AX.25 Modem
CMX7031/CMX7041
Typical stages of Rx operation are depicted in Figure 11 and occur as follows:
1. The host detects the Sync detect IRQ becoming active, at which point the device will start to
transfer received data bits to the internal buffer and transfer the first four words of data to the C-
BUS RxData registers
2. The host detects the RxDataRDY IRQ becoming active
3. The host reads 4 words from the RxData C-BUS registers
4. Steps 2 and 3 can be repeated until the host detects the end of the data burst
5. The host writes to the Mode Control register to either:
a. Disable Rx mode, at which point RxENA will become inactive and the demodulator, NRZI,
synch detect blocks will become inactive
b. Write to the Mode Control register with the mode bits set for Rx, which will cause the
modem to re-start its sync detect process
Note that the demodulator will continue to deliver data until the host shuts it down, even if the signal at the
input is no longer valid.
Modem Command Register
RxENA
1200 Sync Detect
C-BUS
registers
Sync
1200bps
Detect
Demodulator
Rx Data
Buffer
(128 words)
B
9
B
8
A
A
A
9
NRZI
De-
9600bps
scramble
Demodulator
Data Ready
Sync
Detect
9600 Sync Detect
Figure 11 Rx Operation
7.6.4 1200bps AFSK Modem
The 1200bps AFSK encoding operates in accordance with the bit settings in the Modem Command
register ($C1). NRZI encoding is applied to all transmitted bits.
The CMX7031/CMX7041FI-4.x generates its own internal data clock and converts the binary data into the
appropriately phased frequencies, as shown in Figure 12 and Table 5.
2013 CML Microsystems Plc
Page 26
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