FRS Signalling Processor
CMX882
It is possible (although unlikely) that a fade will exactly coincide and obliterate 2 lots of 4 tone sequence
indicating an EOM and the start of a new message. In this case, the host could misinterpret the received
signal as a long fade and enable the voice when the maintenance tone reappears. It is therefore
recommended that the host operates a timer that is started on loss of maintenance tone. If this times out
the host can then assume that the fade is long enough that the original call is lost or has become so
corrupted that it is not worth continuing with. The host could then choose only to restore the audio path
on the next occurrence of a valid XTCSS tone set. Note that the XTCSS detector operates independently
and the host may enable or disable the audio path at any time.
1.5.4
Transmit Mode
The device operates in half duplex, so when the device is in transmit mode the receive path
(discriminator and audio output amplifiers) should be disabled, and can be powered down, by the host
µC.
Two modulator outputs with independently programmable gains are provided to facilitate single or two-
point modulation, separate sub-audio and voice band outputs. If one of the modulator outputs is not used
it can be disabled to conserve power.
To avoid erroneous transmission of out of band frequencies when changing from Rx to Tx the MOD_1
and MOD_2 outputs are ramped to the quiescent modulator output level, VBIAS before switching.
Similarly, when starting a transmission, the transmitted signal strength is ramped up from the quiescent
VBIAS level and when ending a transmission the transmitted signal strength is ramped down to the
quiescent VBIAS level. The ramp rates are set in the Programming register P4.6. When the modulator
outputs are disabled, their outputs will be set to VBIAS. When the modulator output drivers are powered
down, their outputs will be floating (high impedance), so the RF modulator will need to be turned off.
Table 7 Concurrent Tx Modes Supported by the CMX882
Sub-Audio
Voice band
CTCSS
CTCSS
CTCSS
CTCSS
CTCCS^
DCS
+
+
+
+
Voice
In-band tone
FFSK/MSK
XTCSS
DCS
+
+
+
Voice
DCS
In-band tone
FFSK/MSK
Voice
DCS
In-band
FFSK/MSK
XTCSS
^ Special subaudio tone only
For all transmissions the host must only enable signals after the appropriate data and settings for those
signals are loaded into the C-BUS registers. As soon as any signalling is enabled the CMX882 will use
the settings to control the way information is transmitted.
A programmable gain stage in the microphone input path facilitates a host controlled VOGAD capability.
2004 CML Microsystems Plc
25
D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
