Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Associated Control and Status
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Power down control
Mic 1 amp
Mic 2 amp
FM Disc
(inputs)
SD ADC
SD DAC 1
Mod 2 amp
Signal processing block
SD DAC 2
Audio amp
Xtal oscillator
(converters)
(outputs)
Mod 1 amp
RSSI monitor
Bias
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Power down status
1.5.1.2
RSSI Monitoring
The RSSI monitoring facility comprises an 8-bit ADC, a comparator, an 8-bit result data word and two 8-bit
threshold registers, one defining the ‘RSSI high’ level and the other the ‘RSSI low’ level. The two threshold
registers are combined into one 16-bit “C-BUS” register word. The ADC measures the RSSI signal at intervals
that are set by “C-BUS” command. It is advised that the interval be set to < 120µs while waiting for a new
incoming signal, so that the CMX880 and the DSP can be powered up and put into receive mode in time to avoid
missing any message data. The default interval period following a reset is 12.5µs. Power dissipation of the RSSI
monitor can be reduced by increasing the conversion interval. RSSI monitoring should be disabled during
transmit.
The result of the most recent RSSI measurement can be read by “C-BUS” command at any time.
The RSSI monitor continuously compares the conversion results with the values in the ‘RSSI high’ or the ‘RSSI
low’ threshold registers. The CMX880 issues an interrupt to the host µC to wake up the receive path when the
RSSI signal exceeds the ‘high’ level threshold. The CMX880 issues an interrupt to the host µC to indicate a
weak or absent received signal when it falls below the ‘low’ level threshold. The value in the ‘low’ register must
be less than that of the ‘high’ register. This provides a user programmable hysteresis facility. The options for
issuing interrupts and for automatic start-up are selected by “C-BUS” command.
The CMX880 can be programmed to wake up its receive path automatically (automatic start-up) when the RSSI
signal exceeds the ‘high’ level threshold. While the CMX880 is in automatic start-up mode the FM Discriminator
must be selected as the input signal path. When not in automatic start-up mode, it is recommended that the FM
Discriminator be selected as the input signal during RSSI monitoring (in anticipation of receiving a message), to
avoid subsequent switching of the input signal source.
The RSSI Monitoring options are controlled by the $B2, $B3 and $C0 “C-BUS” registers.
Associated Control and Status
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Enable/disable RSSI monitoring.
Read the most recent RSSI level measurement.
Select two level monitoring select.
Enable “Rx Wake Up” mode - used in conjunction with two level monitoring mode to automatically wake up
the device’s powered down receive path and identify the type of the incoming signal when the RSSI exceeds
the ‘high’ threshold. Other paths are woken by mC command, as needed.
High threshold level register; Low threshold level register.
ADC measurement data/result.
Interrupts: mask/status - (1) RSSI risen above level, (2) RSSI fallen below level.
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ã 2001 Consumer Microcircuits Limited
12
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
