PMR Signalling Processor
CMX881
1.5
General Description
The CMX881 is intended for use in half duplex analogue two way land mobile radio (LMR) equipment and
is particularly suited to multi standard PMR systems. The CMX881 provides radio signal encoder and
decoder functions for: Voice, Selcall, Tx DTMF, CTCSS, DCS and FFSK/MSK data permitting simple to
sophisticated levels of tone control and data transfer. Power control facilities allow the device to be
placed in varying levels of sleep allowing the user to fine tune the power depending on system
requirements. The CMX881 includes a crystal clock generator, with buffered output, to provide a
common system clock if required. A block diagram of the CMX881 is shown in Figure 1.
Tx functions
Audio
o
o
o
o
Single/dual microphone inputs with input amplifier and programmable gain adjustment
Filtering selectable for 12.5kHz and 25kHz channels
Selectable pre-emphasis
2-point modulation outputs with programmable level adjustment
Signalling
o
o
o
o
o
o
Pre-programmed 39 tone CTCSS encoder
Programmable 23/24bit DCS encoder
Programmable audio tone generator (for custom audio tones)
Programmable Selcall encoder
DTMF encoder
1200/2400bps MSK modulator
Rx functions
Audio
o
o
o
o
Single/dual demodulator inputs with input amplifier and programmable gain adjustment
Voice-band and sub-audio rejection filtering
Selectable de-emphasis
Software volume control
Signalling
o
o
o
o
o
1 from 39 CTCSS decoder + Tone Clone mode
23/24bit DCS decoder
Programmable Selcall decoder
1200/2400bps MSK demodulator and 16-bit frame sync detector
Signal Monitor (RSSI / Microphone / Rx channel level monitor)
Host Interface
A serial data interface (C-BUS) is used for command, status and data transfers between the CMX881 and
the host µC; this interface is compatible with microwire, SPI etc. Interrupt signals notify the host µC when
a change in status has occurred and the µC should read the status register across the C-BUS and
respond accordingly. Interrupts only occur if the appropriate mask bit has been set. See section 1.6.15.
Auxiliary (Signal Monitor) analogue signal
The CMX881 includes a Signal level monitor. This is an 8-bit successive approximation ADC and a two
level signal sensor. The two level sensor facility can be used in conjunction with the power saving mode
to wake up powered down blocks, and issue an interrupt on the IRQN line when the Signal exceeds the
preset threshold level. The auxiliary ADC voltage reference is taken directly from the VDD(A) supply, so
the Signal level being monitored should be derived from this supply voltage.
2004 CML Microsystems Plc
13
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
