PMR Signalling Processor
CMX881
1.5.1
Sleep Mode and Auto Start Up
Power-on reset or C-BUS general reset places the CMX881 into sleep mode, which results in all internal
blocks, except the xtal clock circuit, being placed in power-saved mode. The xtal clock circuit can be
power-saved but this must be done by an explicit C-BUS command. Power saving is achieved by turning
off bias current sources or disabling local clocks, as appropriate.
During system standby periods, parts of the device can be put into sleep mode by the host to conserve
power. The Auxiliary ADC can be programmed so that when the level exceeds a threshold, an interrupt
is issued over the C-BUS and the selected mode (Tx or Rx) “woken up” within 400µs. If this time is too
long to ensure no part of the signal is lost, the DISC or MIC input and ADC path can be kept powered up
whilst in standby mode. The receive modes and transmit modes can also be activated by commands
from the C-BUS. On wake up, activation of the various signal path stages are phased appropriately to
avoid causing unwanted transients. More details are provided in section 1.6.4 on Signal Routing.
The CMX881 can be programmed to wake up its receive path automatically (automatic start-up) when the
DISC input level exceeds the ‘high’ level threshold. While the CMX881 is in automatic receive start-up
mode the DISC input must also be selected for the signal path. When not in automatic start-up mode it is
recommended that the required input is selected during Auxiliary ADC operation to avoid subsequent
switching of the input signal source.
1.5.2
Auxiliary ADC
This section of the CMX881 operates in both Tx and Rx modes and can be used to monitor one of 4
signal sources: Sig_Monitor pin, MIC1, Input_2 or DISC inputs. Activity on the selected input will
optionally issue an interrupt if host intervention is required. During idle periods the majority of the
CMX881 can be placed into low power mode. If monitoring ac signals connected to the Sig_Monitor pin
they must be rectified and filtered using passive external circuitry.
The Auxiliary ADC facility comprises an 8-bit ADC, a comparator, an 8-bit result data word and two 8-bit
threshold registers, one defining the ‘Signal high’ level and the other the ‘Signal low’ level. The two
threshold registers are combined into one 16-bit C-BUS register word. The ADC measures the Signal
level at intervals that are set by C-BUS command.
It is advised that the interval be set to <125µs while waiting for a new incoming signal so that the
CMX881 and host µC can be powered up and put into the correct mode in time to avoid missing any part
of the signal. The default interval period following a reset is 20.8µs. Power dissipation of the Auxiliary
ADC can be reduced by increasing the conversion interval time.
The result of the most recent Auxiliary ADC measurement can be read over the C-BUS whenever the
Signal Processing and Aux ADC circuits are powered up.
The Auxiliary ADC compares each conversion result with the values in the ‘Signal high’ or the ‘Signal low’
threshold registers. The CMX881 can, for example, issue an interrupt to the host µC to wake up the
receive path when the Auxiliary ADC input exceeds the ‘high’ level threshold. The CMX881 can also
issue an interrupt to the host µC to indicate a weak or absent signal when it falls below the ‘low’ level
threshold. This provides a user programmable hysteresis facility. The host must ensure that the value in
the ‘low’ register is always less than that of the ‘high’ register. The options for issuing interrupts and for
automatic start-up are selected by C-BUS command.
The Auxiliary ADC options are controlled by the $B2, $B3 and $C0 C-BUS registers.
The Auxiliary ADC requires the Auxiliary ADC, BIAS and Xtal clock to all be enabled in the Power Down
Control register.
2004 CML Microsystems Plc
14
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
