Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
$C8 (B1.7)
RAMP RATE CONTROL
Bit:
15
0
14
0
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Ramp UP Rate Control (RRU)
[Range: 0 to 127]
Ramp DOWN Rate Control (RRD)
[Range: 0 to 127]
The ramp-up rate and ramp-down rates can be independently programmed. The ramp rates apply to all the
analogue output ports. They only affect those ports being turned on (ramp-up) or turned off (ramp down). The
ramp rates should be programmed before ramping any outputs.
Ramp-up rate
Ramp-down rate
= 0.833ms/LSB
= 0.833ms/LSB
Time to ramp-up to full gain = (RRU ´ 0.833)ms
Time to ramp down to zero gain = (RRD ´ 0.833)ms
Bit 15 and bit 14 must be cleared to 0.
$C8 (B1.8) RX C4FM UNIT LEVEL
Bit:
15
0
14
0
13
0
12
11
10
9
8
7
6
5
4
3
2
1
0
Unit Level (unsigned integer)
The CMX880 derives the 4 C4FM symbol levels from the unit level programmed into the ‘RX C4FM UNIT LEVEL’
register. These levels are used to decode received C4FM signals and to monitor signal quality.
Symbol level
+3
+1
- 1
- 3
Unit level ´ +3
Unit level ´ +1
Unit level ´ - 1
Unit level ´ - 3
The value programmed into the ‘RX C4FM UNIT LEVEL’ register is derived using the formula:
65336 ´ ‘Unit Level’ (V) / VDD
For optimum performance it is recommended that the FM discriminator be set up so that the value programmed
in the ‘RX C4FM UNIT LEVEL’ register is close to 2048 and the p-p signal level for a repeated symbol sequence
of “+3, +3, - 3, - 3, +3, +3, - 3, - 3 …” is less than VDD, with minimal distortion.
$C8 (B1.9)
RX C4FM FRAME SYNC THRESHOLD
Bit:
15
0
14
0
13
12
11
10
0
9
0
8
0
7
6
5
4
3
2
1
0
FS Tol
Tol En
Frame Sync Threshold
This register has no affect in transmit mode. In receive mode, this register defines the accuracy with which the
CMX880 identifies a C4FM signal and the Frame Sync pattern matching tolerance.
The Frame Sync Threshold defines the level that the cross-correlation of signal with the frame sync pattern must
exceed before the CMX880 will detect a C4FM FS. The resultant FS cross-correlation peak value is made
available in the FS Cross-Correlation Peak register. The value to be programmed into the ‘RX C4FM Frame
Sync Threshold’ field is obtained using the formula:
‘Frame Sync Threshold’ ´ 2
[range 0 to255]
The maximum possible cross-correlation peak is 72 ‘units’, which corresponds to a programmed value of 144.
The cross-correlation level achieved is affected by internal rounding and by the SNR, as shown in Figure 21. It is
advised that the level be set to at least 5 (programmed value 10) less than the nominal level shown in Figure 21
for the minimum SNR in which the system will be expected to operate.
Tol En (bit 11) enables the tolerance checking facility, but does not affect availability of the C4FM FS Tolerance
status. Frame Sync Tolerance (bits 13,12) defines the maximum number of mismatches allowed during a
search for the Frame Sync pattern. The FS Mismatch count is made available in the ‘STATUS’ register.
ã 2001 Consumer Microcircuits Limited
60
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
