PMR Signalling Processor
CMX881
Bits 11 to 9 determine the sub-audio transmission / reception signalling:
Bit 11
Bit 10
Bit 9
Tx - Transmitted signal:
Rx - Monitored signal(s):
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
No Sub-Audio Transmitted
Inverted DCS*
DCS
No Sub-audio Monitoring
Inverted DCS*
DCS
Do not use
DCS + inv DCS*
CTCSS
CTCSS
Do not use
CTCSS + inv DCS*
CTCSS + DCS
CTCSS + DCS + inv DCS*
Do not use
Do not use
* See Table 3 DCS Modulation Modes.
Bit 12 enables Audio tone generation (see section 1.6.14). This operates in transmit and receive modes.
In transmit mode this bit will only enable the Audio Generator when no other voice band signals are being
transmitted i.e. bits 14, 13, 4 and 3 set to '0'.
Bits 14 and 13 determine the voice band tone transmission and reception. When transmitting or
receiving audio band signals the voice path must be disabled by clearing ‘Voice Enable’ bit 15 to '0'.
Bit 14
Bit 13
Tx - Transmitted signal
Rx - Monitored signal
0
0
1
1
0
1
0
1
No voice band tone transmitted
No voice band tones monitored
Selcall
DTMF
Selcall
Reserved
Reserved
Reserved
When set to '1', bit 15 enables the voice path. In transmit mode the selected audio input is routed to the
modulator outputs. In receive mode the voice processing path is enabled to the audio output. In transmit
mode bit 15, if set to '1', will be temporarily disabled (cleared to '0') whenever any of the bits 3, 4, 12, 13
and 14 are set to '1'. In receive mode bit 15, if set to '1', will be temporarily disabled (cleared to '0')
whenever bit 12 is set to '1'. It is up to the host µC to control bit 15 when voice band signals are
received.
The Mode Control register ($C1) may be written to at any time (subject to C-BUS timing restrictions). If
the enable bit of the currently decoded signal is disabled whilst in phase 2 the CMX881 will return to
phase 1 for that band. If the same signal needs to be searched for again then the appropriate bit needs
to be set back to ‘1’ in $C1. However, to de-emphasise Selcall tones, bit 15 must be set to ‘1’.
The CMX881 will only detect signals when their amplitude is above the threshold set for each band (sub-
audio and voice), as set in the program registers. Therefore even if valid tones or signals are present the
CMX881 will ignore them unless they exceed the detect threshold. Time and level hysteresis is applied
to reduce chattering in marginal conditions.
Detection strategies used by the CMX881 whilst in receive mode:
When in receive mode the CMX881 treats the received signal in two bands; Sub-audio (60-260Hz) and
voice band (300-3kHz). For the sub-audio the CMX881 can monitor and decode CTCSS and DCS
signals in parallel. Because certain FFSK bit patterns can mimic some Selcall tones the Selcall receiver
is temporarily disabled when an FFSK frame sync is detected. The host must monitor the received data
and restore Selcall (by setting bits 14 and 13 as required) when it has detected the end of data.
2004 CML Microsystems Plc
35
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
