Digital PMR Radio Processor
CMX7131/CMX7141
CMX7131/CMX7141 can be configured to use its auxiliary C-BUS as an SPI interface and use its built-in
DAC/ADC’s as audio converters as shown in Figure 7.
MOD
Radio
DISC
CMX7131/CMX7141
Host CPU
MIC
C-BUS
MIC
Input2
Speaker
Output1
AUDIO
SPI
EPCSN
SSOUT
EEPROM
SPI
SPI
Serial
SPI
UART
SPI-Codec_ENA = 1
VOC_DIS = 0
AMBE+2
Vocoder
Figure 7 NXDN Vocoder Connection
Table 3 SPI-Codec Format
SPI
EPSCLK
EPSO
EPSI
SSOUT
Note: There are 16 SCLK pulses per data transfer. The default SCLK rate is 2MHz.
5.2.2 Data Transfer
When transmitting, an initial block of payload or control channel data will need to be loaded from the host
into the C-BUS TxData registers. The CMX7131/CMX7141 can then format and transmit that data while at
the same time loading in the following data blocks from the host or vocoder.
When receiving, the host needs to consider that when a signal is received over the air there will be a
processing delay while the CMX7131/CMX7141 filters, demodulates and decodes the output data before
presenting it to the host. For best performance voice payload data can be output in soft-decision (4-bit log-
likelihood ratio) format although this mode increases the data transfer rate over C-BUS by a factor of four.
5.2.3 RSSI Measurement (LD Mode only)
The AuxADC provided by the CMX7131/CMX7141 can be used to detect the squelch or RSSI signal from
the RF section while the device is in Rx or Idle mode. This allows a significant degree of powersaving
within the CMX7131/CMX7141 and avoids the need to wake the host up unnecessarily. The host
programmable AuxADC thresholds allow for user selection of squelch threshold settings.
2014 CML Microsystems Plc
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