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Section 11 DTMF Generator
11.1
Overview
The H8/3637 Series has an on-chip dual-tone multifrequency (DTMF) generator that can generate
DTMF signals.
A DTMF signal accesses a telephone switching system by a pair of sine waves. Figure 11.1 shows
the frequency matrix. The DTMF generator generates frequencies corresponding to the numbers
and symbols on the keypad of a telephone set or facsimile machine.
1
4
7
2
5
8
0
3
6
9
#
A
B
C
D
R1 (697 Hz)
R2 (770 Hz)
R3 (852 Hz)
R4 (941 Hz)
C1 (1,209 Hz)
C2 (1,336 Hz)
C3 (1,477 Hz)
C4 (1,633 Hz)
Figure 11.1 DTMF Frequencies
11.1.1
Features
Features of the DTMF generator are as follows.
•
Generates sine waves with DTMF frequencies from the system clock input at the OSC pins
(f
OSC
)
The OSC clock (1.2 MHz to 10 MHz, selectable in 400-kHz steps) is divided to generate a
400-kHz clock. Input to a feedback loop with a modified programmable divider and sine-wave
counter, this clock is used to generate sine waves with the DTMF frequencies.
•
Stable sine-wave output with low distortion