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AN1816
USING THE HC912B32 TO IMPLEMENT THE DISTRIBUTED
SYSTEMS INTERFACE (DSI) PROTOCOL
By Tracy McHenry
August 1999
Introduction
System design requirements are continually changing as systems
become increasingly complex. In conventional systems where sensors
and actuators are connected directly to a microcontroller (MCU), the
number of pins available on the MCU limits system expansion. As a
result, extra costs can be incurred if another MCU is required or if the
hardware has to be redesigned to accommodate a higher pin count
MCU. An alternative approach is to move to a distributed bus
architecture. This option allows one master MCU to interconnect to
many remote sensors and actuators.
The Distributed Systems Interface (DSI) is one such master/slave
system, with the central control module being the master and the remote
sensors and actuators acting as slave devices. A key feature of the DSI
architecture is that the sensors and actuators can be connected on the
same bus. Another benefit of the DSI is that it promotes the use of
standard components and interfaces which enables maximum re-use
and accelerates time to market. This is a great advantage to systems
designers who are therefore able to develop flexible system solutions. It
was initially developed for the automotive market although it is equally
suited to other applications that require distributed sensors and/or
actuators. Examples of possible DSI applications include occupant
safety systems, body networks, building and industrial controls.
This application note provides an overview of the DSI and describes the
hardware and software design of a demonstrator system.
© Freescale Semiconductor, Inc., 2004. All rights reserved.
© Motorola, Inc., 1999
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Application Note
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Distributed Systems Interface (DSI) Overview
The Distributed Systems Interface (DSI) was designed to interconnect
multiple remote sensor and actuator devices to a central control module.
It provides simultaneous support for sensors and actuators using a
2-wire bus that provides both power and communications. This results in
savings in wiring costs and connector complexity. Unlike conventional
systems, the size of the connector for the control module does not need
to grow to accommodate every new sensor or actuator. A new sensor or
actuator can be added to the bus without reconfiguring the system
design. The DSI, therefore, enables the development of easily
expandible systems.
The DSI communication bus is simple yet robust. Signals are
superimposed onto the power line and, as communication between the
master and slave nodes is bi-directional, the DSI makes efficient use of
bus bandwidth. Also, to ensure message integrity, each message
contains a 4-bit Cyclic Redundancy Check (CRC).
Slave nodes can be attached to the bus in daisy chain or parallel
connections. Each slave device on a bus has a unique address. The
daisy chain connection allows the central module to establish the node
addresses at power-up. The parallel configuration can be used for
devices that have pre-programmed or fixed addresses. It is possible to
have a combination of the two on one bus with the maximum number of
nodes on a DSI bus being 16 (1 master and 15 slaves).
Signalling
The DSI uses two mediums for signalling - voltage mode for messages
from the master to the slaves, and current mode for the responses from
the slaves.
Voltage Mode
Encoding
The voltage mode signal bits are sent on a 2:1 ratio as shown in Figure
1 Voltage Mode Bit Encoding.
Logic 0
Logic 1
Logic 1
Logic 0
Figure 1 Voltage Mode Bit Encoding
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Distributed Systems Interface (DSI) Overview
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The DSI protocol has been designed such that the first third of a voltage
mode signal bit is always low and the last third is always high. The
central third defines the signal value. For a logic zero the master
produces a signal that is low for 2/3 of the bit time and high for the final
1/3. For a logic one the signal is low for 1/3 of the bit time and high for
2/3 of the bit time. Each slave has a built-in oscillator. This feature of the
DSI message protocol has created a high immunity to temporal
distortion. Consequently, each slave IC is simpler and cheaper to design
while still able to capture all messages accurately. An added benefit is
that it allows for a range of operating frequencies.
Bus Voltage Levels
The voltage mode signalling uses a tri-level bus as shown in Figure 2
Tri-level bus.
Idle
High Threshold
Signal High
Low Threshold
Signal Low
Gnd
End of First Bit ("0")
End of Word
Start of Word
End of Second Bit ("1")
Start of Next Word
Figure 2 Tri-level bus
The DSI bus provides power to the slave nodes as well as supporting
bi-directional communication between the slave nodes and the master.
When the bus is at the idle voltage, which ranges from 8 to 25 volts, it
supplies power to the slave nodes. A high threshold level (typically 6V)
and a low threshold level (typically 3V) play a significant part in message
transmission. The start of a word occurs when the bus voltage drops
from the idle voltage below the high threshold level and then below the
low threshold level. Data values are determined by the ratio of time spent
above and below the low threshold. The voltage rising above the high
threshold level signals the end of a word.
Current Mode
Encoding
Slave responses to commands are returned by modulating the amount
of current sunk by the device. This is measured at the end of each bit to
determine a ‘zero’ or ‘one’ response. When responding with a logic one,
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Application Note
the slave draws additional current from the source during the bit time.
Conversely, no extra current is drawn when responding with a logic zero.
Figure 3 Current Waveform shows a representation of the current
waveform referenced to a voltage waveform.
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Voltage
(from Master)
Logic 0
Logic 1
Logic 1
Logic 0
Logic 1
Logic 0
Current
(from Slave)
Logic 1
Logic 0
Figure 3 Current Waveform
Message
Format
DSI messages are composed of individual words separated by a
minimum frame delay. Transfers are full duplex, that is, command
messages from the master occur at the same time as responses from
the slaves. This is an important feature of the DSI as it doubles the
effective signal bandwidth. Slave responses to commands occur during
the next command message. This allows slaves time to decode the
command, act upon it and then respond to the master. The minimum
frame delay is present to allow recharge of energy storage devices in the
slaves. This is necessary because the slave receives its power from the
signal line.
Message
Encoding
Message encoding depends on the direction of the transfer. Command
and control messages are sent from the master to the slave. Response
messages are sent from the slaves to the master. In both cases there
are long word and short word messages. A long word consists of 16-bits
of information followed by a 4-bit cyclic redundancy check (CRC). A
short word is composed of 8-bits of information followed by the 4-bit
CRC.
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Distributed Systems Interface (DSI) Overview
The long word command and control message encoding is shown in
Figure 4 Long Word Command and Control Message.
Command and
Control Messages
DATA
D7
D6
D5
D4
ADDRESS
D3
D2
D1
D0
A3
A2
A1
COMMAND
A0
C3
C2
C1
CRC
C0
X3
X2
X1
X0
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Figure 4 Long Word Command and Control Message
The message consists of 8 bits of data, the encoded 4-bit address of the
intended slave device, a 4-bit encoded command, and the calculated
4-bit CRC.
The short word command and control message encoding is shown in
Figure 5 Short Word Command and Control Message.
ADDRESS
A3
A2
A1
COMMAND
A0
C3
C2
C1
CRC
C0
X3
X2
X1
X0
Figure 5 Short Word Command and Control Message
The message consists of the encoded 4-bit address of the intended
slave device, a 4-bit encoded command, and the calculated 4-bit CRC.
A long word response message is sent from the slave to the master in
response to a long word command and control message to the slave’s
address. The response is transmitted during the next command and
control message. The long word response message encoding is shown
in Figure 6 Long Word Response Message.
Response
Messages
DATA BYTE 1
D15
D14
D13
D12
D11
DATA BYTE 2
D10
D9
D8
D7
D6
D5
D4
D3
CRC
D2
D1
D0
X3
X2
X1
X0
Figure 6 Long Word Response Message
The message consists of two 8-bit data bytes and the calculated 4-bit
CRC
A short word response message is sent from the slave to the master in
response to a short word command and control message to the slave’s
address. The response is transmitted during the next command and
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Application Note
control message. The short word response message encoding is shown
in Figure 7 Short Word Response Message.
DATA
D7
D6
D5
D4
D3
CRC
D2
D1
D0
X3
X2
X1
X0
Figure 7 Short Word Response Message
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The message consists of one 8-bit data byte and the calculated 4-bit
CRC.
Long words are always sent in response to long word commands and
short words are always sent in response to short word commands. When
the word format changes between successive commands, the first
response sent during the new format will be invalid since it will not have
the proper number of bits.
Error Checking
The master and slaves calculate a CRC on the information portion of their
received messages. The message is valid only if the calculated CRC
matches the CRC sent as part of the message. An error bit is set in the
master when it receives an invalid message. The slaves discard and ignore
all invalid received messages and in addition do not respond to them.
Slave Device
Addressing
Each slave device on the bus must be given a unique 4-bit address (from
1 (0001) to 15 (1111)) and assigned to one of four groups. Address 0
(0000) is used to address all 15 slave nodes at once.
Programmable
Devices
After system power up the master must set the address of all daisy chain
slave devices with programmable addresses before network
communications can commence. These devices have a bus switch on
the power/signal line. At power up the programmable device bus
switches are open and only close once an initialisation message has
been received.
With the first bus switch open, the bus only goes as far as the first slave.
When the master sends a slave initialisation command, the first slave
device stores its address information and closes its bus switch. The
second daisy chain slave is now connected to the network. When the
master initialises the second slave’s address, the first device responds
with an initialisation response message. The response message echoes
the programming information back to the master so that it knows that the
address was successfully established. Each slave sends the
initialisation response message only once after receiving a program
address command message.
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HARDWARE DESIGN
An advantage of having programmable devices present in the system is
that they may be replaced and/or the system may be reconfigured by
adding nodes to the bus and the system will automatically reconfigure
itself at the next power up.
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Pre-programmed
Devices
Slaves with pre-programmed addresses do not require a bus switch. On
power-up the stored pre-programmed address becomes the slave
address. However, pre-programmed devices must still receive an
Initialisation Command and reply with an Initialisation Response before
responding to any other bus commands.
HARDWARE DESIGN
Master Board
MC33790 Bus Transceiver
MC68HC55 SPI Peripheral
HC912B32 MCU
DSI0F
PWM pin
SCLK
DSI0S
CH.0
DSI0R
SPI
MOSI
DI
(master)
MISO
DO
I/O PIN
CS*
TWO-WIRE BUS
GND
CLK
SCK
DSI0O
DSI1F
CH.1
Signal
DSI1S
DSI1O
DSI1R
GND
Return
G
N
D
B
U
S
I
N
B
U
S
O
U
T
Slav
Figure 8 Top Level DSI System Connections
Several ICs are used in the development of the system :
•
Master board
– HC912B32
– MC68HC55 SPI Peripheral
– MC33790 Bus Transceiver
•
Slave board
– evaluation slave device, the BEM IC
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Application Note
Master Board
Figure 8 Top Level DSI System Connections shows how the ICs are
connected together. It should be noted that the diagram shows a daisy
chain connected slave but slave nodes can also be connected onto the
bus in parallel. The parallel configuration is used for slave nodes that
have pre-programmed addresses. The SPI of the HC912B32 is set up
such that it acts as the master and all communication from the
HC912B32 to the MC68HC55 is via the SPI. The MC33790, the physical
layer interface to the DSI bus, sends commands to and receives
responses from the slave devices.
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Software required to control the system can be programmed into the
HC912B32’s 32k of FLASH memory. It also has 768 bytes of EEPROM
and 1k of RAM. The PWM is used to provide the MC68HC55 with a
system clock. It is set up such that PWM channel 0 is output on port P
pin 0 (PP0) and connected to the SCLK pin of the MC68HC55 SPI
Peripheral. The HC912B32’s on board SPI is a key element in the DSI
communication protocol and is set up as follows. The MOSI, MISO and
SCLK pins on the HC912B32 are connected to the MC68HC55’s DI, DO,
CLK pins respectively. Port S pin 7 (PS7) is connected to the CS pin on
the MC68HC55. It should then be defined in the software as a
general-purpose I/O pin and set up to drive CS on the MC68HC55. CS
is an active low signal that is controlled by the HC912B32. When driven
low it indicates the start of message transmission from the HC912B32 to
the MC68HC55 and in turn to the MC33790 and then to the slave nodes.
This is the start of what is termed a SPI burst transfer (refer to the sub
section: ‘Initialisation of the PWM and SPI’ in the Software Design
section of this Application Note). The end of a SPI burst transfer is
signalled by the HC912B32 pulling CS high. Commands are sent to and
responses are received from the slave nodes via the SPI during a burst
transfer. The data written to the SPI data register is transferred into the
MC68HC55’s data register, which transmits the message to the slave
nodes via the MC33790 Bus Transceiver.
The MC68HC55 is the protocol controller of the system and controls all
the digital functions of the DSI. It contains 2 independent DSI channels,
each capable of interfacing to up to 15 slave nodes. The MC68HC55 SPI
Peripheral uses 3 pins to transmit a message to the MC33790 Bus
Transceiver. Pin DSIxS (signal) on the MC68HC55 transmits the data
output signal to the MC33790. Data bits on this signal line are pulse
length encoded voltage levels. A logic zero starts with a falling edge on
DSIxS and is low for two thirds of the bit time and then high for one third
of the bit time. A logic one starts with a falling edge on DSIxS and is low
for one third of the bit time and then high for two thirds of the bit time.
DSIxF (frame) output pin idles high and is driven low during each
transfer frame. DSIxR is the data input signal from the MC33790. The
MC68HC55 samples the CMOS level on this pin at the end of a bit time.
This level corresponds to the current sensed by the MC33790.
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HARDWARE DESIGN
The MC33790 Bus Transceiver is an analogue SmartMOS™ device,
which serves as the physical interface to the two-wire DSI bus. It uses a
combination of pulse length encoded voltage levels for transmit data to
slave nodes and current return signals for receive data at the same time.
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DSIxF (frame)
from MC68HC55
DSIxS (signal)
from MC68HC55
Idle
Idle
DSIxO
from MC33790
High
Low
Figure 9 MC68HC55 frame and output signals with respect to MC33790 DSIx0 signal
When the MC33790 receives data for transmission to the slave nodes
from the MC68HC55, its bus transmitter circuit converts the 0 to 5 volt
inputs from the MC68HC55’s DSIxF (frame) and DSIxS (signal) to a
voltage level on the output DSIxO which connects to BUS_IN of the first
slave node. The value output on DSIxO of the MC33790 depends on the
values of DSIxF and DSIxS as shown in Table 1 DSIxO Truth Table.
See also Figure 9 MC68HC55 frame and output signals with respect to
MC33790 DSIx0 signal.
DSIxF
DSIxS
DSIxR
DSIxO
0
0
Return Data
Low (1.5V)
0
1
Return Data
High (4.5V)
1
0
0
High Impedance
1
1
0
Idle
Table 1 DSIxO Truth Table
When the MC33790 is receiving data from the slave nodes, it samples
the current responses on the rising edge of the final third of bit time. The
current response is then compared against a reference point, which
determines whether a low or high has been returned.
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Application Note
Schematics and
Layout
Considerations Master Board
Vdd
Vdd
DSI Ch.0 Rx Data
W17 - TP1
PP0/PW0
1
SCLK
PS6/SCK
2
PS5/MOSI
C24
0.1µF
U11 - DSIP
Vdd
16
CLK
DSI0F
15
1
DSI0F
Vdd
16
3
DI
DSI0S
14
2
DSI0S
Gnd0
15
PS4/MISO
4
DO
DSI0R
13
3
DSI0R
DSI0O
14
PS7/SS
5
CS
n.c.
12
4
DSI1F
Vsup
13
PS3
6
RESET DSI1F
11
5
DSI1S
DSI1O
12
7
INT
DSI1S
10
6
DSI1R
Gnd1
11
DSI1R
9
7
n.c.
n.c.
10
8
CPCAP
n.c.
9
W18
R15
100Ω
8
Gnd
PE1/IRQ
W19 - TP2
DSI Ch.1 Rx Data
C22
0.1µF
1 W20
DSI0
MCU PINS
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U9 - DSID
1 W22
DSI1
C23
0.1µF
W21
25v
Gnd
n.c.
no pin +
C26
10µF
Bus Power
Figure 10 Master Board Schematic
Figure 10 Master Board Schematic shows the schematic for the master
board. The board was based on an existing HC912B32 Evaluation
Board (EVB) with the addition of the required DSI circuitry. The
HC912B32 EVB circuitry allows the board to be used to evaluate
prototype hardware and software. When laying out the DSI circuitry on
the master board, the bus traces out of the MC33790 were made as wide
as possible to deal with the presence of the higher voltage (the DSI bus
maximum idle voltage is 25V). All components were positioned where
clock and bus trace lengths could be kept to a minimum. 0.1µF
capacitors were used to filter the bus supply voltage and to decouple the
power supplies to the MC68HC55 and MC33790. These capacitors were
placed as close to the ICs as possible.
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HARDWARE DESIGN
The slave board discussed in this application note is one that was
designed as part of a DSI evaluation tool kit. It contains the Bus
Evaluation Module (BEM) IC, which is a slave DSI interface evaluation
device. It provides bi-directional communications from the DSI bus. The
slave board contains an on board potentiometer which can be used to
vary the voltage input to the BEM IC. This voltage is converted by the
BEM IC to a digital signal that is then transmitted over the DSI bus.
Alternatively, an accelerometer or other analogue output device could be
connected using the wire wrap area of the slave board and the jumper
setting changed such that the output of the accelerometer is input to the
BEM IC. Multiple slave nodes can be attached to one DSI bus by daisy
chaining them together.
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Slave Board
Schematics and
Layout
Considerations Slave Board
SLAVE BOARD
REV
R E V I S I O N S
DESCRIPTION
DATE
VR1
Bus signals from ECU
GND
U3
BUS_IN
GND
BUS_OUT
CON3WAY
1
2
3
D1
DIODE ZN1
GND
3
1
J1
TP1
2
TEST POINT
TP2
VREG
SIGIN
BUSIN
BUSOUT
TEST POINT
TP3
TEST POINT
TP4
D2
DIODE ZN1
14
1
12
1
R2
10K
R1
10K
1
2
2
GND
GND
TEST POINT
BUS_OUT
16
VREG
10
N/C
N/C
9
8
2
4
FILT_CAP
11
7
N/C
N/C
N/C
3
13
15
5
6
1
U4
BUS_IN
BUS_GND
BUS_GND
AGND
AGND
SIG_IN
C2
4.7uF
GND
C1
1.0uF
BEM IC
C3
2 0.1uF
GND
GND
OCCUPANT SAFETY SYSTEMS
COMPUTER GENERATED DRAWING : DO NOT REVISE MANUALLY
Title
SLAVE BOARD
Size Document Number
B
SLAVE.SCH
Date:
August 10, 1999 Sheet
REV
1 of
1
Figure 11 Slave Board Schematic
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Application Note
Figure 11 Slave Board Schematic shows the schematic for the slave
board. A benefit of the DSI architecture is that the slave board requires
only a few additional components. Zener diodes are used to protect
against ESD damage to signals BusIn and BusOut. C1 (filt_cap)
supplies the power to the BEM IC during signalling. When laying out the
slave board, the pads for capacitor, C1 were enlarged so that they could
accommodate various values of capacitor from 1µF up to 4.7µF
depending on what was required of the evaluation system. For the slave
node described in this Application Note a 1µF capacitor was selected as
being capable of storing enough charge to power the BEM IC during
signalling. All components were positioned where signal trace lengths
could be kept to a minimum and signal traces were made as wide as
possible. Also, analogue and digital grounds were connected together
as close to the BEM IC as possible.
SOFTWARE DESIGN
The initialisation software can be divided into 3 sections - initialisation of
the PWM and SPI, initialisation of the MC68HC55 SPI peripheral’s
registers and initialisation of the slave nodes.
Initialisation of
the PWM and
SPI
The PWM on the HC912B32 provides the system clock (SCLK) for the
MC68HC55 SPI Peripheral. The system software must initialise the
PWM so that it supplies a clock signal to the MC68HC55 SPI Peripheral
with the appropriate duty cycle and period. An example of possible C
source code that can be used to perform this set-up is shown in function
InitPWM in Appendix 1 - Source Code. The PWM registers, in this
example, are set up to generate a clock signal with a duty cycle of 50%
and a period of 3.5µs (frequency of 285kHz).
The SPI is set up such that the slave select (SS) pin on the HC912B32
(connected to chip select (CS) on the MC68HC55 SPI Peripheral) is
configured as a general purpose I/O pin allowing the software to control
it. This is necessary for SPI Burst Transfers, thus enabling the
HC912B32 to communicate with the slave nodes via the MC68HC55
and the MC33790. Figure 12 SPI Burst Transfer Example shows an
example of an SPI burst transfer.
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SOFTWARE DESIGN
CLK
DI
COMMAND – WRT (000)
WRITE DSI0H (000)
WRITE DSI0L (001)
DO
ZEROS (1 transfer after reset)
READ DSI0H (000)
READ DSI0L (001)
ST
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CS*
Figure 12 SPI Burst Transfer Example
When CS is driven low by the HC912B32, the MC68HC55 SPI
Peripheral is enabled and the first SPI transfer after this is a command
transfer. Bit7 of this command determines if it is a read (0) or a write (1)
command. Bits[2:0] specify the address of one of the eight MC68HC55
registers and an internal pointer is established. Data sent back to the
HC912B32 from the MC68HC55 during a command transfer is read data
from the adress previously pointed to (this would be all zeros for the first
transfer after reset). Any additional transfers result in a write-to or
read-from successive registers in the MC68HC55. The internal register
pointer is incremented at the end of the transfer and will roll over from 7
(111) to 0(000). CS remains low throughout the whole burst sequence
and is driven high at the end by the HC912B32. Possible software
routines to perform the SPI set-up and to control the transfer of data in
SPI burst mode are shown in the source code detailed in functions
InitSpi and SpiBurst in Appendix 1 - Source Code.
Initialisation of
the MC68HC55
SPI Peripheral’s
Registers
Table 2 DSI Registers shows the MC68HC55 registers. The HC912B32
can read-from or write-to the MC68HC55’s seven registers through the
SPI interface. Data to be transferred to the slave nodes is written into the
DSI0H - Data Access for DSI/D Channel 0 (high byte)
Bit 7
Bit-15
6
Bit-14
5
4
Bit-13
Bit-12
address - 000
3
2
Bit-11
Bit-10
1
Bit-9
DSI0L- Data Access for DSI/D Channel 0 (low byte)
Bit 7
Bit-7
6
5
4
Bit-6
Bit-5
Bit-4
3
Bit-3
Bit 0
Bit-8
High
address - 001
2
Bit-2
1
Bit-1
Bit 0
Bit-0
Low
Table 2 DSI Registers
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Application Note
DSI1H - Data Access for DSI/D Channel 1(high byte)
Bit 7
Bit-15
6
5
Bit-14
Bit-13
4
Bit-12
address - 010
3
2
Bit-11
Bit-10
1
Bit-9
DSI1L- Data Access for DSI/D Channel 1(low byte)
Bit 7
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Bit-7
6
5
4
Bit-6
Bit-5
Bit-4
3
Bit-3
ER1
6
TFE1
Bit-8
High
address - 011
2
Bit-2
1
Bit-1
DSISTAT- DSI Status Register
Bit 7
Bit 0
Bit 0
Bit-0
Low
address - 100
5
4
TFNF1
RFNE1
3
ER0
2
1
Bit 0
TFE0
TFNF0
RFNE0
DSI0CTRL- DSI Channel 0 Control Register
address - 101
Bit 7
6
5
4
3
2
CDIV0B
CDIV0A
DLY0B
DLY0A
RIE0
TIE0
1
0
Bit 0
MS0
DSI1CTRL- DSI Channel 1 Control Register
address - 110
Bit 7
6
5
4
3
2
CDIV1B
CDIV1A
DLY1B
DLY1A
RIE1
TIE1
1
0
Bit 0
MS1
address - 111
DSIENABL
Bit 7
6
5
4
3
2
1
Bit 0
0
0
0
0
0
0
EN1
EN0
Table 2 DSI Registers (Continued)
DSIxH:DSIxL register pair for 16-bit messages; DSIxL for 8-bit. The
transfer begins once DSIxL has been written to. Similarly, responses
from the slave nodes are written into these registers. The DSISTAT
register provides status information and should be checked before and
after transfers. It contains an error flag, ERx, which indicates if the
master received an invalid CRC value. Also, transmit and receive status
information, TFEx, TFNFx and RFNEx, is available through this register.
The DSIxCTRL register is written to before data is sent over the DSI bus.
This register is used to specify an additional divider between the SCLK
input and the bit timing circuitry, CDIVx[B:A], as well as defining the
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Freescale Semiconductor, Inc...
Summary
interframe delay, DLYx[B:A]. It is also used to enable interrupts, RIEx
and TIEx, and to select 12 -bit (8 data bits plus 4 CRC bits) or 20-bit (16
data bits plus 4 CRC bits) messages, MSx. This register is updated as
soon as new data is received over the SPI interface, however, the new
value does not take affect until the next DSI clock cycle after the
conclusion of the SPI write to this register. Finally, the DSIENABL
register is used to enable or disable each DSI channel. The function
SetupDSI shown in the source code in Appendix 1 - Source Code is an
example of how to set up the MC68HC55 SPI Peripheral’s registers. It
uses the SPI burst routine discussed in sub section: ‘Initialisation of the
PWM and SPI’ and sets up the registers for initialisation of the
programmable slave nodes.
Initialising the
Slave Nodes
Programmable
Devices
The source code shown in the main section of the program that calls
functions PgmAddr, PgmChk and ChkRsp in Appendix 1 - Source Code
details a very simple routine to program the addresses into slave nodes.
It programs 15 slave nodes sequentially starting with address 1 (0001)
and finishing with address 15 (1111). To ensure the response from the
slave node whose address is being set is captured, the MC68HC55’s
clock period is set to SCLK divided by 4 and the interframe delay is set
to 32 bit times. This is achieved by writing $B0 to the MC68HC55’s
DSIxCTRL register (refer to sub section: ‘Initialisation of the MC68HC55
SPI Peripheral’s Registers’).
Pre-programmed
Devices
When the network is configured with pre-programmed devices the
initialisation procedure is similar to that of programmable devices. An
initialisation command that contains the address of the slave node being
initialised is sent to that slave node. The slave node then responds to the
initialisation command to let the master know it is present in the network.
Summary
This Application Note has discussed a total system solution using a full
suite of Freescale ICs. Although the initial target application is automotive
airbag systems it could be used in other applications which require
remote sensors. The Distributed Systems Interface (DSI) has many
advantages in that it allows flexibility of system design, is easily
expandible and allows the central module size to decrease while the
system content grows.
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Application Note
References
1. DSI Specification; internal Freescale document
2. MC68HC55 SPI Peripheral Specification; data sheet, order
number MC68HC55/D
3. BEM Specification; only available with DSI Evaluation System
(contact sales office for more details)
Freescale Semiconductor, Inc...
4. MC33790 Physical Layer ASIC Specification; internal Freescale
document
5. MC68HC912B32 Advance Information; order number
MC68HC912B32/D
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Appendix 1 - Source Code
Appendix 1 - Source Code
/**********************************************************************************
FILE : B32DSI.h
Header file referenced in program InitB32DSI.c to define the register addresses of the
COP, PWM and SPI
Freescale Semiconductor, Inc...
***********************************************************************************/
/* Define COP register */
#define COPBASE (volatile char *const)(0x16)
#define COPCTL (*(COPBASE+0))
/* Define PORTA for general purpose I/O */
#define PTABASE (volatile char *const) (0x00)
#define PORTA (*(PTABASE+0))
#define DDRA (*(PTABASE+2))
/* Define PWM registers */
#define
#define
#define
#define
#define
#define
#define
#define
#define
#define
PWMBASE (volatile char *const)(0x40)
PWCLK (*(PWMBASE+0))
PWPOL (*(PWMBASE+1))
PWEN (*(PWMBASE+2))
PWRES (*(PWMBASE+3))
PWPER0 (*(PWMBASE+0xc))
PWDTY0 (*(PWMBASE+0x10))
PWCTL (*(PWMBASE+0x14))
PORTPP (*(PWMBASE+0x16))
DDRP (*(PWMBASE+0x17))
/* Define SPI Registers */
#define
#define
#define
#define
#define
#define
#define
#define
#define
SPIBASE (volatile char *const)(0xd0)
SP0CR1 (*(SPIBASE+0))
SP0CR2 (*(SPIBASE+1))
SP0BR (*(SPIBASE+2))
SP0SR (*(SPIBASE+3))
SP0DR (*(SPIBASE+5))
PORTS (*(SPIBASE+6))
DDRS (*(SPIBASE+7))
PURDS (*(SPIBASE+0xb))
/**********************************************************************************
FILE : InitB32DSI.c
C code to be programmed into the FLASH of the HC912B32 which initialises the PWM and
SPI of the HC912B32 then sets up the DSI registers on the MC68HC55 SPI Peripheral
I.C. before programming the address into 15 programmable slave nodes
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Application Note
Freescale Semiconductor, Inc...
This source code is example code that could be used to perform initialisation.
Freescale reserves the right to make changes without further notice to any product
herein to improve reliability, function, or design. Freescale does not assume any
liability arising out of the application or use of any product, circuit or software
described herein; neither does it convey any licensed under its patent rights nor the
right of others. Freescale products are not designed, intended or authorised for use
as components intended for surgical implant into the body, or other applications
intended to support life, or any other application in which failure of the Freescale
product could create a situation where personal injury or death may occur. Should
Buyer purchase or use Freescale products for any such intended or unauthorised
application, Buyer shall indemnify and hold Freescale and its officers, employees,
subsidiaries, affiliates, and distributors harmless against all claims, costs,
damages, expenses and reasonable attorney fees arising out of, directly or indirectly,
any claim of personal injury or death associated with such unintended or unauthorised
use, even if such a claim alleges that Freescale was negligent regarding the design
or manufacture of the part. Freescale and the Freescale logo are registered trademarks
of Freescale Semiconductor, Inc.
***********************************************************************************/
#include <stdio.h>
#include “B32DSI.h”
#define ArraySize 0x0F
/* (no. of slave nodes) */
short TBytes [ArraySize]; /* Array of bytes to be transmitted */
short RBytes [ArraySize]; /* Array of received bytes */
short ChkBytes [ArraySize]; /* Array of check bytes to check
received data is correct */
short ErrorCode [ArraySize]; /* Array of Error codes */
/* This function sets up the PWM of the HC912B32 */
void InitPWM(void)
{
PWCLK = 0x00; /* Don’t divide A clock */
PWPOL = 0x00; /* Use A clock and polarity is low until duty
count is reached */
PWPER0 = 0x1b; /* Period of 3.5µs (frequency of 285.71 kHz) */
PWDTY0 = 0x0d; /* Duty cycle is 50% */
DDRP = 0x01;
/* Set up PTP0 as output */
PWCTL = 0x00; /* Left aligned mode as CENTR=0 */
PWEN = 0x01;
/* Enable the PWM */
}
/* This function sets up the SPI of the HC912B32 */
void InitSPI(void)
{
PURDS = 0x00; /* Leave as normal conditions */
SP0BR = 0x00; /* SPI clock frequency is 4 MHz */
SP0CR1 = 0x00; /* CPOL = CPHA = 0 */
DDRS = 0xe0;
/* Set up PTS[7:5] as outputs (SS, SCLK and
MOSI) */
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Appendix 1 - Source Code
SP0CR1 = 0x10; /* Enable master mode and SSOE=0, therefore SS
is GP I/O */
SP0CR1 = 0x50; /* Enable SPI system now */
}
Freescale Semiconductor, Inc...
/* This function is called by the SpiBurst function. It writes information into the
SPI data register and when the SPI transfer complete flag is set it sends back the
information received by the SPI */
short TransmitReceive(short SendByte)
{
int dummy=0;
short result;
SP0DR = SendByte;
while ((SP0SR & 0x80) == 0)
complete flag is set */
{
dummy++;
}
result = SP0DR;
return (result);
}
/* Wait until SPI transfer
/* This function sets up the SPI Burst transfer routine. It enables the MC68HC55 SPI
Peripheral by driving CS low and then transfers the required no. of bytes of data to
complete the burst transfer. It finishes by driving CS high and disabling the MC68HC55
SPI Peripheral */
void SpiBurst(int ByteCount)
{
int count;
/* CS to go LOW */
PORTS = 0x00;
/* Transmit bytes */
for (count=0; count<ByteCount; count++)
{
RBytes[count] = TransmitReceive(TBytes[count]);
}
/* CS to go HIGH */
PORTS = 0x80;
}
/* This function sets up the DSI registers */
int SetupDSI(void)
{
int ErrCnt=0;
int ChkCnt;
/* Set up DSI/D Registers */
TBytes[0] = 0x85;
/* Send 85 (write cmd to reg.
addr. 5) */
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Application Note
TBytes[1] = 0xb0;
/* Rec./Transmit Interrupts
disabled, Msg size 20Bits (16 + 4CRC) */
TBytes[2] = 0x00;
/* Set up ch. 1 ctrl reg to be
the same */
TBytes[3] = 0x01;
/* Only ENABLE ch. 0 (0x03 to
enable ch. 0 & 1, 0x02 to enable ch.1 only) */
Freescale Semiconductor, Inc...
SpiBurst(4);
by TBytes to the DSI/D */
/* Transmits the info. set up
TBytes[0] = 0x05;
/* Cmd. to read DSI/D registers
starting address 5 */
SpiBurst(4);
/* RBytes[x] will return
the contents of DSI/D registers 5 to 7 */
ChkBytes[1] = 0xb0;
contents of DSI0CTRL reg. */
ChkBytes[2] = 0x00;
contents of DSI1CTRL reg. */
ChkBytes[3] = 0x01;
contents of DSIENABL reg. */
/* should match RBytes[1]
/* should match RBytes[2]
/* should match RBytes[3]
/* Compare RBytes[1 to 3] to ChkBytes[1 to 3] if not equal ErrCnt is incremented &
returned to main */
for (ChkCnt=1; ChkCnt<4; ChkCnt ++)
if ((RBytes[ChkCnt] & 0x00ff) != (ChkBytes[ChkCnt] &
0x00ff)) ErrCnt++;
return (ErrCnt);
}
/* This function is here to allow visibility of failures when using debugger tool */
void FlagError(int Err, int Adr)
{
ErrorCode[Err]=Adr;
}
/* This function checks that the TFNF0 flag is set */
void TFFlag(int Address)
{
TBytes[0] = 0x04;
SpiBurst(2);
if (!(RBytes[1] & 0x02)) {
FlagError(2,Address);
for (;;); /* error - loop until device is reset */
}
}
/* This function waits in a loop until the RFNE0 flag is set */
/* & therefore data has been written into the DSI data registers */
void RFFlag(void)
{
TBytes[0] = 0x04;
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Appendix 1 - Source Code
SpiBurst(2);
while ((RBytes[1] & 0x01) == 0)
{
SpiBurst(2);
}
Freescale Semiconductor, Inc...
}
/* This function programs the address into the first slave node (no response is
expected ) */
void PgmAddr(int Addr)
{
TFFlag(Addr);
/* Check TFNF0 flag is set */
TBytes[0] = 0x80;
/* send 80 (write cmd to DSI0H) receive
00 */
TBytes[1] = Addr;
/* Pgm Addr cmd into DSI0H then TBytes[2]
is DSI0L */
TBytes[2] = 0x00;
SpiBurst(3);
RFFlag();
}
/* This function programs the addresses into slave nodes 2 through to 15 and checks
the responses from slave nodes 1 to 14 */
void PgmChk(int Addr, int LastAddr)
{
unsigned short DsiDat;
unsigned short DsiChk;
int dummy=0;
TFFlag(Addr);
/* Check TFNF0 flag is set */
TBytes[0] = 0x80;
/* send 80 (write cmd to DSI0H) */
TBytes[1] = Addr;
/* Pgm Addr cmd written into DSI0H &
DSI0L */
TBytes[2] = 0x00;
SpiBurst(3);
RFFlag();
/* CRC Check */
if (RBytes[1] & 0x08) {
FlagError(4,LastAddr);
for (;;); /* error - loop until device is reset */
}
TBytes[0] = 0x00;
/* Read DSI0H and DSI0L */
SpiBurst(3);
DsiDat = RBytes[1] | 0x0000;
/* Now shift DsiDat 8 times */
DsiDat <<= 8;
DsiDat = DsiDat+RBytes[2];
/* Now need to check the response ie that bits[15:12] = Prev.
Slave Addr */
DsiDat >>= 12;
DsiChk = LastAddr | 0x0000;
if (DsiDat != DsiChk) {
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Application Note
FlagError(6,LastAddr);
for (;;); /* error - loop until device is reset
}
*/
/* This function checks the response to the program address command of the last slave
node (number 15) */
void ChkRsp(int Addr, int ADCcmd))
{
unsigned short DsiDat;
unsigned short DsiChk;
int dummy=0;
Freescale Semiconductor, Inc...
TFFlag(Addr);
/* Check TFNF0 flag is set */
TBytes[0] = 0x80;
/* send 80 (write cmd to DSI0H) */
TBytes[1] = 0x00;
/* cmd written into DSI0H & DSI0L */
TBytes[2] = 0xADCcmd; /*dummy command to allow response from
final pgm addr cmd to be captured */
SpiBurst(3);
RFFlag();
/* CRC Check */
if (RBytes[1] & 0x08) {
FlagError(4, Addr);
for (;;); /*error - loop until device is reset */
}
TBytes[0] = 0x00;
/* Read DSI0H and DSI0L */
SpiBurst(3);
DsiDat = RBytes[1] | 0x0000;
/* Now shift DsiDat 8 times */
DsiDat <<= 8;
DsiDat = DsiDat+RBytes[2];
/* Now need to check the response ie that bits[15:12] = Slave
Addr */
DsiDat >>= 12;
DsiChk = Addr | 0x0000;
if (DsiDat != DsiChk) {
FlagError(6,Addr);
for (;;); /* error - loop until device is reset */
}
int main (int argc, char* argv[] )
{
int Result=-1;
short SlaveNum; /* the address of the slave node to be
programmed */
short PrevAddr;
/* the address of the previous slave node
whose response requires to be checked */
COPCTL = 0x00; /* Disable COP resets */
InitPWM();
/* Set up PWM */
InitSPI();
/* Set up SPI */
Result = SetupDSI();
/* Set up DSI ctrl and enable
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Appendix 1 - Source Code
registers (Result=0) if all OK*/
if (Result != 0) {
FlagError(1,1); /* need to stop program as well */
for (;;); /* error - loop until device is reset */
}
/* Program the address into each slave node and check response */
/* For this version no. of slave nodes is 15 (the maximum) */
Freescale Semiconductor, Inc...
PgmAddr(0x01);
for (SlaveNum=0x02, PrevAddr=0x01; SlaveNum<0x10; SlaveNum++,
PrevAddr++)
{
PgmChk(SlaveNum, PrevAddr);
}
ChkRsp(0x0F, 0x02);
return;
}
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not designed, intended, or authorized for use as components in systems intended for
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or for any other application in which the failure of the Freescale Semiconductor product
could create a situation where personal injury or death may occur. Should Buyer
purchase or use Freescale Semiconductor products for any such unintended or
unauthorized application, Buyer shall indemnify and hold Freescale Semiconductor
and its officers, employees, subsidiaries, affiliates, and distributors harmless against all
claims, costs, damages, and expenses, and reasonable attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associated with such
unintended or unauthorized use, even if such claim alleges that Freescale
Semiconductor was negligent regarding the design or manufacture of the part.
For More Information On This Product,
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