Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
1.5.1.6
“C-BUS” Operation
Instructions, status and data are transferred between the CMX880 and the mC over the “C-BUS”. The
“C-BUS” protocol, shown in Figure 19, complies with CML’s “C-BUS” Hardware Interface specification.
Instruction and data transfers to and from the CMX880 consist of an Address/Command (A/C) byte followed by
either:
(i) a further instruction,
(ii) 1 or 2 bytes of data (write) or
(iii)1 or 2 bytes of status or received data reply (read).
The number of data bytes following an A/C byte is dependent on the value of the A/C byte. The most significant
bit of the address or data are sent first. The “C-BUS” SERIAL_CLOCK input to the CMX880 originates from the
µC.
CSN
SERIAL_CLOCK
7 6 5 4 3 2 1 0
MSB LSB
CMD_DATA
7 6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
First Data Byte
Last Data Byte
Address/Command
Byte
REPLY_DATA
7 6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
MSB
LSB
First Reply Data Byte
Last Reply Data Byte
Logic level is not important
Figure 19 “C-BUS” Protocol
Signal data must be transferred at the rate appropriate to the signal type. The CMX880 buffers signal data in
16-bit word registers, one for data from the CMX880 (REPLY_DATA) and one for data from the host mC
(CMD_DATA). All signal data is transferred as 16-bit words. If the “C-BUS” is being used to transfer signal
data, the CMX880 will issue interrupts to indicate when data is available or required. The mC must respond to
these interrupts within the maximum allowable latency for the signal type. Table 17 shows the maximum
latencies for transferring signal data over the “C-BUS”, to maintain appropriate data throughput.
Table 17 Maximum Data Transfer Latency
Signal type
Max time to replenish
16-bit data buffer
125µs
Effective data
size
1 word/buffer
Baseband rate
Analogue voice band
(from Mic or to Audio)
8000 words/s
C4FM/CQPSK
FFSK - 1200baud
FFSK - 2400baud
1.66ms
13.2ms
6.6ms
8 symbols/buffer 4800 symbols/s
16 bits/buffer
16 bits/buffer
1200 bits/s
2400 bits/s
ã 2001 Consumer Microcircuits Limited
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D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
