GMSK Packet Data Modem and RF Transceiver
CMX990
Task Descriptions:
NULL - No effect
This task is provided so the acquisition commands can be issued without loading a new task.
SFH - Search for Frame Head
Causes the modem to search the received signal for a Frame Head. The Frame Head will
consist of a 16-bit Frame Sync followed by control data (see Figure 11 - Mobitex Over Air
Signal). The search will continue until a Frame Head has been found, or until the RESET task is
loaded.
The search is carried out by first attempting to match the incoming bits against the previously
programmed (task LFSB) 16-bit Frame Sync pattern (allowing up to any one bit (of 16) in error).
When a match has been found, the modem will read the next 3 received bytes as Frame Head
bytes, these bytes will be checked, and corrected if necessary, using the FEC bits. The two
Frame Head Data bytes are then placed into the Data Buffer.
The BFREE and IRQ bits of the Status 1 Register will then be set to a logic ‘1’ to indicate that the
µC may read the 2 Frame Head Data bytes from the Data Buffer and write the next task to the
Command Register. If the FEC indicates uncorrectable errors the modem will set the CRCFEC
bit in the Status 1 Register to a logic ‘1’. The MOBAN bit (Mobile or Base) in the Status 1
Register will be set according to the polarity of the 3 bits preceding the Frame Sync pattern.
R3H - Read 3-byte Frame Head
This task, which would normally follow an SFS task, will place the next 3 bytes directly into the
Data Buffer. It also causes the modem to check the 3 bytes as Frame Head control data bytes
and will set the CRCFEC bit to a logic ‘1’ (high) only if the FEC bits indicate uncorrectable errors.
Note: This task will not correct any errors and, due to the Mobitex FEC specification, will not
detect all possible uncorrectable error patterns. The BFREE and IRQ bits of the Status 1
Register will be set 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 in the Status 1 Register will be set according to the validity of the received FEC
bits.
RDB - Read Data Block
This task causes the modem to read the next 240 bits as a Mobitex Data Block.
It will de-scramble and de-interleave the bits, FEC correct and CRC check the resulting 18 data
bytes and place them into the Data Buffer, setting the BFREE and IRQ bits of the Status 1
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.
SFS - Search for Frame Sync
This task, which is intended for special test and channel monitoring purposes, performs the first
part only of a SFH task. It causes the modem to search the received signal for a 16-bit sequence
which matches the Frame Synchronisation pattern with up to any 1 bit in error.
When a match is found the modem will set the BFREE and IRQ bits of the Status 1 Register to
‘1’ and update the MOBAN bit. The µC may then write the next task to the Command Register.
ã 2004 CML Microsystems Plc
25
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
