FX929AD2 datasheet(11/44 Pages) CMLMICRO

11 / 44 page

4-Level FSK Modem Data Pump

FX929A

D/929A/4

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

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, such as the 'S' Channel

Status symbol.

symbol

ms bit

ls bit

-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

lsb

Bits:

Symbols:

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, TSID,

RHB, RILB and RSID described in Section 1.5.5.2.

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