CMX7131 datasheet(21/69 Pages) CMLMICRO

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Digital PMR Radio Processor

CMX7131/CMX7141

 2013 CML Microsystems Plc

Page 21

D/7131/7141FI-1.x/11

6.2.3

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 CMX618.

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 or CMX618. For best performance voice payload data can be output in soft-

decision (4-bit log-likelihood ratio) format compatible with the CMX618/CMX608 and other third-party

vocoders, although this mode increases the data transfer rate over C-BUS by a factor of four.

6.2.4

RSSI Measurement

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.

6.2.5

Serial Memory Connection

In all cases, the auxiliary C-BUS/SPI-Codec bus is shared with the serial memory bus, which may be used

to load

the contents of the Function Image™. Bus conflicts are avoided by the use of an additional Chip

Select signal (SSOUT). If this feature is not used then the EPSCSN pin should be left un-connected.

6.3

Introduction

This modem can run at either 4800bps or 9600bps, occupying a 6.25kHz or a 12.5kHz bandwidth RF

channel respectively. It has been designed such that, when combined with suitable RF, host controller,

CMX618/CMX608 Vocoder and appropriate control software, it meets the requirements of the EN 301 166

or EN 300 113 standards as appropriate. See www.etsi.org for details of these standards.

TS 102 490 is available on the ETSI web site (www.etsi.org) which describes a 6.25kHz channel spacing

FDMA dPMR system. This standard uses a 4FSK modulation scheme with an over-air bit rate of 4800bps

(ie. 2400 symbols per second). With respect to dPMR formatted modes of operation, this document should

be read in conjunction with the ETSI standard.

The dPMR standard does not specify a voice coding algorithm, but the CMX618 or CMX608 (also

available from CML) are both suitable devices for this purpose. In the rest of this document these two

devices are referred to generically as the CMX6x8, as the only significant difference between them is the

inclusion of an on-chip audio codec in the CMX618 while the CMX608 requires an external Audio Codec.

Version 1.5.1 of TS 102 490 introduces two additional vocoder control bits which specify which vocoder is

in use for a particular voice call. Function Image 1.1.0.0 onwards supports these additional bits, however

care should be taken if backwards compatibility with earlier implementations is to be maintained.

Note that the TS 102 490 (dPMR) standard is NOT compatible with the TS 102 361 (DMR)

12.5kHz/9600baud TDMA system.

The 9600bps option is made available for customer-specific applications only

– this mode does not

support automated control of the CMX6x8

– in which case all data should be routed via the host.

6.3.1

Modulation

The dPMR 4FSK modulation scheme operates in a 6.25kHz channel bandwidth with a deviation index of

0.29 and has an over-air bit rate of 4800bps (2400 symbols per second). RRC filters are implemented in

both Tx and Rx with a filter “alpha” of 0.2. The maximum frequency error is +/-625Hz and the

CMX7131/CMX7141 can adapt to the maximum time-base clock drift of 2ppm over the duration of a 180-

second burst. Figure 12 shows the basic parameters of the 4FSK modulation, symbol mapping and

filtering requirements.

The 9600bps mode provided by the CMX7131/CMX7141 is essentially the same as the 4800bps mode,

but with all timings modified by a factor of two.

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