AIS Baseband IC with/without RF Synthesiser
CMX7032/CMX7042
Rx Timing
Received Burst
Time
Training
RSSI1
RSSI2
Wait
Sleep (if no burst received)
sequence
Measurement
search
Wake up in time for
next slotclock
Figure 10 Rx Burst Timing
7.4.8 CSTDMA Threshold Measurement
The CMX7032/CMX7042 provides support for the CSTDMA mode of operation by measuring the signal
level at the start of a slot period and aborting the transmission if a level over a user defined threshold is
found. This fact is reported to the host in Status2 C-BUS register $C5.
The CSTDMA threshold measurement window is setup by the Tx_Sequence Config task using timing
values based on the number of 24kHz “tick” increments from the rising edge of the SLOTCLK pulse.
During this period (if Tx_Status and CS-EN are both “active”) the CMX7032/CMX7042 will measure the
RSSI level of the appropriate channel (set in C-BUS Mode register $C1):
1. sample RSSI at 48k samples/sec
2. scale values to 0 - 127
3. apply the look-up table values
4. apply the CSTDMA_gain value
5. accumulate values over the defined period
6. compare the value with the value in the C-BUS CS-Threshold register $C2 and abort the
transmission if it is exceeded.
Note that the same lookup table is used for both CSTDMA Threshold measurement as the RSSI
measurement.
7.4.9 RSSI Measurement
RSSI can not be determined directly from the baseband signal output of a limiter-discriminator device so
the CMX7032/CMX7042 has two dedicated inputs (one for each Rx channel) for separate external
analogue RSSI signals. Suitable outputs are available on many limiter-discriminator ICs, e.g. Philips
SA605. The ADCs sample the signals during a user-defined window and apply an averaging algorithm and
present the results in C-BUS registers $BA and $BB.
The RSSI measurement windows are set up using the RSSI_Window config task using timing values
based on the number of 24kHz “tick” increments from the rising edge of the SLOTCLK plulse. During this
period (if Tx_Status is not “active”) the device will:
1. sample RSSI at 48k samples/sec
2. scale values to 0 -127
3. apply the look-up table values
2012 CML Microsystems Plc
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D/7032/42_FI1.2/13
CMLMICRO [ CML MICROCIRCUITS ]
