Digital PMR Radio Processor
CMX7131/CMX7141
In common with other FIs developed for the CMX7131/CMX7141, this device is equipped with two signal
processing paths. However, in this implementation of the FI, Input 2 is ONLY used in SPI-Codec mode for
the Tx audio input signal. Input 1 should be routed to one other of the three input sources (ALT, DISC or
MIC) which should be connected to the radio’s discriminator output. The internal signals Output 1 and 2
are used to provide either 2-point or I/Q signals and should be routed to the MOD1 and MOD2 pins as
required. In I/Q mode Input 1 should be routed to the DISC input source for the I channel input and Input 2
should be routed to the ALT input source for the Q channel input.
It is important to correctly attach the signal from the CMX994 I/Q outputs to the CMX7141 DISC and ALT
inputs. Crossing these connections will cause the CMX7141 dc offset calibration to fail, as attempted
corrections to the I signal will be made to the Q signal and vice versa. Crossed connections can be
swapped using the Input Gain and Signal Routing register ($B1:b5-2). Likewise, it is important that the
sense of connection is correct between the CMX994 and CMX7141. If the input signals are inverted then
attempts by the CMX7141 to remove the dc offset will, in fact, increase the dc offset. The inputs may be
inverted by using the ‘Input Invert’ bit in the Analogue Output Gain register ($B0:b7). When demodulating
the received signal (internally to the CMX7141), it is possible that the signal could be inverted, resulting in
no framesync detection and, in effect, inverted data. Often this can be corrected by swapping the I and Q
signals (changing the signal that leads in phase to the one that lags). However, the relationship between
I/Q outputs of the CMX994 and the CMX7141 DISC and ALT inputs must be maintained as described
above. Therefore the demodulated signal can be inverted using Programming Register block 0, P0.10 bit1
(IFD).
6.5.4 Modem Control
The CMX7131/CMX7141 operates in one of these operational modes:
o
o
o
o
o
o
Idle
Rx
Tx
CMX994 Pass-through
Rx with CMX994 I/Q Cal.
Rx with Powersave.
At power-on or following a Reset, the device will automatically enter Idle mode, which allows for maximum
powersaving whilst still retaining the capability of monitoring the AuxADC inputs (if enabled).
It is only possible to write to the Programming register whilst in Idle mode.
See:
o
Modem Control - $C1 write
The RXENA and TXENA pins (GPIO1 and GPIO2) reflect bits 0 and 1 of the Modem Control register, as
shown in Table 12. These can be used to drive external hardware without the host having to intervene.
There are also two additional GPIO pins that are programmable under host control.
2014 CML Microsystems Plc
Page 38
D/7141_FI-3.x/6
CMLMICRO [ CML MICROCIRCUITS ]
