4-Level FSK Modem Data Pump
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
ms bit
ls bit
+3
+1
-1
1
1
0
0
1
0
0
1
-3
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.
msb
7
lsb
0
Bits:
6
5
4
3
2
1
Symbols:
a
b
c
d
sent first
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.
ã 1996 Consumer Microcircuits Limited
11
D/919A/4
CMLMICRO [ CML MICROCIRCUITS ]
