AX.25 Modem
CMX7031/CMX7041
Table 3 DTMF Tone Pairs
Tone Code
(binary)
Low Tone
(Hz)
High Tone
Tone Code (Hex)
Key Pad Position
(Hz)
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
00000
1xxxx
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
00
1x
1
2
3
4
5
6
7
8
9
0
*
#
A
B
C
D
null
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
941
-
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
1633
-
7.6
AFSK/GMSK Data Modem
The CMX7031/CMX7041 supports both 1200bps AFSK and 9600bps GMSK data modes. In Rx mode, the
device can be set to look for either of the AFSK or GMSK modes, however, once a valid mode has been
found, it will stay in that mode until the host resets it.
See:
o
Mode Control – $C1 write
7.6.1 Receiving AFSK/GMSK Signals
The CMX7031/CMX7041 can decode incoming AFSK/GMSK signals at either 1200 or 9600 baud data
rates, automatically detecting the rate from the received signal. The form of AFSK/GMSK signals for these
baud rates is shown in Figure 12.
In 1200bps AFSK mode, the received signal is filtered and data is extracted with the aid of a PLL to
recover the clock from the serial data stream. The bit clock is not output externally.
The extracted data is compared with the 32-bit sync pattern which corresponds to either $0000007E or
$7E7E7E7E (both are required to ensure compatibility with existing devices already in service) and then
NRZI decoded. An interrupt will be flagged when the sync pattern is detected. The host µC may stop the
sync search by disabling the AFSK demodulator. Once a valid sync pattern has been detected, the sync
search algorithm is disabled; it may be re-started by the host re-writing to the Mode register ($C1:b4,5)
with the modem and mode bits set appropriately. The recovered data is NRZI decoded and is held in an
256-byte internal buffer, from where it can be read by the host over C-BUS using the RxData block. A
DataRDY flag will be raised whenever there is data available in the buffer. Data is transferred over the C-
BUS under host µC control. The host should ensure that the data is transferred at an adequate rate
following data ready being flagged.
The host µC must keep track of the message length, or otherwise determine the end of reception, and
disable the demodulator at the appropriate time.
In 9600bps GMSK mode, the received signal is fed through a Gaussian filter with a Bt of 0.5. The device
extracts timing and level information from the 24-bit sync pattern. Once a valid sync pattern has been
detected, the extracted data is de-scrambled and NRZI decoded in a similar manner to the 1200bps mode.
2013 CML Microsystems Plc
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