GMSK Packet Data Modem
CMX909B
RSD - Read Short Data Block
This task causes the modem to read the next 72 bits as a Mobitex Short Data Block.
It will de-scramble and de-interleave the bits, FEC correct and CRC check the resulting 4 data bytes
and place them into the Data Buffer, 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 Buffer and write
the next task to the modem’s Command Register. The CRCFEC bit will be set according to the
outcome of the CRC check.
Note: in receive mode the CRC checksum circuits are initialised on completion of any task other
than NULL.
SFSZ - Search for Frame Sync with Zero Errors
This performs the same task as SFS task but allowing no bits to be in error over the 16-bit Frame
Sync pattern.
T7H - Transmit 7-byte Frame Head
This task takes 6 bytes of data from the Data Buffer, calculates and appends 8 bits of FEC from
bytes ‘4’ and ‘5’ then transmits the result as a complete Mobitex Frame Head.
Bytes ‘0’ and ‘1’ form the bit sync pattern, bytes ‘2’ and ‘3’ form the frame sync pattern and bytes ‘4’
and ‘5’ are the frame head control bytes. Bit 7 of byte ‘0’ of the Data Buffer is sent first, bit 0 of the
FEC byte last.
Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the
Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data to the
modem.
TQB - Transmit 4 Bytes
This task takes 4 bytes of data from the Data Buffer and transmits them, bit 7 first.
Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the
Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data to the
modem.
TDB - Transmit Data Block
This task takes 18 bytes of data from the Data Buffer, calculates and applies a 16-bit CRC and
forms the FEC for the 18 data bytes and the CRC. This data is then interleaved and passed through
the scrambler, if enabled, before being transmitted as a Mobitex Data Block.
Once the modem has read the data bytes from the Data Buffer, the BFREE and IRQ bits of the
Status Register will be set to ‘1’, indicating to the µC that it may write the next task and its data to the
modem.
Note: In transmit mode the CRC checksum circuit is initialised on completion of any task other than
NULL.
TSB - Transmit Single Byte
This task takes a byte from the Data Buffer and transmits the 8 bits, bit 7 first.
Once the modem has read the data byte from the Data Buffer, the BFREE and IRQ bits of the Status
Register will be set to ‘1’, indicating to the µC that it may write the next task and its data to the
modem.
ã 2001 Consumer Microcircuits Limited
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D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
