic "1"
to signal the microcontroller that the reception is
complete.
Receive Sequence
A receive sequence begins with the RI status bit
becoming a logic "1", which indicates that a
previous
reception
has
concluded.
The
microcontroller will be interrupted if the
corresponding bit in the Interrupt Mask Register is
set to logic "1". Otherwise, the microcontroller
must periodically check the Status Register. Once
the microcontroller is alerted to the fact that the
previous reception has concluded, it may issue
the "Enable Receive to Page fnn" command,
which resets the RI bit to logic "0" and selects a
new page in the RAM buffer.
Again, the
appropriate buffer size is specified in the "Define
Configuration" command. Typically, the page
which just received the data packet will be read by
the microcontroller at this point. Once the "Enable
Receive to Page fnn" command is issued, the
microcontroller attends to other duties. There is
no way of knowing how long the new reception will
take, since another node may transmit a packet at
any time. When another node does transmit a
packet to this node, and if the "Define
MSB
TRI
LSB
RI
TA
POR
TEST
TRI
RECON
TMA
TTA
TMA
TTA
FIGURE 10 - COMMAND CHAINING STATUS REGISTER QUEUE
43
COMMAND CHAINING
#$
The Command Chaining operation allows
consecutive transmissions and receptions to occur
without host microcontroller intervention.
#$
Through the use of a dual two-level FIFO,
commands to be transmitted and received, as well
as the status bits, are pipelined.
In order for the COM20020 to be compatible with
previous SMSC ARCNET device drivers, the
device defaults to the non-chaining mode. In order
to take advantage of the Command Chaining
operation, the Command Chaining Mode must be
enabled via a logic "1" on bit 6 of the Configuration
Register.
#$
Up to two outstanding transmit interrupts and
two outstanding receive interrupts are stored
by the device, along with their respective
status bits.
The Interrupt Mask bits act on TTA (Rising
Transition on Transmitter Available) for
transmit operations and TRI (Rising
Transition of Receiver Inhibited) for receive
operations. TTA is set upon completion of a
packet transmission only. TRI is set upon
completion of a packet reception only.
Typically there is no need to mask the TTA
and TRI bits after clearing the interrupt.
The traditional TA and RI bits are still
available to reflect the present status of the
device.
Transmit Command Chaining
In Command Chaining, the Status Register
appears as in Figure 10.
When the processor issues the first "Enable
Transmit to Page fnn" command, the COM20020
responds in the usual manner by resetting the TA
and TMA bits to prepare for the transmission from
the specified page. The TA bit can be used to see
if there is currently a transmission pending, but the
TA bit is really meant to be used in the nonchaining mode only. The TTA bits provide the
relevant information for the device in the
Command Chaining mode.
The following is a list of Command Chaining
guidelines for the software programmer. Further
detail can be found in the Transmit Command
Chaining and Receive Command Chaining
sections.
#$ The device is designed such that the interrupt
service routine latency does not affect
performance.
#$ Up to two outstanding transmissions and two
outstanding receptions can be pending at any
given time. The commands may be given in
any order.
44
In the Command Chaining Mode, at any time after
the first command is issued, the processor can
issue a second "Enable Transmit from Page fnn"
command. The COM20020 stores the fact that
the second transmit command was issued, along
with the page number.
(at EF=1) interrupt inactive time interval before the
following edge.
The Transmitter Available (TA) bit of the Interrupt
Mask Register now masks only the TTA bit of the
Status Register, not the TA bit as in the nonchaining mode. Since the TTA bit is only set upon
transmission of a packet (not by RESET), and
since the TTA bit may easily be reset by issuing a
"Clear Transmit Interrupt" command, there is no
need to use the TA bit of the Interrupt Mask
Register to mask interrupts generated by the TTA
bit of the Status Register.
After the first transmission is completed, the
COM20020 updates the Status Register by setting
the TTA bit, which generates an interrupt. The
interrupt service routine should read the Status
Register. At this point, the TTA bit will be found to
be a logic "1" and the TMA (Transmit Message
Acknowledge) bit will tell the processor whether
the transmission was successful. After reading
the Status Register, the "Clear Transmit Interrupt"
command is issued, thus resetting the TTA bit and
clearing the interrupt.
Note that only the
"Clear Transmit Interrupt" command will clear the
TTA bit and the interrupt. It is not necessary,
however, to clear the bit or the interrupt right away
because the status of the transmit operation is
double buffered in order to retain the results of the
first transmission for analysis by the processor.
This information will remain in the Status Register
until the "Clear Transmit Interrupt" command is
issued. Note that the interrupt will remain active
until the command is issued, and the second
interrupt will not occur until the first interrupt is
acknowledged. The COM20020 guarantees a
minimum of 200nS (at EF=1) interrupt inactive
time interval between interrupts. The TMA bit is
also double buffered to reflect whether the
appropriate transmission was a success. The
TMA bit should only be considered valid after the
corresponding TTA bit has been set to a logic "1".
The TMA bit never causes an interrupt.
In Command Chaining mode, the "Disable
Transmitter" command will cancel the oldest
transmission. This permits canceling a packet
destined for a node not ready to receive. If both
packets should be canceled, two "Disable
Transmitter" commands should be issued.
Receive Command Chaining
Like the Transmit Command Chaining operation,
the processor can issue two consecutive "Enable
Receive from Page fnn" commands.
After the first packet is received into the first
specified page, the TRI bit of the Status Register
will be set to logic "1", causing an interrupt.
Again, the interrupt need not be serviced
immediately.
Typically, the interrupt service
routine will read the Status Register. At this point,
the RI bit will be found to be a logic "1". After
reading the Status Register, the "Clear Receive
Interrupt" command should be issued, thus
resetting the TRI bit and clearing the interrupt.
Note that only the "Clear Receive Interrupt"
command will clear the TRI bit and the interrupt. It
is not necessary, however, to clear the bit or the
interrupt right away because the status of the
receive operation is double buffered in order to
retain the results of the first reception for analysis
by the processor, therefore the information will
remain in the Status Register until the "Clear
Receive Interrupt" command is issued. Note that
the interrupt will remain active until the "Clear
Receive Interrupt" command is issued, and the
second interrupt will be stored until the first
interrupt is acknowledged. A minimum of 200nS
When the token is received again, the second
transmission will be automatically initiated after
the first is completed by using the stored "Enable
Transmit from Page fnn" command. The operation
is as if a new "Enable Transmit from Page fnn"
command has just been issued. After the first
Transmit status bits are cleared, the Status
Register will again be updated with the results of
the second transmission and a second interrupt
resulting from the second transmission will occur.
The COM20020 guarantees a minimum of 200ns
45
(at EF=1) interrupt inactive time interval between
interrupts is guaranteed.
Registers, the Configuration Register, or the
Setup1 Register. A hardware reset occurs when a
low signal is asserted on the nRESET input. The
minimum reset pulse width is 5TXTL. This pulse
width is used by the internal digital filter, which
filters short glitches to allow only valid resets to
occur.
The second reception will occur as soon as a
second packet is sent to the node, as long as the
second "Enable Receive to Page fnn" command
was issued. The operation is as if a new "Enable
Receive to Page fnn" command has just been
issued. After the first Receive status bits are
cleared, the Status Register will again be updated
with the results of the second reception and a
second interrupt resulting from the second
reception will occur.
Upon reset, the transmitter portion of the device is
disabled and the internal registers assume those
states outlined in the Internal Registers section.
After the nRESET signal is removed the user may
write to the internal registers. Since writing a nonzero value to the Node ID Register wakes up the
COM20020 core, the Setup1 Register should be
written before the Node ID Register. Once the
Node ID Register is written to, the COM20020
reads the value and executes two write cycles to
the RAM buffer. Address 0 is written with the data
D1H and address 1 is written with the Node ID.
The data pattern D1H was chosen arbitrarily, and
is meant to provide assurance of proper
microsequencer operation.
In the COM20020, the Receive Inhibit (RI) bit of
the Interrupt Mask Register now masks only the
TRI bit of the Status Register, not the RI bit as in
the non-chaining mode. Since the TRI bit is only
set upon reception of a packet (not by RESET),
and since the TRI bit may easily be reset by
issuing a "Clear Receive Interrupt" command,
there is no need to use the RI bit of the Interrupt
Mask Register to mask interrupts generated by
the TRI bit of the Status Register. In Command
Chaining mode, the "Disable Receiver" command
will cancel the oldest reception, unless the
reception has already begun. If both receptions
should be canceled, two "Disable Receiver"
commands should be issued.
INITIALIZATION SEQUENCE
Bus Determination
Writing to and reading from an odd address
location from the COM20020's address space
causes the COM20020 to determine the
appropriate bus interface. When the COM20020
is powered on the internal registers may be
written to. Since writing a non-zero value to the
Node ID Register wakes up the core, the Setup1
Register should be written to before the Node ID
Register. Until a non-zero value is placed into the
NID Register, no microcode is executed, no
tokens are passed by this node, and no
reconfigurations are generated by this node.
Once a non-zero value is placed in the register,
the core wakes up, but the node will not attempt to
join the network until the TX Enable bit of the
Configuration Register is set.
RESET DETAILS
Internal Reset Logic
The COM20020 includes special reset circuitry to
guarantee smooth operation during reset. Special
care is taken to assure proper operation in a
variety of systems and modes of operation. The
COM20020 contains digital filter circuitry and a
Schmitt Trigger on the nRESET signal to reject
glitches in order to ensure fault-free operation.
The COM20020 supports two reset options;
software and hardware reset. A software reset is
generated when a logic "1" is written to bit 7 of the
Configuration Register. The device remains in
reset as long as this bit is set. The software reset
does not affect the microcontroller
interface
modes determined after hardware reset, nor does
it affect the contents of the Address Pointer
Before setting the TX Enable bit, the software may
make some determinations. The software may
first observe the Receive Activity and the Token
Seen bits of the Diagnostic Status Register to
verify the health of the receiver and the network.
46
Next, the uniqueness of the Node ID value placed
in the Node ID Register is determined. The TX
Enable bit should still be a logic "0" until it is
ensured that the Node ID is unique. If this node
ID already exists, the Duplicate ID bit of the
Diagnostic Status Register is set after a maximum
of 420mS (or 840mS if the ET1 and ET2 bits are
other than 1,1). To determine if another node on
the network already has this ID, the COM20020
compares the value in the Node ID Register with
the DID's of the token, and determines whether
there is a response to it. Once the Diagnostic
Status Register is read, the DUPID bit is cleared.
The user may then attempt a new ID value, wait
420mS before checking the Duplicate ID bit, and
repeat the process until a unique Node ID is
found. At this point, the TX Enable bit may be set
to allow the node to join the network. Once the
node joins the network, a reconfiguration occurs,
as usual, thus setting the MYRECON bit of the
Diagnostic Status Register.
interrupt service routine will typically read the
MYRECON bit of the Diagnostic Status Register.
Reading the Diagnostic Status Register resets the
MYRECON bit. Successive occurrences of a
logic "1" on the MYRECON bit indicates that a
problem exists with this node. At that point, the
transmitter should be disabled so that the entire
network is not held down while the node is being
evaluated.
The Duplicate ID (DUPID) bit is used before the
node joins the network to ensure that another
node with the same ID does not exist on the
network. Once it is determined that the ID in the
Node ID Register is unique, the software should
write a logic "1" to bit 5 of the Configuration
Register to enable the basic transmit function.
This allows the node to join the network.
The Receive Activity (RCVACT) bit of the
Diagnostic Status Register will be set to a logic "1"
whenever activity (logic "1") is detected on the
RXIN pin.
The Tentative ID Register may be used to build a
network map of all the nodes on the network, even
once the COM20020 has joined the network.
Once a value is placed in the Tentative ID
Register, the COM20020 looks for a response to a
token whose DID matches the Tentative ID
Register. The software can record this information
and continue placing Tentative ID values into the
register to continue building the network map. A
complete network map is only valid until nodes are
added to or deleted from the network. Note that a
node cannot detect the existence of the next
logical node on the network when using the
Tentative ID. To determine the next logical node,
the software should read the Next ID Register.
The Token Seen (TOKEN) bit is set to a logic "1"
whenever any token has been seen on the
network (except those tokens transmitted by this
node).
The RCVACT and TOKEN bits may help the user
to troubleshoot the network or the node. If unusual
events are occurring on the network, the user may
find it valuable to use the TXEN bit of the
Configuration Register to qualify events. Different
combinations of the RCVACT, TOKEN, and TXEN
bits, as shown indicate different situations:
Normal Results:
IMPROVED DIAGNOSTICS
RCVACT=1, TOKEN=1, TXEN=0: The node is
not part of the network. The network is operating
properly without this node.
The COM20020 allows the user to better manage
the operation of the network through the use of the
internal Diagnostic Status Register.
RCVACT=1, TOKEN=1, TXEN=1: The node sees
receive activity and sees the token. The basic
transmit function is enabled. Network and node
are operating properly.
A high level on the My Reconfiguration
(MYRECON) bit indicates that the Token
Reception Timer of this node expired, causing a
reconfiguration by this node.
After the
Reconfiguration (RECON) bit of the Status
Register interrupts the microcontroller, the
MYRECON=0, DUPID=0, RCVACT=1, TXEN=0,
TOKEN=1: Single node network.
47
Abnormal Results:
The New Next ID bit permits the software to
detect the withdrawal or addition of nodes to the
network.
RCVACT=1, TOKEN=0, TXEN=X: The node
sees receive activity, but does not see the token.
Either no other nodes exist on the network, some
type of data corruption exists, the media driver is
malfunctioning, the topology is set up incorrectly,
there is noise on the network, or a reconfiguration
is occurring.
The Tentative ID bit allows the user to build a
network map of those nodes existing on the
network.
This feature is useful because it
minimizes the need for human intervention. When
a value placed in the Tentative ID Register
matches the Node ID of another node on the
network, the TENTID bit is set, telling the software
that this NODE ID already exists on the network.
The software should periodically place values in
the Tentative ID Register and monitor the New
Next ID bit to maintain an updated network map.
RCVACT=0, TOKEN=0, TXEN=1: No receive
activity is seen and the basic transmit function is
enabled. The transmitter and/or receiver are not
functioning properly.
RCVACT=0, TOKEN=0, TXEN=0: No receive
activity and basic transmit function disabled. This
node is not connected to the network.
OSCILLATOR
The Excessive NAK (EXCNAK) bit is used to
replace
a
timeout
function
traditionally
implemented in software.
This function is
necessary to limit the number of times a sender
issues a FBE to a node with no available buffer.
When the destination node replies to 128 FBEs
with 128 NAKs or 4 FBEs with 4 NAKs, the
EXCNAK bit of the sender is set, generating an
interrupt. At this point the software may abandon
the transmission via the "Disable Transmitter"
command. This sets the TA bit to logic "1" when
the node next receives the token, to allow a
different transmission to occur. The timeout value
for the EXNACK bit (128 or 4) is determined by
the FOUR-NAKS bit on the Setup1 Register.
The COM20020 contains circuitry which, in
conjunction with an external parallel resonant
crystal or TTL clock, forms an oscillator.
If an external crystal is used, two capacitors are
needed (one from each leg of the crystal to
ground). No external resistor is required, since
the COM20020 contains an internal resistor. The
crystal must have an accuracy of 0.020% or
better. The oscillation frequency range is from 10
MHz to 20 MHz.
The crystal must have an accuracy of 0.010% or
better when the internal clock multiplier is turned
on. The oscillation frequency must be 20MHz
when the internal clock multiplier is turned on.
The user may choose to wait for more NAK's
before disabling the transmitter by taking
advantage of the wraparound counter of the
EXCNAK bit. When the EXCNAK bit goes high,
indicating 128 or 4 NAKs, the "POR Clear Flags"
command maybe issued to reset the bit so that it
will go high again after another count of 128 or 4.
The software may count the number of times the
EXCNAK bit goes high, and once the final count is
reached, the "Disable Transmitter" command may
be issued.
The XTAL2 side of the crystal may be loaded with
a single 74HC-type buffer in order to generate a
clock for other devices.
The user may attach an external TTL clock, rather
than a crystal, to the XTAL1 signal. In this case, a
390! pull-up resistor is required on XTAL1, while
XTAL2 should be left unconnected.
48
OPERATIONAL DESCRIPTION
MAXIMUM GUARANTEED RATINGS*
o
o
Operating Temperature Range ................................................................................................. -40 C to +85 C
o
o
Storage Temperature Range .................................................................................................. -55 C to +150 C
o
Lead Temperature (soldering, 10 seconds)..........................................................................................+325 C
Positive Voltage on any pin, with respect to ground ......................................................................... VDD+0.3V
Negative Voltage on any pin, with respect to ground ...............................................................................-0.3V
Maximum VDD ............................................................................................................................................. +7V
*Stresses above those listed may cause permanent damage to the device. This is a stress rating only and
functional operation of the device at these or any other condition above those indicated in the operational
sections of this specification is not implied.
Note: When powering this device from laboratory or system power supplies, it is important that the
Absolute Maximum Ratings not be exceeded or device failure can result. Some power supplies exhibit
voltage spikes or "glitches" on their outputs when the AC power is switched on or off. In addition, voltage
transients on the AC power line may appear on the DC output. If this possibility exists it is suggested that
a clamp circuit be used.
DC ELECTRICAL CHARACTERISTICS
VDD=3.3V±0.3V
o
o
TA=-40 C to +85 C
PARAMETER
SYMBOL
Low Input Voltage 1
VIL1
(All inputs except A2,
XTAL1, nRESET, nRD,
nWR, and RXIN)
High Input Voltage 1
VIH1
(All inputs except A2,
XTAL1, nRESET, nRD,
nWR, and RXIN)
Low Input Voltage 2
VIL2
(XTAL1)
High Input Voltage 2
VIH2
(XTAL1)
Low to High Threshold
VILH
Input Voltage
(A2, nRESET, nRD, nWR,
and RXIN)
High to Low Threshold
VIHL
Input Voltage
(A2, nRESET, nRD, nWR,
and RXIN)
MIN
TYP
MAX
0.8
2.0
1.0
2.4
49
UNIT
V
COMMENT
TTL Levels
V
TTL Levels
V
TTL Clock Input
V
1.8
V
1.2
V
Schmitt Trigger,
All Values at VDD =
3.3V
PARAMETER
Low Output Voltage 1
(nPULSE1 in Push/Pull
Mode, nPULSE2,
NTXEN)
High Output Voltage 1
(nPULSE1 in Push/Pull
Mode, nPULSE2,
nTXEN)
Low Output Voltage 2
(D0-D7)
High Output Voltage 2
(D0-D7)
Low Output Voltage 3
(nINTR)
High Output Voltage 3
(nINTR)
Low Output Voltage 4
(nPULSE1 in Open-Drain
Mode)
Dynamic VDD Supply
Current
Input Pull-up Current
(nPULSE1 in Open-Drain
Mode, A1, AD0-AD2,
D3-D7)
Input Leakage Current
(All inputs except A1,
AD0-AD2, D3-D7,
XTAL1, XTAL2
SYMBOL
VOL1
MIN
VOH1
2.4
TYP
VOL2
VOH2
0.4
2.4
VOL3
VOH3
MAX
0.4
0.8
2.4
UNIT
V
COMMENT
ISINK=2mA
V
ISOURCE=-1mA
V
ISINK=8mA
V
ISOURCE=-6mA
V
ISINK=12mA
V
ISOURCE=-5mA
VOL4
0.5
V
ISINK=24mA
Open Drain Driver
IDD
TBD
mA
200
"A
5 Mbps
All Outputs Open
VIN=0.0V
±10
"A
VSS < VIN < VDD
80
IP
IL
50
CAPACITANCE (TA = 25%C; fC = 1MHz; VDD = 0V)
Output and I/O pins capacitive load specified as follows:
PARAMETER
SYMBOL
MIN
TYP
MAX
UNIT
CIN
5.0
pF
COUT1
45
pF
COUT2
400
pF
Input Capacitance
Output Capacitance 1
(All outputs except
XTAL2, nPULSE1 in
Push/Pull Mode)
Output Capacitance 2
(nPULSE1, in BackPlane
Mode Only - Open
Drain)
COMMENT
Maximum
Capacitive Load
which can be
supported by each
output.
AC Measurements are taken at the following points:
Inputs:
Outputs:
t
t
2.0V
2.4V
1.4V
50%
0.8V
0.4V
t
2.4V
2.0V
1.4V
0.4V
50%
0.8V
t
Inputs are driven at 2.4V for logic "1" and 0.4 V for logic "0" except XTAL1 pin.
Outputs are measured at 2.0V min. for logic "1" and 0.8V max. for logic "0".
51
TIMING DIAGRAMS
AD0-AD2,
D3-D7
VALID DATA
VALID
t1
t2,
t4
nCS
t3
t12
t11
ALE
t6
t5
nDS
t7
t13
t14
Note 2
t8
t9
DIR
t10
MUST BE: RBUSTMG bit = 0
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
min
Address Setup to ALE Low
Address Hold from ALE Low
nCS Setup to ALE Low
nCS Hold from ALE Low
ALE Low to nDS Low
nDS Low to Valid Data
nDS High to Data High Impedance
Cycle Time (nDS Low to Next Time Low)
DIR Setup to nDS Active
DIR Hold from nDS Inactive
ALE High Width
ALE Low Width
nDS Low Width
nDS High Width
20
10
10
10
15
0
4TARB*
10
10
20
20
60
20
max
40
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
Note 2: !
FIGURE 11 - MULTIPLEXED BUS, 68XX-LIKE CONTROL SIGNALS; READ CYCLE
52
AD0-AD2,
D3-D7
VALID DATA
VALID
t1
t2,
t4
nCS
t3
ALE
t10
t9
nRD
t6
t7
t5
nWR
t11
t13 "
#
t8
t12
"
$
MUST BE: RBUSTMG bit = 0
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
min
Address Setup to ALE Low
Address Hold from ALE Low
nCS Setup to ALE Low
nCS Hold from ALE Low
ALE Low to nRD Low
nRD Low to Valid Data
nRD High to Data High Impedance
Cycle Time (nRD Low to Next Time Low)
ALE High Width
ALE Low Width
nRD Low Width
nRD High Width
nWR
to nRD Low
max
20
10
10
10
15
0
4TARB*
20
20
60
20
20
40
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
Note 2: !
Note 3: %
!
FIGURE 12 - MULTIPLEXED BUS, 80XX-LIKE CONTROL SIGNALS; READ CYCLE
53
AD0-AD2,
D3-D7
VALID DATA
VALID
t1
t2,
t4
nCS
t3
t12
t11
ALE
t7
t5
t6
nDS
Note 2
t8**
t13
t14
DIR
t9
t10
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
Address Setup to ALE Low
Address Hold from ALE Low
nCS Setup to ALE Low
nCS Hold from ALE Low
ALE Low to nDS Low
Valid Data Setup to nDS High
Data Hold from nDS High
Cycle Time (nDS
to Next
DIR Setup to nDS Active
DIR Hold from nDS Inactive
ALE High Width
ALE Low Width
nDS Low Width
nDS High Width
t8
20
10
10
10
15
30
10
4TARB*
10
10
20
20
20
20
)**
max
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
The Microcontroller typically accesses the COM20020 on every other cycle.
Note 1:
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
** Note 2:
Any cycle occurring after a write to Address Pointer Low Register requires a
minimum of 4TARB from the trailing edge of nDS to the leading edge of the
next nDS.
Write cycle for Address Pointer Low Register occurring after an access to
Data Register requires a minimum of 5TARB from the trailing edge of nDS to
the leading edge of the next nDS.
FIGURE 13 - MULTIPLEXED BUS, 68XX-LIKE CONTROL SIGNALS; WRITE CYCLE
54
AD0-AD2,
D3-D7
VALID
t1
VALID DATA
t2,
t4
nCS
t3
t10
t9
ALE
t7
t5
nWR
t6
Note 2
t8**
nRD
t13
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t12
t13
Address Setup to ALE Low
Address Hold from ALE Low
nCS Setup to ALE Low
nCS Hold from ALE Low
ALE Low to nDS Low
Valid Data Setup to nDS High
Data Hold from nDS High
Cycle Time (nWR
to Next
ALE High Width
ALE Low Width
nWR Low Width
nWR High Width
nRD
to nWR Low
t12
t11
Note 3
20
10
10
10
15
30
10
4TARB*
)**
20
20
20
20
20
max
t8
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
The Microcontroller typically accesses the COM20020 on every other cycle.
Note 1:
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
** Note 2:
Any cycle occurring after a write to Address Pointer Low Register requires a
minimum of 4TARB from the trailing edge of nWR to the leading edge of the
next nWR.
Write cycle for Address Pointer Low Register occurring after a write to Data
Register requires a minimum of 5TARB from the trailing edge of nWR to the
leading edge of the next nWR.
Note 3:
%
%!
FIGURE 14 - MULTIPLEXED BUS, 80XX-LIKE CONTROL SIGNALS; WRITE CYCLE
55
VALID
A0-A2
t1
t2
nCS
t4
t3
t5
Note 3
t10
nRD
t8
t7
t6
nWR
D0-D7
t9
Note 2
VALID DATA
CASE 1: RBUSTMG bit = 0
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
Address Setup to nRD Active
Address Hold from nRD Inactive
nCS Setup to nRD Active
nCS Hold from nRD Inactive
Cycle Time (nRD Low to Next Time Low)
nRD Low to Valid Data
nRD High to Data High Impedance
nRD Low Width
nRD High Width
nWR
to nRD Low
max
15
10
5**
0
4TARB*
0
60
20
20
40**
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
** nCS may become active after control becomes active, but the access time (t6)
will now be 45nS measured from the leading edge of nCS.
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
&& Note 2: !
Note 3: %
!
FIGURE 15A - NON-MULTIPLEXED BUS, 80XX-LIKE CONTROL SIGNALS; READ CYCLE
56
VALID
A0-A2
t1
t2
nCS
t4
t3
t5
Note 3
t10
nRD
t8
t7
t6
nWR
D0-D7
t9
Note 2
VALID DATA
CASE 2: RBUSTMG bit = 1
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
Address Setup to nRD Active
Address Hold from nRD Inactive
nCS Setup to nRD Active
nCS Hold from nRD Inactive
Cycle Time (nRD Low to Next Time Low)
nRD Low to Valid Data
nRD High to Data High Impedance
nRD Low Width
nRD High Width
nWR
to nRD Low
max
-5
0
-5
0
4TARB*+30
0
100
30
20
60**
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
** t6 is measured from the latest active (valid) timing among nCS, nRD, A0-A2.
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
Note 2: !
Note 3: %
!
FIGURE 15B - NON-MULTIPLEXED BUS, 80XX-LIKE CONTROL SIGNALS; READ CYCLE
57
A0-A2
VALID
t1
t2
nCS
t4
t3
DIR
t7
t5
t6
t10
nDS
t9
t8
D0-D7
t11
Note 2
VALID DATA
CASE 1: RBUSTMG bit = 0
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
Address Setup to nDS Active
Address Hold from nDS Inactive
nCS Setup to nDS Active
nCS Hold from nDS Inactive
DIR Setup to nDS Active
Cycle Time (nDS Low to Next Time Low)
DIR Hold from nDS Inactive
nDS Low to Valid Data
nDS High to Data High Impedence
nDS Low Width
nDS High Width
max
15
10
5**
0
10
4TARB*
10
0
60
20
40**
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
** nCS may become active after control becomes active, but the access time (t8) will
now be 45nS measured from the leading edge of nCS.
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
Note 2: !
FIGURE 16A - NON-MULTIPLEXED BUS, 68XX-LIKE CONTROL SIGNALS; READ CYCLE
58
A0-A2
VALID
t1
t2
nCS
t4
t3
DIR
t7
t5
t6
t10
nDS
t9
t8
D0-D7
t11
Note 2
VALID DATA
CASE 2: RBUSTMG bit = 1
min
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
Address Setup to nDS Active
Address Hold from nDS Inactive
nCS Setup to nDS Active
nCS Hold from nDS Inactive
DIR Setup to nDS Active
Cycle Time (nDS Low to Next Time Low)
DIR Hold from nDS Inactive
nDS Low to Valid Data
nDS High to Data High Impedence
nDS Low Width
nDS High Width
max
-5
0
-5
0
10
4TARB*+30
10
0
100
30
60**
20
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
** t8 is measured from the latest active (valid) timing among nCS, nDS, A0-A2.
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
Note 2: !
FIGURE 16B - NON-MULTIPLEXED BUS, 68XX-LIKE CONTROL SIGNALS; READ CYCLE
59
A0-A2
VALID
t1
t2
nCS
t4
nRD
Note 3
t10
t3
t9
t8
t5
nWR
t6
D0-D7
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t7
Note 2
t5**
VALID DATA
Parameter
Address Setup to nWR Active
Address Hold from nWR Inactive
nCS Setup to WR Active
nCS Hold from nWR Inactive
Cycle Time (nWR
to Next
)**
Valid Data Setup to nWR High
Data Hold from nWR High
nWR Low Width
nWR High Width
nRD
to nWR Low
min
15
10
5
0
4TARB*
30***
10
20
20
20
max
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
***: nCS may become active after control becomes active, but the data setup time will now
be 30 nS measured from the later of nCS falling or Valid Data available.
Note 1: The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
&&Note 2: Any cycle occurring after a write to the Address Pointer Low Register
requires a minimum of 4TARB from the trailing edge of nWR to the leading edge
of the next nWR.
Write cycle for Address Pointer Low Register occurring after a write to Data
Register requires a minimum of 5TARB from the trailing edge of nWR to the
leading edge of the next nWR.
Note 3: %
%!
FIGURE 17 - NON-MULTIPLEXED BUS, 80XX-LIKE CONTROL SIGNALS; WRITE CYCLE
60
A0-A2
VALID
t1
t2
nCS
t4
DIR
t3
t5
t7
t10
nDS
t8
t6
t9
VALID DATA
D0-D7
t1
t2
t3
t4
t5
t6
t7
t8
t9
t10
t11
t11
Note 2
t6**
Parameter
Address Setup to nDS Active
Address Hold from nDS Inactive
nCS Setup to nDS Active
nCS Hold from nDS Inactive
DIR Setup to nDS Active
Cycle Time (nDS
to Next Time
DIR Hold from nDS Inactive
Valid Data Setup to nDS High
Data Hold from nDS High
nDS Low Width
nDS High Width
)**
min
15
10
5
0
10
4TARB*
10
30***
10
20
20
max
units
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
nS
* TARB is the Arbitration Clock Period
TARB is identical to Topr if SLOW ARB = 0
TARB is twice Topr if SLOW ARB = 1
Topr is the period of operation clock. It depends on CKUP1 and CKUP0 bits
***: nCS may become active after control becomes active, but the data setup time will now
be 30 nS measured from the later of nCS falling or Valid Data available.
Note 1:
The Microcontroller typically accesses the COM20020 on every other cycle.
Therefore, the cycle time specified in the microcontroller's datasheet
should be doubled when considering back-to-back COM20020 cycles.
**Note 2: Any cycle occurring after a write to the Address Pointer Low Register
requires a minimum of 4TARB from the trailing edge of nDS to the leading edge
of the next nDS.
Write cycle for Address Pointer Low Registers occurring after an access to
Data Register requires a minimum of 5TARB from the trailing edge of nDS to
the leading edge of the next nDS.
FIGURE 18 - NON-MULTIPLEXED BUS, 68XX-LIKE CONTROL SIGNALS; WRITE CYCLE
61
nTXEN
t4
t5
t2
t1
nPULSE1
LAST BIT
(400 nS BIT TIME)
t3
t2
t1
nPULSE2
t6
t8
RXIN
t7
Parameter
t1
t2
t3
t4
t5
t6
t7
t8
min
nPULSE1, nPULSE2 Pulse Width
nPULSE1, nPULSE2 Period
nPULSE1, nPULSE2 Overlap
nTXEN Low to nPULSE1 Low
Beginning of Last Bit Time to nTXEN High
RXIN Active Pulse Width
RXIN Period
RXIN Inactive Pulse Width
-10
850
250
10
typ
100
400
0
max
+10
950
350
100
400
20
Note: Use Only 2.5 Mbps
FIGURE 19 – NORMAL MODE TRANSMIT OR RECEIVE TIMING
(These signals are to and from the hybrid)
62
units
nS
nS
nS
nS
nS
nS
nS
nS
nTXEN
t1
t13
t3
nPULSE1
t2
t9
t8
LAST BIT
(400 nS BIT TIME)
t4
t5
nPULSE2
(Internal Clk)
t6
t7
t10
t12
RXIN
t11
Parameter
min
t9
t13
nPULSE2 High to nTXEN Low
nPULSE1 Pulse Width
nPULSE1 Period
nPULSE2 Low to nPULSE1 Low
nPULSE2 High Time
nPULSE2 Low Time
nPULSE2 Period
nPULSE2 High to nTXEN High
(First Rising Edge on nPULSE2 after Last Bit Time)
nTXEN Low to first nPULSE1 Low**
Beginning Last Bit Time to nTXEN High**
t10
t11
t12
RXIN Active Pulse Width
RXIN Period
RXIN Inactive Pulse Width
t1
t2
t3
t4
t5
t6
t7
t8
typ
-25
max
50
-25
50
nS
nS
nS
nS
nS
nS
nS
nS
650
450
750
550
nS
nS
200*
400*
-25
50
100*
100*
200*
10
200*
400*
20
Above values are for 2.5 Mbps.
Other Data Rates are shown below.
TDR is the Data Rate Period
*t5, t6 = TDR/4
*t2, t7, t10 = TDR/2
*t3, t11 = TDR
**t9 = 47 x TDR +/- 50 nS
**t13 =
5
4
x TDR +/- 50 nS
FIGURE 20 – BACKPLANE MODE TRANSMIT OR RECEIVE TIMING
(These signals are to and from the differential driver or the cable)
63
units
nS
nS
nS
t1
t3
t2
4.0V
50% of VDD
1.0V
XTAL1
Parameter
t1
t2
t3
t4
t5
min
Input Clock High Time
Input Clock Low Time
Input Clock Period
Input Clock Frequency
Frequency Accuracy*
typ
10
10
25
10
-200
max
100
40
200
units
nS
nS
nS
MHz
''
Note*: Input clock frequency must be 20 MHz (()100ppm or better) to use the internal Clock Multiplier.
''
!
FIGURE 21 – TTL INPUT TIMING ON XTAL1 PIN
t1
nRESET
nINTR
t2
Parameter
t1
t2
nRESET Pulse Width***
nINTR High to Next nINTR Low
min
EF = 0
EF = 1
typ
5TXTL*
TDR**/2
4TXTL*
Note*: TXTL is period of external XTAL oscillation frequency.
Note**: TDR is period of Data Rate (i.e. at 2.5 Mbps, TDR = 400 nS)
Note***: When the power is turned on, t1 is measured from stable XTAL
* +,!*!
FIGURE 22 – RESET AND INTERRUPT TIMING
64
max
units
G
PIN N O.
1
E
J
A
F
B
B1
D3
D2
R
J
J
D1
D
C
D IM
A
A1
B
B1
C
D
D1
D2
D3
E
F
G
J
R
Seating
Plane
Base
Plane
A1
28L
.160-.180
.090-.120
.013-.021
.026-.032
.020-.045
.485-.495
.450-.456
.390-.430
.300 R EF
.050 BSC
.042-.056
.042-.048
.000-.020
.025-.045
N OTES:
1. A ll dim ensions are in inches.
2. C ircle indicating pin 1 can appear on a top surface as shown on the drawing or
right above it on a beveled edge.
FIGURE 23 - 28 PIN PLCC PACKAGE DIMENSIONS
65
E1 E
C
eA
D
eB
A2
Base
Plane
A
A1
Seating
Plane
L
B1
S
e
B
24L
.090-.150
.020-.065
.145-.155
.016-.021
.060-.070
.010-.014
1.245-1.265
.590-.630
.530-.545
.100BSC
.600REF
.610-.670
.120-.140
.065-.085
DIM
A
A1
A2
B
B1
C
D
E
E1
e
eA
eB
L
S
Note: All dimensions are in inches.
FIGURE 24 - 24 PIN DIP PACKAGE DIMENSIONS
66
APPENDIX A
This appendix describes the function of the
NOSYNC and EF bits.
Thus, by setting the NOSYNC bit, the line does
not have to be idle for the RAM initialization
sequence to be written.
NOSYNC Bit
EF Bit
The NOSYNC bit controls whether or not the
RAM initialization sequence requires the line to
be idle by enabling or disabling the SYNC
command during initialization. It is defined as
follows:
The EF bit controls several modifications to
internal operation timing and logic. It is defined
as follows:
EF: Enable/Disable the new internal operation
timing and logic refinements. EF=0: (Default)
Disable the new internal operation timing (the
timing is the same as in the COM20020 Rev. B);
EF=1: Enable the new internal operation timing.
NOSYNC: Enable/Disable SYNC command
during initialization.
NOSYNC=0, Enable
(Default): the line has to be idle for the RAM
initialization sequence to be written, NOSYNC=1,
Disable: the line does not have to be idle for the
RAM initialization sequence to be written.
The EF bit controls the following timing/logic
refinements in the COM20020:
The following discussion describes the function
of this bit:
A)
Extend Interrupt Disable Time
During initialization, after the CPU writes the
Node ID, the COM20020 will write "D1"h data to
Address 000h and Node-ID to Address 001h of
its internal RAM within 6uS. These values are
read as part of the diagnostic test. If the D1 and
Node-ID initialization sequence cannot be read,
the initialization routine will report it as a device
diagnostic failure. These writes are controlled by
a micro-program which sometimes waits if the
line is active; SYNC is the micro-program
command that causes the wait. When the microprogram waits, the initial RAM write does not
occur, which causes the diagnostic error. Thus
in this case, if the line is not idle, the initialization
sequence may not be written, which will be
reported as a device diagnostic failure.
While the interrupt is active (nINTR pin=0), the
interrupt is disabled by writing the Clear Tx/Rx
interrupt and Clear Flag command and by
reading the Next-ID register. This minimum
disable time is changed by the Data Rate. For
example, it is 200 nS at 2.5 Mbps and 100 nS at
5 Mbps. The 100 nS width will be too short to for
the Interrupt to be seen.
However, the initialization sequence and
diagnostics of the COM20020 should be
independent of the network status. This is
accomplished through some additional logic to
decode the program counter, enabled by the
NOSYNC bit. When it finds that the microprogram is in the initialization routine, it disables
the SYNC command.
In this case, the
initialization will not be held up by the line status.
B)
Setting the EF bit will change the minimum
disable time to always be more than 200 nS
even if the Data Rate is 5 Mbps . This is done by
changing the clock which is supplied to the
Interrupt Disable logic. The frequency of this
clock is always less than 20MHz even if the data
rate is 5 Mbps.
Synchronize the Pre-Scalar Output
The Pre-Scalar is used to change the data rate.
The output clock is selected by CKP3-1 bits in
the Set-Up register. The CKP3-1 bits are
changed by writing the Set-Up register from
outside the CPU. It's not synchronized between
the CPU and COM20020. Thus, changing the
CKP3-1 timing does not synchronize with the
internal clocks of Pre-Scalar, and changing
67
CKP3-1 may cause spike noise to appear on the
output clock line.
data rate, such that the minimum interval time
becomes 100 nS.
D) Eliminate The Write Prohibition Period For
The Enable Tx/Rx Commands
Setting the EF bit will include flip-flops inserted
between the Configuration register and PreScalar for synchronizing the CKP3-1 with PreScalar’s internal clocks.
The COM20020 has a write prohibition period for
writing the Enable Transmit/Receive Commands.
This period is started by the TA or RI bit (Status
Reg.) returning to High. This prohibition period is
caused by setting the TA/RI bit with a pulse
signal. It is 3.2 "S at 156.25 Kbps. This period
may be a problem when using interrupt
processing. The interrupt occurrs when the RI bit
returns to High. The CPU writes the next Enable
Receive Command to the other page
immediately. In this case, the interval time
between the interrupt and writing Command is
shorter than 3.2 "S.
Never change the CKP3-1 when the data rate is
over 5 Mbps. They must all be zero.
C) Shorten The Write Interval Time To The
Command Register
The COM20020 limits the write interval time for
continuous writing to the Command register. The
minimum interval time is changed by the Data
Rate. It's 100 nS at the 2.5 Mbps and 1.6 "S at
the 156.25 Kbps. This 1.6 "S is very long for
CPU.
Setting the EF bit will cause the TA/RI bit to
return to High upon release of the pulse signal
for setting the TA/RI bit, instead of at the start of
the pulse. This is illustrated in figure 25 on the
following page.
Setting the EF bit will change the clock source
from OSCK clock (8 times frequency of data
rate) to XTAL clock which is not changed by the
68
EF=0
Tx/Rx completed
TA/RI bit
Setting Pulse
nINTR pin
prohibition period
EF=1
Tx/Rx completed
TA/RI bit
Setting Pulse
nINTR pin
FIGURE 25 - EFFECT OF THE EF BIT ON THE TA/RI BIT
69
The EF bit also controls the resolution of the
following issues from the COM20020 Rev. B:
A)
B)
Mask Register Reset
The Mask register is reset by a soft reset in the
COM20020 Rev. A, but is not reset in Rev. B.
The Mask register is related to the Status and
Diagnostic register, so it should be reset by a
soft reset. Otherwise, every time the soft reset
happens, the COM20020 Rev. B generates an
unnecessary interrupt since the status bits RI
and TA are back to one by the soft reset.
Network MAP Generation
Tentative ID is used for generating the Network
MAP, but it sometimes detects a non-existent
node. Every time the Tentative-ID register is
written, the effect of the old Tentative-ID remains
active for a while, which results in an incorrect
network map. It can be avoided by a carefully
coded software routine, but this requires the
programmer to have deep knowledge of how the
COM20020 works. Duplicate-ID is mainly used
for generating the Network MAP. This has the
same issue as Tentative-ID.
This is resolved by changing the logic to reset
the Mask register both by the hard reset and by
the soft reset. The soft reset is activated by the
Node-ID register going to 00h or by the RESET
bit going to High in the Configuration register.
This solution is Enabled/Disabled by the EF bit.
A minor logic change clears all the remaining
effects of the old Tentative-ID and the old
Duplicate-ID, when the COM20020 detects a
write operation to Tentative-ID or Node-ID
register. With this change, programmers can use
the Tentative-ID or Duplicate-ID for generating
the network MAP without any issues. This
change is Enabled/Disabled by the EF bit.
70
APPENDIX B
Software Identification of the COM20020 Rev B, Rev C and Rev D
In order to properly write software to work with the COM20020 Rev B, C and D it is necessary to be
able to identify the different revisions of the part.
To identify the COM20020 Revision follow the following procedure:
1.
2.
3.
Write 0x98 to Register-6 (Address = 6)
Write 0x02 to Register-5 (Address = 5)
Read Register-6
* If the value read from Register-6 is 0x98 then the part is a COM20020 Rev B or earlier
* If the value read from Register-6 is 0x9A then go to next step below
4.
5.
Write 0x80 to Register-5
Read Register-5
* If the value read from Register-5 is 0x00 then the part is a COM20020 Rev C
* If the value read from Register-5 is 0x80 then the part is a COM20020 Rev D
71
COM20020 REVISIONS
PAGE(S)
SECTION/FIGURE/ENTRY
DATE
REVISED
CORRECTION
1
Features
+5V changed to +3.3V
03/09/00
3
Table of Contents
Deleted ISA BUS example
03/09/00
4
Pin Configuration
Deleted Commercial Upgrade
03/09/00
5
Description of Pin Functions
+5V changed to +3.3V
03/09/00
14
Figure 2
75176B changed to LTC1480 (for 3.3V Operation). Inserted 3.3V-5V level converter
03/09/00
15
Figure 3
75176B changed to LTC1480 (for 3.3V Operation). Inserted 3.3V-5V level converter
03/09/00
16
High Speed CPU Bus Timing Support
SH-1 changed to SuperH
03/09/00
19
Figure 5
75176B changed to LTC1480 (for 3.3V Operation). +VCC changed to +3.3V
03/09/00
49
DC Electrical Characteristics
Supply only industrial grade device. +5V changed to +3.3V, 4.0V changed to 2.4V
03/09/00
49
DC Electrical Characteristics
Deleted Voh1c definition (this spec is for HYC3500 only)
03/09/00
71
Appendix B
Deleted Figure, ISA bus is not valid for 3.3V device
03/09/00
71
Appendix C
Changed to Appendix B
03/09/00
© 2000 STANDARD MICROSYSTEMS CORPORATION (SMSC)
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COM20020 Rev. 03/24/2000
72