GMSK Packet Data Modem and RF Transceiver
CMX990
5.1.5
CRC, FEC, Interleaving and Scrambling Information:
5.1.5.1 CRC
This is a 16-bit CRC code used in both the Mobitex Data Block and Short Data Block. In
transmit it is calculated by the modem from the data block bytes using the following generator
polynomial:
16
12
5
g(x) = x + x + x + 1
i.e.
CRC - CCITT X.25.
This code detects all (single) error bursts of up to 16 bits in length and about 99.998% of all
other error patterns.
The CRC register is initialised to all ‘1s’ and the CRC is calculated octet by octet starting with
the least significant bit of ‘byte 0’. The CRC calculated is bit-wise inverted and appended to the
data bytes with the most significant bit transmitted earliest.
In receive mode, a 16-bit CRC code is generated from the data bytes of each Mobitex Data
Block or Short Data Block as above and the bit-wise inverted value is compared with the
received CRC bytes. If a mis-match is present, then an error has been detected.
ã 2004 CML Microsystems Plc
37
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
