Digital PMR Radio Processor
CMX7131/CMX7141
6.6
NXDN Mode Operation
The CMX7131/CMX7141 performs all frame building/splitting and FEC coding/decoding, which relieves
the host controller of a significant processing load. During voice calls voice is transferred to and from the
vocoder under host control. In Rx mode the CMX7131/CMX7141 monitors the incoming control channel
fields and will only accept calls if the programmed address requirements are satisfied. This allows the host
to remain in a power-down or “sleep” state until it is really necessary to wake up, enhancing the battery life
of the final product design.
6.6.1 Frame Format
NXDN calls contain an optional preamble sequence (controlled by $C7:b11) followed by a continuous
stream of 384-bit (80ms) frames. Three frame formats are defined by the standard: “Audio
Communications Format”, “Data Communications Format” and “Data Communications Burst Format”.
All frames start with a 20-bit Sync Word followed by a 16-bit Link Information Channel (LICH) block. The
LICH fields indicate the frame format and payload type, and the CMX7131/CMX7141 uses this information
to determine how to process each frame in both Tx and Rx. The “Communications mode identification”
field sets the frame format and the “Option” field sets the payload field type (TCH, FACCH1, FACCH2,
UDCH1).
In an “Audio Communications Format” call the 60-bit Slow Associated Control Channel (SACCH) block is
present in all frames. The “Start” and “End” frames carry two 144-bit “Fast Associated Control Channel 1”
(FACCH1) blocks in the payload fields instead of Traffic Channel (TCH) data. The CMX7131/CMX7141
scans the “SU” and “Message Classification” fields contained in the SACCH and FACCH1 blocks and uses
these to perform address matching (in Rx mode) and to decide whether to route the following TCH data
via the host or to enable audio sample routing when in SPI-CODEC mode. The CMX7131/CMX7141 also
checks the “Message Classification” field for the “Clearing” value which indicates an “End” frame. When
this is found the CMX7131/CMX7141 terminates call processing.
Note that in an “Audio Communications Format” call the TCH bearing frames are grouped into
superframes, in which the information contained in SACCH may be split across four consecutive frames.
Dis-assembly and re-assembly of the contents must be done by the transmitting and receiving hosts; this
function is not performed by the CMX7131/CMX7141.
Note that all block sizes given above refer to “over-air” bits, some of which are FEC coded. Because the
CMX7131/CMX7141 performs all FEC coding functions, the block sizes of data transfers between the
CMX7131/CMX7141 and host are smaller in most cases except for uncoded TCH blocks.
Table 15 RxData 0/TxData 0 Block ID settings
RxData 0
TxData 0 Block ID
b5-4
00
01
10
11
reserved
Payload Data
LICH
SACCH and LICH
2014 CML Microsystems Plc
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