Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
1.5.1.7
Fast Serial Bus Operation
The Fast Serial Bus is provided to maximise throughput when transferring data between the CMX880 and an
external DSP. It is based on the industry standard three wire serial interface, to allow communication with
standard DSP ICs using a minimum of external components. This interface is capable of full duplex operation,
so that data can be transferred to and from a DSP concurrently. This facilitates transferring data at different
word rates corresponding (for example) the received C4FM symbol rate of 4800 symbols/s to the DSP and the
digitised voice sample rate to the audio output path at 8000S/s from the DSP.
The Fast Serial Bus comprises 5 signals
1. FSB_CLOCK
[output from the CMX880]
[output from the CMX880]
[output from the CMX880]
[output from the CMX880]
[input to the CMX880]
2. FSB_SYNC_OUT
3. FSB_DATA_OUT
4. FSB_SYNC_IN
5. FSB_DATA_IN
Data is transferred from the CMX880 to the DSP over the FSB_DATA_OUT port. Data is transferred from the
DSP to the CMX880 over the FSB_DATA_IN port. The FSB_CLOCK is driven by the CMX880 and is common
for both ports. The data transfer protocol is shown in Figure 20. Data is transferred over the Fast Serial Bus as
16-bit words. The Sync signals flag the start of a transfer. The most significant bit of the data to be transferred
follows on the next clock cycle after the Sync pulse. If the sync pulse is on FSB_SYNC_OUT, the data will be
transferred from the CMX880 to the DSP on FSB_DATA_OUT. If the sync pulse is on FSB_SYNC_IN, the data
will be transferred from the DSP to the CMX880 on FSB_DATA_IN.
FSB CLOCK
S Y N C
B i t 1 5
M S B
B i t 1 4
Bit 1
Bit 0
L S B
D A T A
Figure 20 Fast Serial Bus Data Transfer Protocol
Two 16-bit registers form part of the CMX880 Fast Serial Bus interface; a serial to parallel register (RI) for data
transfers from the DSP to the CMX880 and a parallel to serial register (RO) for transferring data from the
CMX880 to the DSP.
The CMX880 takes data from RI to be processed for the output path, either to drive the modulator outputs or the
audio outputs. Whenever RI is read by the device a new FSB In transfer is initiated. Thus the input word
transfer rate is determined by the type of signal being processed (voice, C4FM symbol or FFSK bit stream); see
Table 17.
Processed input path data, either from the FM Discriminator input (FM_DISC) or from one of the Microphone
inputs (MIC_1 or MIC_2), is placed in RO by the CMX880 when it is available. Whenever RO is loaded by the
device a new FSB Out transfer is initiated. Thus the output word transfer rate is determined by the type of signal
being processed (voice, C4FM symbol or FFSK bit stream).
ã 2001 Consumer Microcircuits Limited
40
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
