Wireless Modem Data Pump
FX909A
Control Register B7, B6: CKDIV - Clock Division Ratio and
B5: HILON - Xtal Range Selection
These bits control a frequency divider driven from the clock signal present at the XTALN pin, and
hence determine the nominal bit rate. The table below shows how bit rates of 4000/8000/16000 or
4800/9600/19200 bits/sec may be obtained from common Xtal frequencies:
B5
Xtal/Clock Frequency (MHz)
8.192
9.8304
4.9152
4.096
(12.288/3)
2.048
4.9152
2.048
(6.144/3)
1.024
2.4576
(12.288/5)
1.2288
1
4.096
2.4576
0
(12.288/3)
(6.144/3)
(12.288/5)
Division Ratio:
XtalFrequency
DataRate
B7 B6
Data Rate (bits per second)
0
0
1
1
0
1
0
1
256
512
1024
2048
128
256
512
16000
8000
4000
19200
9600
4800
8000
4000
9600
4800
16000
8000
4000
19200
9600
4800
1024
Note: Device operation is not guaranteed below 4000 or above 19200 bits/sec.
The values used for C3 and C4 should be suitable for the frequency of the crystal X1. As a guide;
C3 = C4 = 33pF for X1 < 5MHz, and C3 = C4 = 18pF for X1 > 5MHz.
Control Register B4: DARA - Data Rate
This bit operates in both transmit and receive modes, optimising the modem's internal signal filtering
according to the relevent bit rate.
If the bit rate used is above 10k bits/sec this bit should be set to '1' else it should be set to '0'.
Control Register B3, B2: LEVRES - Level Measurement Response Time
These two bits have no effect in transmit mode.
In receive mode, they set the 'normal' response time of the Rx signal amplitude and dc offset
measuring circuits. This setting will be temporarily overridden by the automatic sequencing of an
AQLEV command.
B3 B2
0
0
1
1
0
1
0
1
Hold
Keep current values of amplitude and offset
Track input signal using bit peak averaging
Track input signal using peak detect
Peak Averaging
Peak Detect
Lossy Peak Detect
Track input signal using lossy peak detection
For Mobitex systems, and most general purpose applications using the modem, these bits should
normally be set to 'Peak Averaging', except when the µC detects a receive signal fade, when 'Hold'
should be selected.
ã 1996 Consumer Microcircuits Limited
23
D/909A/4
CMLMICRO [ CML MICROCIRCUITS ]
