CMX882 View Datasheet(PDF) - CML Microsystems Plc

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CMX882

FRS Signalling Processor

CML Microsystems Plc 

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 + Voice

CTCSS + In-band tone

CTCSS + FFSK/MSK

CTCCS^ + XTCSS

DCS

DCS + Voice

DCS + In-band tone

DCS + FFSK/MSK

Voice

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

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