CM8888 Datasheet, PDF (8/13 Pages) California Micro Devices Corp – CMOS INTEGRATED DTMF TRANCEIVER

CALIFORNIA MICRO DEVICES
CM8888/8888-2
Guard Time Adjustment
The simple steering circuit shown in Figure 3 is adequate
for most applications. Component values are chosen
according to the formula:
tREC = tDP + tGTP
tID = tDA + tGTA
The value of tDP is a device parameter and tREC is the
minimum signal duration to be recognized by the
receiver. A value for C of 0.1 uF is recommended for
most applications, leaving R to be selected by the
designer. Different steering arrangements may be used
to select independently the guard times for tone present
(tGPT) and tone absent (tGTA). This may be necessary
to meet system specifications which place both accept
and reject limits on both tone duration and interdigital
pause. Guard Time adjustment also allows the designer
to tailor system parameters such as talk-off and noise
immunity. Increasing tREC improves talk-off performance
since it reduces the probability that tones simulated by
speech will maintain signal condition long enough to
be registered. Alternatively, a relatively short tREC with
a ling tDO would be appropriate for extremely noisy
environments where fast acquisition time and immunity
to tone drop-outs are required. Design information for
Guard Time adjustments is shown in
Figure 4.
Figure 3. Basic Steering Circuit
Call Progress Filter (CM8888)
A Call Progress (CP) Mode can be selected allowing the
detection of various tones which identify the progress of a
telephone call on the network. The Call Progress tone input
and DTMF input are common, however, call progress tones
can only be detected when the CP Mode has been selected.
DTMF signals cannot be detected if the CP Mode has been
selected (see Table 5). Figure 6. indicates the useful detect
bandwidth of the Call Progress filter. Frequencies presented
to the input (IN + and IN-) which are within the ‘accept’
bandwidth limits of the filter are hard-limited by a high-
gain comparator with the IRQ/CP pin serving as the output.
The square wave output obtained from the schmitt trigger
can be analyzed by a microprocessor or counter arrangement
to determine the nature of the Call Progress tone being
detected. Frequencies which are in the ‘reject’ area will
not be detected, and consequently there will be no activity
on IRQ/CP as a result of these frequencies.
Figure 4. Guard Time Adjustment
FLOW FHIGH DIGIT D3 D2 D3 D0
697 1209
1
0
001
697 1336
2
0 0 10
697 1477 3
770 1209
4
770 1336
5
770 1477
6
852 1209
7
852 1336
8
852 1477
9
941 1336
0
941 1209
941 1477 #
697 1633 A
770 1633
B
852 1633
C
941 1633
D
0 0 11
0
100
0
10
1
0
1 10
0
1 11
10 00
100 1
1
0
10
1
0
11
1 100
1 10 1
1 1 10
1 111
0 000
0=LOGIC LOW, 1=LOGIC HIGH
Figure 5. Function Encode/Decode
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