FSK Modem and DTMF Codec
CMX865
On powerup or after a reset, the tone pairs TA-TC are set to notone, and TD set to generate 2130Hz +
2750Hz at approximately –20dBm each.
Receive Tone Pair Programming
The programmable tone pair detector is implemented as shown in Figure 10a. The filters are 4th order IIR
sections. The frequency detectors measure the time taken for a programmable number of complete input
signal cycles, and compare this time against programmable upper and lower limits.
Figure 10a Programmable Tone Detectors
Figure 10b Filter Implementation
Programming is done by writing a sequence of twenty-seven 16-bit words to the Programming Register.
The first word should be 32769 (8001 hex), the following twenty-six 16-bit words set the frequencies and
levels and are in the range 0 to 32767 (0000-7FFF hex).
Word Value written
Word Value written
1
2
3
4
5
6
7
8
9
10
11
12
13
14
32769
Filter #1 coefficient b2
Filter #1 coefficient b1
Filter #1 coefficient b0
Filter #1 coefficient a2
Filter #1 coefficient a1
Filter #1 coefficient b2
Filter #1 coefficient b1
Filter #1 coefficient b0
Filter #1 coefficient a2
Filter #1 coefficient a1
15
16
17
18
19
20
21
22
23
24
25
26
27
Filter #2 coefficient b2
1
1
1
1
1
2
2
2
2
2
1
1
1
1
1
2
2
2
2
2
Filter #2 coefficient b1
Filter #2 coefficient b0
Filter #2 coefficient a2
Filter #2 coefficient a1
Filter #2 coefficient b2
Filter #2 coefficient b1
Filter #2 coefficient b0
Filter #2 coefficient a2
Filter #2 coefficient a1
Freq measurement #1 ncycles
Freq measurement #1 mintime
Freq measurement #1 maxtime
Freq measurement #2 ncycles
Freq measurement #2 mintime
Freq measurement #2 maxtime
© 2005 CML Microsystems Plc
30
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
