FRS Signalling Processor
CMX882
1.7 Application Notes
Radio Section
Receive
Discriminator
input
FFSK
Audio output
Rx
Voice+SubAudio
Demodulator
CMX882
RF
M odulator
output 1
Audio input 1
Audio input 2
2-point Tx
Modulator
Transmit
MIC 1
MIC 2
M odulator
output 2
FFSK
Voice+SubAudio
Host
GPS
MicroController
Keypad
Figure 13 Possible FRS + GPS Configuration
1.7.1
For messages with FEC coding the following matrix is used to calculate and decode bytes:
Data bits FEC bits
CRC and FEC encoding information
7
1
1
1
0
6
1
1
0
1
5
1
0
1
1
4
0
1
1
1
3
1
0
1
0
2
1
0
0
1
1
0
1
1
0
0
0
1
0
1
3
1
0
0
0
2
0
1
0
0
1
0
0
1
0
0
0
0
0
1
8 bit CRC is used in all frame heads with the following generator polynomial (GP):
x8 + x7 + x4 + x3 + x1 + x0
16 bit CRC is used at the end of sized data messages of up to 16 bytes with the following GP:
x16 + x12 + x5 + x0
32 bit CRC is used at the end of sized data messages of over 16 bytes with the following GP:
x32 + x31 + x30 + x28 + x27 + x25 + x24 + x22 + x21 + x20 + x16 + x10 + x9 + x6 + x0
The scrambler uses a 16 bit pseudo random number generator with the following feed back taps:
x16 + x14 + x1 + x0
2004 CML Microsystems Plc
57
D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
