CML Semiconductor Products
4-Level FSK Modem Data Pump
FX919A
D/919A/4 June 1996
1.0
Features
Advance Information
• 4-Level FSK Modulation
• Flexible Operating Modes
• Half Duplex, 4800 to 19.2k bits/sec
• Host Bus Interface
• Full Packet Data Framing
• Low Power 3.3V to 5V Operation
• 24-Pin Small Form Package Option
1.1
Brief Description
The FX919A is a CMOS integrated circuit that contains all of the baseband signal processing and Medium
Access Control (MAC) protocol functions required for a high performance 4-level FSK Wireless Packet Data
Modem. It interfaces with the modem host processor and the radio modulation/demodulation circuits to deliver
reliable two-way transfer of the application data over the wireless link.
The FX919A assembles application data received from the processor, adds forward error correction (FEC) and
error detection (CRC) information and interleaves the result for burst-error protection. After adding symbol and
frame sync codewords, it converts the packet into filtered 4-level analogue signals for modulating the radio
transmitter.
In receive mode, the FX919A performs the reverse function using the analogue signals from the receiver
discriminator. After error correction and removal of the packet overhead, the recovered application data is
supplied to the processor. Any residual uncorrected errors in the data will be flagged. A readout of the SNR
value during receipt of a packet is also provided.
The FX919A uses data block sizes and FEC/CRC suitable for applications which require the high-speed
transfer of data over narrow-band wireless links. The device is programmable to operate at most standard bitrates from a wide choice of Xtal/clock frequencies.
1996 Consumer Microcircuits Limited
4-Level FSK Modem Data Pump
FX919A
CONTENTS
Section
Page
1.0 Features.......................................................................................................... 1
1.1 Brief Description............................................................................................ 1
1.2 Block Diagram................................................................................................ 3
1.3 Signal List....................................................................................................... 4
1.4 External Components.................................................................................... 6
1.5 General Description....................................................................................... 7
1.5.1 Description of Blocks ....................................................................... 7
1.5.2 Modem - µC Interaction ................................................................. 10
1.5.3 Binary to Symbol Translation......................................................... 11
1.5.4 Frame Structure ............................................................................ 12
1.5.5 The Programmer's View ................................................................ 13
1.5.5.1 Data Block Buffer ....................................................................... 13
1.5.5.2 Command Register .................................................................... 14
1.5.5.3 Control Register ......................................................................... 22
1.5.5.4 Mode Register............................................................................ 24
1.5.5.5 Status Register........................................................................... 25
1.5.5.6 Data Quality Register ................................................................. 27
1.5.6 CRC, FEC, and Interleaving .......................................................... 28
1.6 Application Notes ........................................................................................ 29
1.6.1 Transmit Frame Examples ............................................................ 29
1.6.2 Receive Frame Examples ............................................................. 32
1.6.3 Clock Extraction & Level Measurement Systems.......................... 35
1.6.4 AC Coupling .................................................................................. 36
1.6.5 Radio Performance........................................................................ 38
1.7 Performance Specification ......................................................................... 39
1.7.1 Electrical Performance .................................................................. 39
1.7.2 Packaging...................................................................................... 43
Note: As this product is still in development, it is likely that a number of changes and additions will be
made to this specification. Items marked TBD or left blank will be included in later issues.
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1.2
FX919A
Block Diagram
Figure 1 Block Diagram
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1.3
FX919A
Signal List
Package
P4/D2/D5
Signal
Description
Pin No.
Name
Type
1
IRQN
O/P
2
3
4
5
6
7
8
9
D7
D6
D5
D4
D3
D2
D1
D0
BI
BI
BI
BI
BI
BI
BI
BI
)
)
)
)
)
)
)
)
10
RDN
I/P
Read. An active low logic level input used to
control the reading of data from the modem into
the host µC.
11
WRN
I/P
Write. An active low logic level input used to
control the writing of data into the modem from
the host µC.
12
Vss
Power
13
CSN
I/P
Chip Select. An active low logic level input to the
modem, used to enable a data read or write
operation.
14
15
A0
A1
I/P
I/P
)
)
16
XTALN
O/P
The output of the on-chip oscillator.
17
XTAL/CLOCK
I/P
The input to the on-chip oscillator, for external
Xtal circuit or clock.
18
19
DOC 2
DOC 1
O/P
O/P
)
)
)
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A 'wire-ORable' output for connection to the host
µC's Interrupt Request input. This output has a
low impedance pull down to VSS when active
and is high impedance when inactive.
8-bit bidirectional 3-state µC interface data
lines.
The negative supply rail (ground).
4
Two logic level modem register select
inputs.
Connections to the Rx level measurement
circuitry. A capacitor should be connected
from each pin to VSS.
D/919A/4
4-Level FSK Modem Data Pump
FX919A
Package
P4/D2/D5
Signal
Description
Pin No.
Name
Type
20
TXOP
O/P
The Tx signal output from the modem.
21
VBIAS
O/P
A bias line for the internal circuitry, held at
½ VDD. This pin must be decoupled to VSS by a
capacitor mounted close to the device pins.
22
RXIN
I/P
The input to the Rx input amplifier.
23
RXFB
O/P
The output of the Rx input amplifier and the input
to the Rx RRC filter.
24
VDD
Power
The positive supply rail. Levels and voltages are
dependent upon this supply. This pin should be
decoupled to VSS by a capacitor.
Notes: I/P =
O/P =
BI
=
Input
Output
Bidirectional
Internal protection diodes are connected from each signal pin to VDD and VSS.
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1.4
FX919A
External Components
Figure 2 Recommended External Components
R1
R2
R3
R4
X1
See Section 1.5.1
C1
C2
C3
C4
100k ohm ± 5%
1M ohm ± 20%
100k ohm ± 5%
See Section 1.5.5.3
0.1 µF ± 20%
0.1 µF ± 20%
± 20%, see Note 1
± 20%, see Note 1
C5
C6
C7
C8
± 5%, see Note 3
± 20%, see Note 2
± 20%, see Note 2
± 5%, see Note 3
Note 1: The values used for C3 and C4 should be suitable for the frequency of the crystal X1.
As a guide, values (including stray capacitances) of 33pF at 1MHz falling to 18pF at 10MHz will
generally prove suitable. The 'Phase-Locked Loop Modes' part of section 1.5.5.3 discusses crystal
frequency tolerances.
Note 2: C6 and C7 values (in nano Farads) should be equal to 50000 ÷ symbol rate, e.g.
Symbol Rate
2400 symbols/second
4800 symbols/second
9600 symbols/second
C6/C7 (nF)
22.0
10.0
4.7
Note 3: C5 and C8 values (in pico Farads) should be equal to 750000 ÷ symbol rate, e.g.
Symbol Rate
2400 symbols/second
4800 symbols/second
9600 symbols/second
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C5/C8 (pF)
330
150
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1.5
General Description
1.5.1
Description of Blocks
Data Bus Buffers
Eight bidirectional 3-state logic level buffers between the modem's internal registers and the host µC's data bus
lines.
Address and R/W Decode
This block controls the transfer of data bytes between the µC and the modem's internal registers, according to
the state of the Write and Read Enable inputs (WRN and RDN), the Chip Select input (CSN) and the Register
Address inputs A0 and A1.
The Data Bus Buffers, Address and R/W Decode blocks provide a byte-wide parallel µC interface, which can
be memory-mapped, as shown in Figure 3.
Figure 3 Typical Modem µC Connections
Status and Data Quality Registers
Eight-bit registers which the µC can read to determine the status of the modem and the received data quality.
Command, Mode and Control Registers
The values written by the µC to these 8-bit registers control the operation of the modem.
Data Buffer
A 12-byte buffer used to hold receive or transmit data to or from the µC.
CRC Generator/Checker
A circuit which generates (in transmit mode) or checks (in receive mode) the Cyclic Redundancy Checksum
bits, which may be included in transmitted data blocks so that the receive modem can detect transmission
errors.
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FX919A
FEC Generator/Checker
In transmit mode, this circuit adds Forward Error Correction bits to the transmitted data, then converts the
resulting binary data to 4-level symbols. In receive mode, it translates received 4-level symbols to binary data,
using the FEC information to correct a large proportion of transmission errors.
Interleave/De-interleave Buffer
This circuit interleaves data symbols within a block before transmission and de-interleaves the received data so
that the FEC system is best able to handle short noise bursts or fades.
Frame Sync Detect
This circuit, which is only active in receive mode, is used to look for the 24-symbol Frame Synchronisation
pattern which is transmitted to mark the start of every frame.
Rx I/P Amp
This amplifier allows the received signal input to the modem to be set to the optimum level by suitable selection
of the external components R1 and R2. The value of R1 should be calculated to give 0.2 x VDD pk-pk at the
RXFB pin for a received '...+3 +3 -3 -3 ...' sequence.
A capacitor may be fitted in series with R1 if ac coupling of the received signal is desired (see Section 1.6.4),
otherwise the dc level of the received signal should be adjusted so that the signal at the modem's RXFB pin is
centred around VBIAS (½ VDD).
RRC Low Pass Filter
This filter, which is used in both transmit and receive modes, is a linear-phase lowpass filter with a 'Root Raised
Cosine' frequency response defined by:
H(f)
=1
for
0
<= f < (1-b)/(2T)
= square root of {0.5 [1 - sin(π T (f - 0.5/T)/b)]}
for
(1-b)/(2T)
<= f <= (1+b)/(2T)
=0
for
(1+b)/(2T) <
f
where b = 0.2, T = 1/symbol rate
This frequency response is illustrated in Figure 5.
In transmit mode, the 4-level symbols are passed through this filter to eliminate the high frequency components
which would otherwise cause interference into adjacent radio channels.
Figure 4 Translation of Binary Data to Filtered 4-Level Symbols in Tx Mode
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FX919A
In receive mode, the filter is used to reject HF noise and to equalise the received signal to a form suitable for
extracting the 4-level symbols.
Tx Output Buffer
This is a unity gain amplifier used in transmit mode to buffer the output of the Tx low pass filter. In receive
mode, the input of this buffer is connected to VBIAS unless the RXEYE bit of the Control Register is '1', when it
is connected to the received signal. When changing from Rx to Tx mode the input to this buffer will be
connected to VBIAS for 8 symbol times while the RRC filter settles.
Note: The RC low pass filter formed by the external components R4 and C5 between the TXOP pin and the
input to the radio's frequency modulator forms an important part of the transmit signal filtering. These
components may form part of any dc level-shifting and gain adjustment circuitry. The value used for C5 should
take into account stray circuit capacitances, and its ground connection should be positioned to give maximum
attenuation of high frequency noise into the modulator.
The signal at the TXOP pin is centred around VBIAS and is approx 0.2 x VDD pk-pk for a continuous '+3 +3 -3
-3 ...' pattern.
A capacitor may be fitted in series with the input to the frequency modulator if ac coupling is desired, see
Section 1.6.4.
0
-5
-10
dB -15
-20
-25
-30
0
0.2
0.4
0.6
0.8
1
Frequency / Symbol rate
Figure 5 RRC Filter Frequency Response (including the external RC filter R4/C5)
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FX919A
Figure 6 Transmitted Signal Eye Diagram
Rx Level/Clock Extraction
These circuits, which operate only in receive mode, derive a symbol rate clock from the received signal and
measure the received signal amplitude and dc offset. This information is then used to extract the received 4level symbols and also to provide an input to the received Data Quality measuring circuit. The external
capacitors C6 and C7 form part of the received signal level measuring circuit.
The capacitors C6 and C7 are driven from a very high impedance source so any measurement of the voltages
on the DOC pins must be made via high input impedance (MOS input) voltage followers to avoid disturbance of
the level measurement circuits.
Further details of the level and clock extraction functions are given in section 1.6.3.
Clock Oscillator and Dividers
These circuits derive the transmit symbol rate (and the nominal receive symbol rate) by frequency division of a
reference frequency which may be generated by the on-chip Xtal oscillator or applied from an external source.
Note: If the on-chip xtal oscillator is to be used, then the external components X1, C3, C4 and R3 are
required. If an external clock source is to be used, then it should be connected to the XTAL/CLOCK input pin,
the XTALN pin should be left unconnected, and X1, C3, C4 and R3 not fitted.
1.5.2
Modem - µC Interaction
In general, data is transmitted over-air in the form of messages, or 'Frames', consisting of a 'Frame Preamble'
followed by one or more formatted data blocks. The Frame Preamble includes a Frame Synchronisation pattern
designed to allow the receiving modem to identify the start of a frame. The following data blocks are
constructed from the 'raw' data using a combination of CRC (cyclic redundancy checksum) generation, Forward
Error Correction coding and Interleaving. Details of the message formats handled by the modem are given in
Section 1.5.3 and Figures 7 and 7a.
To reduce the processing load on the associated µC, the FX919A modem has been designed to perform as
much as possible of the computationally intensive work involved in Frame formatting and de-formatting and when in receive mode - in searching for and synchronising onto the Frame Preamble. In normal operation the
modem will only require servicing by the µC once per received or transmitted block.
Thus, to transmit a block, the controlling µC has only to load the - unformatted - 'raw' binary data into the
modem's Data Block Buffer then instruct the modem to format and transmit that data. The modem will then
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FX919A
calculate and add the CRC bits as required, encode the result as 4-level symbols (with Forward Error
Correction coding) and interleave the symbols before transmission.
In receive mode, the modem can be instructed to assemble a block's worth of received symbols, de-interleave
the symbols, translate them to binary - using the FEC coding to correct as many errors as possible - and check
the resulting CRC before placing the received binary data into the Data Block Buffer for the µC to read.
The modem can also transmit and receive un-formatted data using the T4S, T24S and R4S tasks described in
sections 1.5.3 and 1.5.5.2. These are normally used for the transmission of Symbol and Frame Synchronisation
sequences. They may also be used for the transmission and reception of special test patterns or even for
special data formats - although in this case care should be taken to ensure that the transmitted signal contains
enough level and timing information for the receiving modem's level and clock extraction circuits to function
correctly (see section 1.6.3).
1.5.3
Binary to Symbol Translation
Although the over-air signal, and hence the signals at the modem TXOP and RXIN pins, consists of 4-level
symbols, the raw data passing between the modem and the µC is in binary form. Translation between binary
data and the 4-level symbols is done in one of two ways, depending on the task being performed.
Direct: the simplest form, which converts between 2 binary bits and a single symbol.
symbol
+3
+1
-1
-3
ms bit
1
1
0
0
ls bit
1
0
0
1
This is expanded so that an 8-bit byte translates to four symbols for the T4S, T24S and R4S tasks described in
Section 1.5.5.2.
Bits:
Symbols:
msb
7
6
a
sent first
5
4
b
3
2
c
1
lsb
0
d
sent last
With FEC: This is more complicated, but essentially translates groups of 3 binary bits to pairs of 4-level
symbols using a Forward Error Correcting coding scheme for the block oriented tasks THB, TIB, TLB, RHB
and RILB described in Section 1.5.5.2.
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1.5.4
FX919A
Frame Structure
Figure 7 below shows how an over-air message frame may be constructed from Frame and Symbol Sync
patterns, followed by one or more 'Header', 'Intermediate' or 'Last' blocks.
Header Block
7
6
5
4 3 2
Intermediate Blocks
1
0
7
6
5
4 3
2
Last Block
1
0
7
6
5 4
3
2
1
0
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Data Bytes
(10)
Data Bytes
(8)
Data Bytes
(12)
Byte 6
Byte 7
Byte 8
Byte 9
Byte 10
CRC2
(4 bytes)
CRC1
(2 bytes)
Byte 11
7
0 7
Byte 0
0
7
Byte 1
0
Byte 11
'000'
tri-bits
0
1
2
3
4
5
29
30
31
32
64
65
FEC TRELLIS CODING / DECODING
( ERROR CORRECTION )
4-level
symbols
0
1
2
INTERLEAVING / DE-INTERLEAVING
Over-air
signal
(symbols)
SYMBOL
SYNC
FRAME
SYNC
'HEADER'
BLOCK
24
66
'LAST'
BLOCK
INTERMEDIATE BLOCKS
66
66
66
FRAME
PREAMBLE
FRAME
Frame Sync:
-1
+1
-1
+1
-1
+3
-3
+3
-3
-1
+1
-3
+3
+3
-1
+1
-3
-3
+1
sent first
+3
-1
-3
+1
+3
last
Symbol Sync : at least 24 symbols of '..+3 +3 -3 -3 ...' sequence
Figure 7 Over Air Signal Format
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FX919A
The 'Header' block is self-contained in that it includes its own checksum (CRC1), and would normally carry
information such as the addresses of the called and calling parties, the number of following blocks in the frame
(if any) and miscellaneous control information. The 'Intermediate' block(s) contain only data, the checksum for
all of the data in the 'Intermediate' and 'Last' blocks (CRC2) being contained at the end of the 'Last' block.
This arrangement, whilst efficient in terms of data capacity, may not be optimum for poor signal-to-noise
conditions, since a reception error in any one of the 'Intermediate' or 'Last' blocks would invalidate the whole
frame. In these conditions, increased throughput may be obtained by using the 'Header' block format for all
blocks of the frame, so that blocks which are received correctly can be identified as such, and need not be retransmitted. This, and some other possible frame structures, are shown in Figure 7a below.
Figure 7a Some Alternative Frame Structures
The FX919A performs all of the block formatting and de-formatting, the binary data transferred between the
modem and its µC being that enclosed by the thick dashed rectangles near the top of Figure 7.
1.5.5
The Programmer's View
The modem appears to the programmer as 4 write only 8-bit registers shadowed by 3 read only registers,
individual registers being selected by the A0 and A1 chip inputs:
A1
0
0
1
1
A0
0
1
0
1
Write to Modem
Data Buffer
Command Register
Control Register
Mode Register
Read from Modem
Data Buffer
Status Register
Data Quality Register
not used
Note that there is a minimum allowable time between accesses of the modem's registers, see Section 1.7.1 for
details.
1.5.5.1
Data Block Buffer
This is a 12-byte read/write buffer which is used to transfer data (as opposed to command, status, mode, data
quality or control information) between the modem and the host µC.
It appears to the µC as a single 8-bit register; the modem ensuring that sequential µC reads or writes to the
buffer are routed to the correct locations within the buffer.
The µC should only access this buffer when the Status Register BFREE (Buffer Free) bit is '1'.
The buffer should only be written to while in Tx mode and read from while in Rx mode. Note that in receive
mode the modem will function correctly even if the received data is not read from the Data Buffer by the µC.
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1.5.5.2
FX919A
Command Register
Writing to this register tells the modem to perform a specific action or actions, depending on the setting of the
TASK, AQLEV and AQSC bits.
When it has no action to perform, the modem will be in an 'idle' state. If the modem is in transmit mode the input
to the Tx RRC filter will be connected to VBIAS. In receive mode the modem will continue to measure the
received data quality and extract symbols from the received signal, supplying them to the de-interleave buffer,
but will otherwise ignore the received data.
Command Register B7: AQSC - Acquire Symbol Clock
This bit has no effect in transmit mode.
In receive mode, whenever a byte with the AQSC bit set to '1' is written to the Command Register, and TASK is
not set to RESET, it initiates an automatic sequence designed to achieve symbol timing synchronisation with
the received signal as quickly as possible. This involves setting the Phase Locked Loop of the received bit
timing extraction circuits to its widest bandwidth, then gradually reducing the bandwidth as timing
synchronisation is achieved, until it reaches the 'normal' value set by the PLLBW bits of the Control Register.
Setting this bit to '0' (or changing it from '1' to '0') has no effect, however note that the acquisition sequence will
be re-started every time that a byte written to the Command Register has the AQSC bit set to '1'.
The use of the symbol clock acquisition sequence is described in section 1.6.3.
Command Register B6: AQLEV - Acquire Receive Signal Levels
This bit has no effect in transmit mode.
In receive mode, whenever a byte with the AQLEV bit set to '1' is written to the Command Register and TASK
is not set to RESET, it initiates an automatic sequence designed to measure the amplitude and dc offset of the
received signal as rapidly as possible. This sequence involves setting the measurement circuits to respond
quickly at first, then gradually increasing their response time, hence improving the measurement accuracy, until
the 'normal' value set by the LEVRES bits of the Control Register is reached.
Setting this bit to '0' (or changing it from '1' to '0') has no effect, however note that the acquisition sequence will
be re-started every time that a byte written to the Command Register has the AQLEV bit set to '1'.
The use of the level measurement acquisition sequence (AQLEV) is described in section 1.6.3.
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FX919A
Command Register B5: CRC
This bit allows the user to select between two different forms of the CRC1 and CRC2 checksums. When this
bit is set to '0' the CRC generators are initialised to 'all ones' as for CCITT X25 CRC calculations. When this bit
is set to '1' the CRC generators are initialised to 'all zeros'. Setting this bit to '0' gives compatibility with the
older 'non-A' version of the FX919, other systems may set this bit as required.
Command Register B4, B3
These bits should always be set to '0'.
Command Register B2, B1, B0: TASK
Operations such as transmitting or receiving a data block are treated by the modem as 'tasks' and are initiated
when the µC writes a byte to the Command Register with the TASK bits set to anything other than the 'NULL'
code.
The µC should not write a task (other than NULL or RESET) to the Command Register or write to or read from
the Data Buffer when the BFREE (Buffer Free) bit of the Status Register is '0'.
Different tasks apply in receive and transmit modes.
When the modem is in transmit mode, all tasks other than NULL or RESET instruct the modem to transmit data
from the Data Buffer, formatting it as required. The µC should therefore wait until the BFREE (Buffer Free) bit of
the Status Register is '1', before writing the data to the Data Block Buffer, then it should write the desired task
to the Command Register. If more than 1 byte needs to be written to the Data Block Buffer, byte number 0 of
the block should be written first.
Once the byte containing the desired task has been written to the Command Register, the modem will:
Set the BFREE (Buffer Free) bit of the Status Register to '0'.
Take the data from the Data Block Buffer as quickly as it can - transferring it to the Interleave Buffer for
eventual transmission. This operation will start immediately if the modem is 'idle' (i.e. not transmitting
data from a previous task), otherwise it will be delayed until there is sufficient room in the Interleave
Buffer.
Once all of the data has been transferred from the Data Block Buffer the modem will set the BFREE
and IRQ bits of the Status Register to '1', (causing the chip IRQN output to go low if the IRQNEN bit of
the Mode Register has been set to '1') to tell the µC that it may write new data and the next task to the
modem.
This lets the µC write a task and the associated data to the modem while the modem is still transmitting the
data from the previous task.
Task 2
Task 1
Data from µC to Block Buffer
Task from µC to Command Register
BFREE bit of Status Register
IRQ bit of Status Register
IRQN o/p (IRQNEN = '1')
TXOP signal
from task 1
from task 2
Figure 8 Transmit Task Overlapping
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FX919A
When the modem is in receive mode, the µC should wait until the BFREE bit of the Status Register is '1', then
write the desired task to the Command Register.
Once the byte containing the desired task has been written to the Command Register, the modem will:
Set the BFREE bit of the Status Register to '0'.
Wait until enough received symbols are in the De-interleave Buffer.
Decode them as needed, and transfer the resulting binary data to the Data Block Buffer
Then the modem will set the BFREE and IRQ bits of the Status Register to '1', (causing the IRQN
output to go low if the IRQNEN bit of the Mode Register has been set to '1') to tell the µC that it may
read from the Data Block Buffer and write the next task to the modem. If more than 1 byte is contained
in the buffer, byte number 0 of the data will be read out first.
In this way the µC can read data and write a new task to the modem while the received symbols needed for this
new task are being received and stored in the De-interleave Buffer.
RXIN signal
for task 1
for task 2
IRQN o/p (IRQNEN = '1')
IRQ bit of Status Register
BFREE bit of Status Register
Task 1
Task from µC to Command Register
Task 2
Task 1 data
Data from Block Buffer to µC
Figure 9 Receive Task Overlapping
Detailed timings for the various tasks are given in Figures 10 and 11.
FX919A Modem Tasks:
B2
B1
B0
0
0
0
0
0
0
1
1
0
1
0
1
NULL
SFSH
RHB
RILB
Receive Mode
1
1
1
1
0
0
1
1
0
1
0
1
SFS
R4S
NULL
RESET
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Transmit Mode
Search for FS + Header
Read Header Block
Read Intermediate or Last
Block
Search for Frame Sync
Read 4 symbols
Cancel any current action
16
NULL
T24S
THB
TIB
TLB
T4S
NULL
RESET
Transmit 24 symbols
Transmit Header Block
Transmit Intermediate Block
Transmit Last Block
Transmit 4 symbols
Cancel any current action
D/919A/4
4-Level FSK Modem Data Pump
FX919A
NULL: No effect
This 'task' is provided so that a AQSC or AQLEV command can be initiated without loading a new task.
SFSH: Search for Frame Sync plus Header Block
This task causes the modem to search the received signal for a valid 24-symbol Frame Sync sequence
followed by Header Block which has a correct CRC1 checksum.
The task continues until a valid Frame Sync plus Header Block has been found.
The search consists of two stages:
First of all the modem will attempt to match the incoming symbols against the 24-symbol Frame
Synchronisation pattern to within the tolerance defined by the FSTOL bits of the Control Register.
Once a match has been found, the modem will read in the next 66 symbols as if they were a 'Header'
block, decoding the symbols and checking the CRC1 checksum. If this is incorrect, the modem will
resume the search, looking for a fresh Frame Sync pattern.
If the received CRC1 is correct, the 10 decoded data bytes will be placed into the Data Block Buffer,
the BFREE and IRQ bits of the Status Register will be set to '1' and the CRCERR bit cleared to '0'.
On detecting that the BFREE bit of the Status Register has gone to '1', the µC should read the 10 bytes from
the Data Block Buffer then write the next task to the modem's Command Register.
RHB: Read Header Block
This task causes the modem to read the next 66 symbols as a 'Header' Block, decoding them, placing the
resulting 10 data bytes and the 2 received CRC1 bytes into the Data Block Buffer, and setting the BFREE and
IRQ bits of the Status Register to '1' when the task is complete to indicate that the µC may read the data from
the Data Block Buffer and write the next task to the modem's Command Register.
The CRCERR bit of the Status Register will be set to '1' or '0' depending on the validity of the received CRC1
checksum bytes.
RILB: Read 'Intermediate' or 'Last' Block
This task causes the modem to read the next 66 symbols as an 'Intermediate' or 'Last' block (the µC should be
able to tell from the 'Header' block how many blocks are in the frame, and hence when to expect the 'Last'
block).
In each case, it will decode the 66 symbols and place the resulting 12 bytes into the Data Block Buffer, setting
the BFREE and IRQ bits of the Status Register to '1' when the task is complete.
If an 'Intermediate' block is received then the µC should read out all 12 bytes from the Data Block Buffer and
ignore the CRCERR bit of the Status Register, for a 'Last' block the µC need only read the first 8 bytes from the
Data Block Buffer, and the CRCERR bit in the Status Register will reflect the validity of the received CRC2
checksum.
SFS: Search for Frame Sync
This task causes the modem to search the received signal for a 24-symbol sequence which matches the Frame
Synchronisation pattern to within the tolerance defined by the FSTOL bits of the Mode Register.
When a match is found the modem will set the BFREE and IRQ bits of the Status Register to '1' to indicate to
the µC that it should write the next task to the Command Register.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
R4S: Read 4 Symbols
This task causes the modem to read the next 4 symbols and translate them directly (without de-interleaving or
FEC) to an 8-bit byte which is placed into the Data Block Buffer. The BFREE and IRQ bits of the Status
Register will then be set to '1' to indicate that the µC may read the data byte from the Data Block Buffer and
write the next task to the Command Register.
This task is intended for special tests and channel monitoring - perhaps preceded by SFS task.
T24S: Transmit 24 Symbols
This task, which is intended to facilitate the transmission of Symbol and Frame Sync patterns as well as special
test sequences, takes 6 bytes of data from the Data Block Buffer and transmits them as 24 4-level symbols
without any CRC or FEC.
Byte 0 of the Data Block Buffer is sent first, byte 5 last.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1', indicating to the µC that it may write the data and command byte for the next task to
the modem.
The tables below show what data has to be written to the Data Block Buffer to transmit the FX919A Symbol and
Frame Sync sequences:
+3
+3
+3
+3
+3
+3
'Symbol Sync'
Symbols
+3
-3
+3
-3
+3
-3
+3
-3
+3
-3
+3
-3
-1
-1
-3
+3
-3
-1
'Frame Sync'
Symbols
+1
-1
+3
-3
-1
+1
+3
-1
-3
+1
-3
+1
-3
-3
-3
-3
-3
-3
Values written to Data Block Buffer
Binary
Hex
Byte 0:
11110101
F5
Byte 1:
11110101
F5
Byte 2:
11110101
F5
Byte 3:
11110101
F5
Byte 4:
11110101
F5
Byte 5:
11110101
F5
+1
+3
-3
+1
+3
+3
Values written to Data Block Buffer
Binary
Hex
Byte 0:
00100010
22
Byte 1:
00110111
37
Byte 2:
01001001
49
Byte 3:
11110010
F2
Byte 4:
01011011
5B
Byte 5:
00011011
1B
THB: Transmit Header Block
This task takes 10 bytes of data (Address and Control) from the Data Block Buffer, calculates and appends the
2-byte CRC1 checksum, translates the result to 4-level symbols (with FEC), interleaves the symbols and
transmits the result as a formatted 'Header' Block.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1'.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
TIB: Transmit Intermediate Block
This task takes 12 bytes of data from the Data Block Buffer, updates the 4-byte CRC2 checksum for inclusion
in the 'Last' block, translates the 12 data bytes to 4-level symbols (with FEC), interleaves the symbols and
transmits the result as a formatted 'Intermediate' Block.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1'.
TLB: Transmit Last Block
This task takes 8 bytes of data from the Data Block Buffer, updates and appends the 4-byte CRC2 checksum,
translates the resulting 12 bytes to 4-level symbols (with FEC), interleaves the symbols and transmits the result
as a formatted 'Last' Block.
Once the modem has read the data bytes from the Data Block Buffer, the BFREE and IRQ bits of the Status
Register will be set to '1'.
T4S: Transmit 4 Symbols
This command is similar to T24S but takes only one byte from the Data Block Buffer, transmitting it as four 4level symbols.
RESET: Stop any current action
This 'task' takes effect immediately, and terminates any current action (task, AQSC or AQLEV) the modem may
be performing and sets the BFREE bit of the Status Register to '1', without setting the IRQ bit. It should be used
when VDD is applied, to set the modem into a known state.
Note that due to delays in the transmit filter, it will take several symbol times for any change to appear at the
TXOP pin.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Task Timings
Data to Data Block Buffer
1
Task to Command Register
3
2
1
3
2
t4
t4
t4
IBEMPTY bit
BFREE bit
t2
t1
t2
t3
t2
t3
t3
Symbols to
RRC filter
from task #1
from task #2
from task #3
Modem Tx output
Figure 10 Transmit Task Timing Diagram
t1
t2
t3
t4
Modem in idle state. Time from writing first
task to application of first transmit bit to Tx
RRC filter
Time from application of first symbol of the
task to the Tx RRC filter until BFREE goes
to a logic '1' (high).
Time to transmit all symbols of the task
Max time allowed from BFREE going to a
logic '1' (high) for next task (and data) to
be written to modem
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Task
Time
(symbol times)
Any
1 to 2
T24S
THB/TIB/TLB
T4S
T24S
THB/TIB/TLB
T4S
T24S
THB/TIB/TLB
T4S
5
16
0
24
66
4
18
49
3
D/919A/4
4-Level FSK Modem Data Pump
FX919A
Modem Rx input
for task #1
Symbols to
De-interleave circuit
for task #2
t3
for task #3
t3
Data from Data Block Buffer
t3
1
1
Task to Command Register
t6
2
2
t6
3
3
t7
t6
t7
BFREE bit
t7
Figure 11 Receive Task Timing Diagram
t3
Time to receive all symbols of task
t6
Maximum time between first symbol of task
entering the de-interleave circuit and the
task being written to modem.
t7
Maximum time from the last bit of the task
entering the de-interleave circuit to BFREE
going to a logic '1' (high)
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Task
SFS
SFSH
RHB/RILB
R4S
SFS
SFSH
RHB/RILB
R4S
Any
Time
(symbol times)
24 (minimum)
90 (minimum)
66
4
21
21
49
3
1
D/919A/4
4-Level FSK Modem Data Pump
FX919A
RRC Filter Delay
The previous task timing figures are based on the signal at the input to the RRC filter (in transmit mode) or the
input to the de-interleave buffer (in receive mode). There is an additional delay of about 8 symbol times through
to the RRC filter in both transmit and receive modes, as illustrated below:
T x S y m b o l t o RRC Filter
Tx Symbol at Txop pin / Rx S y m b o l f r o m FM discriminator
Rx Symbol to De-interleave Buffer
Symbol-times
Figure 12 RRC Low Pass Filter Delay
1.5.5.3
Control Register
This 8-bit write-only register controls the modem's symbol rate, the response times of the receive clock
extraction and signal level measurement circuits and the Frame Sync pattern recognition tolerance.
Control Register B7, B6: CKDIV - Clock Division Ratio
These bits control a frequency divider driven from the clock signal present at the XTALN pin, and hence
determine the nominal symbol rate. The table below shows how symbol rates of 2400/4800/9600 symbols/sec
may be obtained from common Xtal frequencies:
2.4576
B7
0
0
1
1
B6
0
1
0
1
Division Ratio:
Xtal Frequency/Symbol Rate
512
1024
2048
4096
Xtal Frequency (MHz)
4.9152
9.8304
Symbol Rate (symbols/sec)
4800
2400
9600
4800
2400
9600
4800
2400
Note: Device operation is not guaranteed below 2400 or above 9600 symbols/sec.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Control Register B5, B4: FSTOL - Frame Sync Tolerance
These two bits have no effect in transmit mode. In receive mode, they define the maximum number of
mismatches which will be allowed during a search for the Frame Sync pattern:
B5
0
0
1
1
B4
0
1
0
1
Mismatches allowed
0
2
4
6
Note: A single 'mismatch' is defined as the difference between two adjacent symbol levels, thus if the symbol
'+1' were expected, then received symbol values of '+3' and '-1' would count as 1 mismatch, a received symbol
value of '-3' would count as 2 mismatches. A setting of '4 mismatches' is recommended for normal use.
Control Register B3, B2: LEVRES - Level Measurement Modes
These two bits have no effect in transmit mode. In receive mode they set the 'normal' operating mode of the
received signal amplitude and dc offset measuring circuits (the automatic sequencing of an AQLEV command
may temporarily override the 'normal' setting).
B3
B2
0
0
1
1
0
1
0
1
Mode
Hold
Slow Peak Detect
Lossy Peak Detect
Clamp
In normal use the LEVRES bits should be set to 'Slow Peak Detect', the other modes are intended for special
test purposes or are invoked automatically during an AQLEV sequence.
In the 'Slow' and 'Lossy' Peak Detect modes the positive and negative excursions of the received signal (after
filtering) are measured by peak rectifiers driving the DOC1 and DOC2 capacitors, to establish the amplitude of
the signal and any dc offset wrt VBIAS . The peak rectifiers are disabled in 'Hold' mode.
In 'Clamp' mode the DOC1 and DOC2 pins are connected directly to the output of the circuit that normally
drives the peak detectors.
Control Register B1, B0: PLLBW - Phase-Locked Loop Modes
These two bits have no effect in transmit mode. In receive mode, they set the 'normal' bandwidth of the Rx
clock extraction Phase Locked Loop circuit. This setting will be temporarily overridden by the automatic
sequencing of an AQSC command.
B1
0
0
1
1
B0
0
1
0
1
PLL Mode
Hold
Narrow Bandwidth
Medium Bandwidth
Wide Bandwidth
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
The normal setting for the PLLBW bits should be 'Medium Bandwidth' when the received symbol rate and the
frequency of the receiving modem's Xtal are both within ±100ppm of nominal, except at the start of a symbol
clock acquisition sequence (AQSC) when 'WIde Bandwidth' should be selected as described in section 1.6.3.
If the received symbol rate and Xtal frequency are both within ±20ppm of nominal then selection of the 'Narrow
Bandwidth' setting will give better performance, especially through fades or noise bursts which might otherwise
pull the PLL away from its optimum timing, but in this case it is recommended that the PLLBW bits are only set
to 'Narrow Bandwidth' after the modem has been running in 'Medium Bandwidth' mode for about 200 symbol
times.
The 'Hold' setting disables the feedback loop of the PLL, which continues to run at a rate determined only by
the actual Xtal frequency and the setting of the Control Register CKDIV bits.
1.5.5.4 Mode Register
The contents of this 8-bit write only register control the basic operating modes of the modem:
Mode Register B7: IRQNEN - IRQN Output Enable
When this bit is set to '1', the IRQN chip output pin is pulled low (to Vss) whenever the IRQ bit of the Status
Register is a '1'.
Mode Register B6: INVSYM - Invert Symbols
This bit controls the polarity of the transmitted and received symbol voltages.
B6
0
Symbol
'+3'
'-3'
Signal at TXOP
Above VBIAS
Below VBIAS
Signal at RXFB
Below VBIAS
Above VBIAS
1
'+3'
'-3'
Below VBIAS
Above VBIAS
Above VBIAS
Below VBIAS
Mode Register B5: TXRXN - Tx/Rx Mode
Setting this bit to '1' puts the modem into Transmit mode, clearing it to '0' puts the modem into Receive mode.
Note that changing between receive and transmit modes will cancel any current task.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Mode Register B4: RXEYE - Show Rx Eye
This bit should normally be set to '0'. Setting it to '1' when the modem is in receive mode configures the modem
into a special test mode, in which the input of the Tx o/p buffer is connected to the Rx Symbol/Clock extraction
circuit at a point which carries the equalised receive signal. This may be monitored with an oscilloscope (at the
TXOP pin itself), to assess the quality of the complete radio channel including the Tx and Rx modem filters, the
Tx modulator and the Rx IF filters and FM demodulator.
The resulting eye diagram (for reasonably random data) should ideally be as shown in the following Figure 13,
with 4 'crisp' and equally spaced crossing points.
Figure 13 Ideal 'RXEYE' Signal
Mode Register B3: PSAVE - Powersave
When this bit is a '1', the modem will be in a 'powersave' mode in which the internal filters, the Rx Symbol &
Clock extraction circuits and the Tx o/p buffer will be disabled, and the TxOp pin will be connected to VBIAS
through a high value resistance. The Xtal Clock oscillator, Rx i/p amplifier and the µC interface logic will
continue to operate.
Setting the PSAVE bit to '0' restores power to all of the chip circuitry. Note that the internal filters - and hence
the TxOp pin in transmit mode - will take about 20 symbol-times to settle after the PSAVE bit is taken from '1' to
'0'.
Mode Register B2, B1, B0
These bits should be set to '000'.
1.5.5.5 Status Register
This register may be read by the µC to determine the current state of the modem.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Status Register B7: IRQ - Interrupt Request
This bit is set to '1' by:
The Status Register BFREE bit going from '0' to '1', unless this is caused by a RESET task or by a
change to the Mode Register TXRXN or PSAVE bits.
or
The Status Register IBEMPTY bit going from '0' to '1', unless this is caused by a RESET task or by
changing the Mode Register TXRXN or PSAVE bits.
or
The Status Register DIBOVF bit going from '0' to '1'.
The IRQ bit is cleared to '0' immediately after a read of the Status Register.
If the IRQNEN bit of the Mode Register is '1', then the chip IRQN output will be pulled low (to VSS) whenever
the IRQ bit is set to '1', and will go high impedance when the Status Register is read.
Status Register B6: BFREE - Data Block Buffer Free
This bit reflects the availability of the Data Block Buffer and is cleared to '0' whenever a task other than NULL or
RESET is written to the Command Register.
In transmit mode, the BFREE bit will be set to '1' (also setting the Status Register IRQ bit to '1') by the modem
when the modem is ready for the µC to write new data to the Data Block Buffer and the next task to the
Command Register.
In receive mode, the BFREE bit is set to '1' (also setting the Status Register IRQ bit to '1') by the modem when
it has completed a task and any data associated with that task has been placed into the Data Block Buffer. The
µC may then read that data and write the next task to the Command Register.
The BFREE bit is also set to '1' - but without setting the IRQ bit - by a RESET task or when the Mode Register
TXRXN or PSAVE bits are changed.
Status Register B5: IBEMPTY - Interleave Buffer Empty
In transmit mode, this bit will be set to '1' - also setting the IRQ bit - when less than two symbols remain in the
Interleave Buffer. Any transmit task written to the modem after this bit goes to '1' will be too late to avoid a gap
in the transmit output signal.
The bit is also set to '1' by a RESET task or by a change of the Mode Register TXRXN or PSAVE bits, but in
these cases the IRQ bit will not be set.
The bit is cleared to '0' within one symbol time after a task other than NULL or RESET is written to the
Command Register.
Note: When the modem is in transmit mode and the Interleave Buffer is empty, a mid level (half-way between
'+1' and '-1') signal will be sent to the RRC filter.
In receive mode this bit will be '0'.
Status Register B4: DIBOVF - De-Interleave Buffer Overflow
In receive mode this bit will be set to '1' - also setting the IRQ bit - when a RHB, RILB or R4S task is written to
the Command Register too late to allow continuous reception.
The bit is cleared to '0' immediately after reading the Status Register, by writing a RESET task to the Command
Register or by changing the TXRXN or PSAVE bits of the Mode Register.
In transmit mode this bit will be '0'.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Status Register B3: CRCERR - CRC Checksum Error
In receive mode this bit will be updated at the end of a SFSH, RHB or RILB task to reflect the result of the
receive CRC check. '0' indicates that the CRC was received correctly, '1' indicates an error.
Note that this bit should be ignored when an 'Intermediate' block (which does not have an integral CRC) is
received.
The bit is cleared to '0' by a RESET task, or by changing the TXRXN or PSAVE bits of the Mode Register. In
transmit mode this bit is '0'.
Status Register B2, B1, B0
These bits are reserved for future use.
1.5.5.6 Data Quality Register
In receive mode, the FX919A continually measures the 'quality' of the received signal, by comparing the actual
received waveform over the previous 64 symbol times against an internally generated 'ideal'.
The result is placed into bits 3-7 of the Data Quality Register for the µC to read at any time, bits 0-2 being
always set to '0'. Figure 14 shows how the value (0-255) read from the Data Quality Register varies with
received signal-to-noise ratio:
250
200
150
DQ
100
50
0
5
7
9
11
13
15
S/N dB (noise in 2* symbol-rate bandwidth)
Figure 14 Typical Data Quality Reading vs S/N
The Data Quality readings are only valid when the modem has successfully acquired signal level and timing
lock for at least 64 symbol times. It is invalid when an AQSC or AQLEV sequence is being performed or when
the LEVRES setting is 'Clamp' or 'Lossy Peak Detect'. A low reading will be obtained if the PLLBW bits are set
to 'Wide' or if the received signal waveform is distorted in any significant way.
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D/919A/4
4-Level FSK Modem Data Pump
1.5.6
FX919A
CRC, FEC and Interleaving
Cyclic Redundancy Codes
CRC1
This is a sixteen-bit CRC check code contained in bytes 10 and 11 of the Header Block. It is calculated by the
modem from the first 80 bits of the block ( Bytes 0 to 9 inclusive) using the generator polynomial:
x16 + x12 + x5 + 1
CRC2
This is a thirty-two-bit CRC check code contained in bytes 8 to 11 of the 'Last' Block. It is calculated by the
modem from all of the data and pad bytes in the Intermediate Blocks and in the first 8 bytes of the Last Block
using the generator polynomial:
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1
Note: In receive mode the CRC2 checksum circuits are initialised on completion of any task other than NULL or
RILB. In transmit mode the CRC2 checksum circuits are initialised on completion of any task other than NULL,
TIB or TLB.
Forward Error Correction
In transmit mode, the FX919A uses a Trellis Encoder to translate the 96 bits (12 bytes) of a 'Header',
'Intermediate' or 'Last' Block into a 66-symbol sequence which includes FEC information.
In receive mode, the FX919A decodes the received 66 symbols of a block into 96 bits of binary data using a
'Soft Decision' Viterbi algorithm to perform decoding and error correction.
Interleaving
The 66 symbols of a 'Header', 'Intermediate' or 'Last' block are interleaved by the modem before transmission
to give protection against the effects of noise bursts and short fades.
In receive mode, the FX919A de-interleaves the received symbols prior to decoding.
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D/919A/4
4-Level FSK Modem Data Pump
1.6
Application Notes
1.6.1
Transmit Frame Examples
FX919A
The operations needed to transmit a single Frame consisting of Symbol and Frame Sync sequences, and one
each Header, Intermediate and Last blocks are shown below:
1.
Ensure that the Control Register has been loaded with a suitable CKDIV value, that the IRQNEN and
TXRXN bits of the Mode Register are '1', the RXEYE and PSAVE bits are '0' and the INVSYM bit is
set appropriately.
2.
Read the Status Register to ensure that the BFREE bit is '1', then write 6 Symbol Sync bytes to the
Data Block Buffer and a T24S task to the Command Register.
3.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the IBEMPTY bit should be '0'.
4.
Write 6 Frame Sync bytes to the Data Block Buffer and a T24S task to the Command Register.
5.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the IBEMPTY bit should be '0'.
6.
Write 10 Header Block bytes to the Data Block Buffer and a THB task to the Command Register.
7.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the IBEMPTY bit should be '0'.
8.
Write 12 Intermediate Block bytes to the Data Block Buffer and a TIB task to the Command Register.
9.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the IBEMPTY bit should be '0'.
10.
Write 8 Last Block bytes to the Data Block Buffer and a TLB task to the Command Register.
11.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the IBEMPTY bit should be '0'.
12.
Wait for another interrupt from the modem, read the Status Register; the IRQ, BFREE and IBEMPTY
bits should be '1'.
Note: The final symbol of the frame will start to appear approximately 2 symbol times after the Status Register
IBEMPTY bit goes to '1'; a further 16 symbol times should be allowed for the symbol to pass completely
through the RRC filter.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Figures 15a and 15b illustrate the host µC routines needed to send a single Frame consisting of Symbol and
Frame Sync patterns, a Header block, any number of Intermediate blocks and one Last block. It is assumed
that the Tx Interrupt Service Routine (Figure 15b) is called whenever the FX919A's IRQN output line goes low.
Figure 15a Transmit Frame Example Flowchart, Main Program
Note that the RESET command in Figure 15a and the practice of disabling the FX919A's IRQN output when not
needed are not essential but can eliminate problems during debugging and if errors occur in operation.
Note also that the CRC bit should be set appropriately whenever a byte is written to the Command Register.
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Figure 15b Tx Interrupt Service Routine
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4-Level FSK Modem Data Pump
1.6.2
FX919A
Receive Frame Examples
The operations needed to receive a single Frame consisting of Symbol and Frame Sync sequences and one
each Header, Intermediate and Last blocks are shown below;
1.
Ensure that the Control Register has been loaded with suitable CKDIV, FSTOL, LEVRES and
PLLBW values, and that the IRQNEN bit of the Mode Register is '1', the TXRXN, PSAVE and RXEYE
bits are '0', and the INVSYM bit is set appropriately.
2.
Wait until the received carrier has been present for at least 8 symbol times (see Section 1.6.3).
3.
Read the Status Register to ensure that the BFREE bit is '1'.
4.
Write a byte containing a SFSH task and with the AQSC and AQLEV bits set to '1' to the Command
Register.
5.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the CRCERR and DIBOVF bits should be '0'.
6.
Check that the CRCERR bit of the Status Register is '0' and read 10 Header Block bytes from the
Data Block Buffer.
7.
Write a RILB task to the Command Register.
8.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the DIBOVF bit '0'.
9.
Read 12 Intermediate Block bytes from the Data Block Buffer.
10.
Write a RILB task to the Command Register.
11.
Wait for an interrupt from the modem, read the Status Register; the IRQ and BFREE bits should be
'1' and the DIBOVF bit '0'.
12.
Check that the CRCERR bit of the Status Register is '0' and read the 8 Last Block bytes from Data
Buffer.
Figures 16a and 16b illustrate the host µC routines needed to receive a single Frame consisting of Symbol and
Frame Sync patterns, a Header block, any number of Intermediate blocks and one Last block. It is assumed
that the Rx Interrupt Service Routine (Figure 16b) is called whenever the FX919A's IRQN output line goes low.
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4-Level FSK Modem Data Pump
FX919A
Figure 16a Receive Frame Example Flowchart, Main Program
Note that the RESET command in Figure 16a and the practice of disabling the FX919A's IRQN output when not
needed are not essential but can eliminate problems during debugging and if errors occur in operation.
Note also that the CRC bit should be set appropriately whenever a byte is written to the Command Register.
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4-Level FSK Modem Data Pump
FX919A
Figure 16b Rx Interrupt Service Routine
Note: This routine assumes that the number of Intermediate blocks in the Frame is contained within the Header
Block data.
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4-Level FSK Modem Data Pump
1.6.3
FX919A
Clock Extraction and Level Measurement Systems
The FX919A is intended for use in systems where:
-
The Symbol Sync pattern is transmitted immediately on start-up of the transmitter, before the first
Frame Sync pattern (see Figure 17).
-
A terminal may remain powered up indefinitely, transmitting concatenated Frames with or without
intervening Symbol Sync patterns (each Frame having a Frame Synch pattern and symbol timing being
maintained from one Frame to the next).
-
A receiving modem may be switched onto a channel before the distant transmitter has started up, or
may be switched onto a channel where the transmitting station is already sending concatenated
Frames.
Whenever the receiving modem is enabled or switched onto a channel it needs to establish the received
symbol levels and timing and look for a Frame Sync pattern in the incoming signal. This is best done by the
following procedure.
1.
Ensure that the Control Register's PLLBW bits are set to 'Wide' and the LEVRES bits to 'Slow Peak
Detect'.
2.
Wait until a received carrier has been present for 8 symbol times. This 8-symbol delay gives time for
the received signal to propagate through the modem's RRC filter and can usefully be included in the
radio's carrier detect circuitry.
3.
Write a SFS or SFSH task to the Command Register with the AQSC and AQLEV bits set to '1'.
4.
When the modem interrupts to signal that it has recognised a Frame Sync pattern (or completed the
SFSH task) then change the PLLBW bits to 'Medium'.
Once the receiving modem has achieved level and symbol timing synchronisation with a particular channel - as
evidenced by recognition of a Frame Sync pattern - then subsequent concatentated Frames can be read by
simply issuing SFS or SFSH tasks at appropriate times, keeping the ASQSC and AQLEV bits at zero, and the
PLLLBW and LEVRES bits at their current 'Medium' and 'Slow Peak Detect' settings.
noise
Symbol Sync
Frame Sync
rest of frame
Received signal
from FM discriminator
to Modem :
8-symbol delay
Set AQSC and AQLEV bits
to start acquisition sequences :
Level Measurement and
Clock Extraction circuits :
Increasing accuracy and lengthening response times
Figure 17 Acquisition Sequence Timing (Transmitter Power-Up)
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
It is also possible to use the modem in a system where there is an indeterminate delay between the transmitter
start-up and the Symbol Sync pattern, or where a receive carrier detect signal is not available to the controlling
µC, or where the transmitting terminal can send separate unsynchronised Frames. In these cases each Frame
should be preceded by a Symbol Sync pattern which should be extended to about 100 symbols, and the
procedure given in paragraphs (1) to (4) above used at all times.
Setting the AQSC and AQLEV bits to '1' triggers the modem's automatic Symbol Clock Extraction and Level
Measurement acquisition sequences, which are designed to measure the received symbol timing, amplitude
and dc offset as quickly as possible before switching to more accurate - but slower - measurement modes.
These acquisition sequences act very quickly if triggered at the start of a received Symbol Sync pattern (as
shown in Figure 17), but will still function correctly - although more slowly - if started any time during a normal
Frame, as when the receiver is switched onto a channel where the transmitter is operating continuously.
The automatic AQLEV Level Measurement acquisition sequence starts with the level measurement circuits
being put into 'Clamp' mode for one symbol time to set the voltages on the DOC pins to some point within the
range of the received signal excursions. The level measurement circuits are then automatically set to 'Lossy
Peak Detect' mode for 15 symbol times, after which the sequence ends and the level measurement circuit
modes reverts to the mode set by the LEVRES bits of the Control Register (normally 'Slow Peak Detect').
The peak detectors used in both 'Slow' and 'Lossy Peak Detect' modes include additional low pass filtering of
the received signal which greatly reduces the effect of pattern noise on the reference voltages held on the
external DOC capacitors, but means that pairs of '+3' (and '-3') symbols need to be received to establish the
correct levels. 2 pairs of '+3' and two pairs of '-3' symbols received after the start of an AQLEV sequence are
sufficient to set the levels on the DOC capacitors to their optimum levels.
The automatic AQSC Symbol Clock acquisition sequence sets the PLL to 'Extra Wide Bandwidth' mode for 16
symbol times (this mode is not one of those which can be selected by the Control Register PLLBW bits) then
changes to 'Wide' bandwidth. After 45 symbol times the PLL mode will revert to that set by the Control Register
PLLBW bits.
1.6.4
AC Coupling
For a practical circuit, ac coupling from the modem's transmit output to the frequency modulator and between
the receiver's frequency discriminator and the receive input of the modem may be desired. There are, however,
two problems:
Firstly, ac coupling of the signal degrades the Bit Error Rate performance of the modem. The following graph
illustrates the typical bit error rates at 4800 symbols/sec (without FEC) for differing degrees of ac coupling:
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
1E-1
BER
1E-2
Tx & Rx DC coupled
Tx 5Hz, Rx DC
1E-3
Tx 5Hz, Rx 5Hz
Tx 5Hz, Rx 10Hz
1E-4
4
5
6
7
8
9
10
11
12
13
14
S/N dB (Noise in 20 to 9600Hz band)
Figure 18 Effect of AC Coupling on BER
Secondly, any ac coupling at the receive input will transform any step in the voltage at the discriminator output
to a slowly decaying pulse which can confuse the modem's level measuring circuits. As illustrated in Figure 19
below, the time for this step to decay to 37% of its original value is 'RC' where:
RC = 1/( 2 x π x the 3dB cut-off frequency of the RC network )
which is 32 msec, or 153 symbol times at 4800 symbols/sec, for a 5Hz network.
Step input
to RC circuit
100%
Output of
RC circuit
37%
T=RC
Figure 19 Decay Time - AC Coupling
In general, it will be best to dc couple the receiver discriminator to the modem, and to ensure that any ac
coupling to the transmitter's frequency modulator has a -3dB cut-off frequency of no higher than 5Hz (for 4800
symbols/sec).
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D/919A/4
4-Level FSK Modem Data Pump
1.6.5
FX919A
Radio Performance
The maximum data rate that can be transmitted over a radio channel using these modems depends on:
-
RF channel spacing.
Allowable adjacent channel interference.
Symbol rate.
Peak carrier deviation (modulation index).
Tx and Rx reference oscillator accuracies.
Modulator and demodulator linearity.
Receiver IF filter frequency and phase characteristics.
Use of error correction techniques.
Acceptable error rate.
As a guide, 4800 symbols/sec can be achieved - subject to local regulatory requirements - over a system with
12.5kHz channel spacing if the transmitter frequency deviation is set to ±2.5kHz peak for a repetitive ' +3 +3 3 -3 ... ' pattern and the maximum difference between transmitter and receiver 'carrier' frequencies is less than
2400Hz.
The modulation scheme employed by these modems is designed to achieve high data throughput by exploiting
as much as possible of the RF channel bandwidth. This does, however, place constraints on the performance
of the radio. In particular, attention must be paid to:
-
Linearity, frequency and phase response of the Tx Frequency Modulator. For a 4800
symbols/sec system, the frequency response should be within ±2dB over the range 3Hz to
5kHz, relative to 2400Hz.
-
The bandwidth and phase response of the receiver's IF filters.
-
Accuracy of the Tx and Rx reference oscillators, as any difference will shift the received signal
towards the skirts of the IF filter response and cause a dc offset at the discriminator output.
Viewing the received signal eye - using the Mode Register RXEYE function - gives a good indication of the
overall transmitter/receiver performance.
Figure 20 Typical Connections Between Radio and FX919A
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D/919A/4
4-Level FSK Modem Data Pump
1.7
Performance Specification
1.7.1
Electrical Performance
FX919A
Absolute Maximum Ratings
Exceeding these maximum ratings can result in damage to the device.
Min.
-0.3
-0.3
-30
-20
Max.
7.0
VDD + 0.3
+30
+20
Units
V
V
mA
mA
D2 Package
Total Allowable Power Dissipation at Tamb = 25°C
... Derating
Storage Temperature
Operating Temperature
Min.
Max.
800
13
+125
+85
Units
mW
mW/°C
°C
°C
D5 Package
Total Allowable Power Dissipation at Tamb = 25°C
... Derating
Storage Temperature
Operating Temperature
Min.
Max.
550
9
+125
+85
Units
mW
mW/°C
°C
°C
P4 Package
Total Allowable Power Dissipation at Tamb = 25°C
... Derating
Storage Temperature
Operating Temperature
Min.
Max.
800
13
+125
+85
Units
mW
mW/°C
°C
°C
Supply (VDD - VSS)
Voltage on any pin to VSS
Current into or out of VDD and VSS pins
Current into or out of any other pin
-55
-40
-55
-40
-55
-40
Operating Limits
Correct operation of the device outside these limits is not implied.
Notes
Supply (VDD - VSS)
Operating Temperature
Symbol Rate
Xtal Frequency
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Min.
3.0
-40
2400
1.0
Max.
5.5
+85
9600
10.0
Units
V
°C
Symbols/sec
MHz
D/919A/4
4-Level FSK Modem Data Pump
FX919A
Operating Characteristics
For the following conditions unless otherwise specified:
Xtal Frequency = 4.9152MHz, Symbol Rate = 4800 symbols/sec,
Noise Bandwidth = 0 to 9600Hz, VDD = 3.3V to 5.0V, Tamb = - 40°C to +85°C.
Notes
DC Parameters
IDD
(VDD = 5.0V)
(VDD = 3.3V)
IDD
(Powersave Mode VDD = 5.0V)
IDD
(Powersave ModeVDD = 3.3V)
IDD
1
1
1
1
AC Parameters
Tx Output
TXOP Impedance
Signal Level
Output DC Offset wrt VDD/2
2
3
4
Rx Input
RXIN Impedance (at 100Hz)
RXIN Amp Voltage Gain (I/P = 1mVrms at 100Hz)
Input Signal Level
DC Offset wrt VDD/2
Xtal/Clock Input
'High' Pulse Width
'Low' Pulse Width
Input Impedance (at 100Hz)
Gain (I/P = 1mVrms at 100Hz)
µC Interface
Input Logic "1" Level
Input Logic "0" Level
Input Leakage Current (Vin = 0 to VDD)
Input Capacitance
Output Logic "1" Level (lOH = 120µA)
Output Logic "0" Level (lOL = 360µA)
'Off' State Leakage Current (Vout = VDD)
Notes:
Min.
0.8
-0.25
5
5
0.7
-0.5
6
6
40
40
10.0
20
7, 8
7, 8
7, 8
7, 8
8
8,9
9
70%
Typ.
Max.
Units
4.0
2.5
1.5
0.6
10.0
6.3
mA
mA
mA
mA
1.0
1.0
2.5
1.2
+0.25
kΩ
V pk-pk
V
1.3
+0.5
MΩ
V/V
V pk-pk
V
10.0
300
1.0
ns
ns
MΩ
dB
30%
+5.0
5.0
10.0
92%
8%
10
VDD
VDD
µA
pF
VDD
VDD
µA
1. At 25°C. Not including any current drawn from the modem pins by external circuitry other than
the Xtal oscillator.
2. Small signal impedance, at VDD = 5.0V and Tamb = 25°C.
3. Measured after the external RC filter (R4/C5) for a "+3 +3 -3 -3...." symbol sequence, at VDD
= 5.0V and Tamb = 25°C (output level is proportional to VDD).
4. Measured at the TXOP pin with the modem in the Tx idle mode.
5. For optimum performance, measured at RXFB pin, for a "...+3 +3 -3 -3..." symbol sequence,
at VDD = 5.0V and Tamb = 25°C. Optimum level is proportional to VDD.
6. Timing for an external input to the CLOCK/XTAL pin.
7. WRN, RDN, CSN, A0 and A1 pins.
8. D0 - D7 pins.
9. IRQN pin.
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4-Level FSK Modem Data Pump
FX919A
Notes
µC Parallel Interface Timings (ref. Figure 21)
tACSL
Address valid to CSN low time
Address hold time
tAH
CSN hold time
tCSH
CSN high time
tCSHI
CSN to WRN or RDN low time
tCSRWL
Read data hold time
tDHR
Write data hold time
tDHW
Write data setup time
tDSW
RDN high to CSN low time (write)
tRHCSL
Read access time from CSN low
tRACL
Read access time from RDN low
tRARL
RDN low time
tRL
RDN high to D0-D7 3-state time
tRX
WRN high to CSN low time (read)
tWHCSL
WRN low time
tWL
Notes:
10
Min.
Typ.
Max.
0
0
0
6
0
0
0
90
0
11
11
175
145
200
50
0
200
Units
ns
ns
ns
clock cycles
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
10. Xtal/Clock cycles at the XTAL/CLOCK pin.
11. With 30pF max to VSS on D0 - D7 pins.
Figure 21 µC Parallel Interface Timings
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
1E-1
1E-2
BER
Without FEC
1E-3
With FEC
1E-4
1E-5
4
5
6
7
8
9
10
11
12
13
14
S/N dB (Noise in 2 x Symbol Rate Bandwidth)
Figure 22 Typical Bit Error Rate With and Without FEC
Measured under nominal working conditions, LEVRES bits set to 'Slow Peak Detect' and PLLBW bits set to
'Narrow Bandwidth' , with pseudo-random data.
Note:
S/N calculated as 20 × LOG10( Signal Voltage ÷ Noise Voltage)
Where Signal Voltage is the measured rms. voltage of a random 4-level signal.
Noise Voltage is the rms. voltage of a flat Gaussian noise signal having a bandwidth from a few
Hz to twice the symbol rate (e.g. to 9600Hz when measuring a 4800 symbol/sec system).
Both signals are measured at the same point in the test circuit.
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D/919A/4
4-Level FSK Modem Data Pump
1.7.2
FX919A
Packaging
Figure 23 D2 Mechanical Outline: Order as part no. FX919AD2
Figure 24 D5 Mechanical Outline: Order as part no. FX919AD5
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D/919A/4
4-Level FSK Modem Data Pump
FX919A
Figure 25 P4 Mechanical Outline: Order as part no. FX919AP4
Handling precautions: This product includes input protection, however, precautions should be taken to prevent
device damage from electro-static discharge. CML does not assume any responsibility for the use of any
circuitry described. No IPR or circuit patent licences are implied. CML reserves the right at any time without
notice to change the said circuitry and this product specification. CML has a policy of testing every product
shipped using calibrated test equipment to ensure compliance with this product specification. Specific testing
of all circuit parameters is not necessarily performed.
CONSUMER MICROCIRCUITS LIMITED
1 WHEATON ROAD
WITHAM - ESSEX CM8 3TD - ENGLAND
Telephone:
Telefax:
+44 1376 513833
+44 1376 518247