HD404654 Datasheet, PDF (78/109 Pages) Hitachi Semiconductor – 4-Bit Single-Chip Microcomputer

HD404654 Series
DTMF Generator Circuit
The MCU provides a dual-tone multifrequency (DTMF) generator circuit. The DTMF signal consists of
two sine waves to access the switching system.
Figure 63 shows the DTMF keypad and frequencies. Each key enables tones to be generated corresponding
to each frequency. Figure 64 shows a block diagram of the DTMF circuit.
The OSC clock (400 kHz, 800 kHz, 2 MHz, 3.58 MHz or 4 MHz) is changed into four clock signals
through the division circuit (1/2, 1/5, 1/9 and 1/10). The DTMF circuit uses one of the four clock signals,
which is selected by system clock select register 1 (SSR1: $029) and system clock select register 2 (SSR2:
$02A) depending on the OSC clock frequency. The DTMF circuit has transformed programmable dividers,
sine wave counters, and control registers.
The DTMF generation circuit is controlled by the following three registers.
1
2
3
A
R1 (697 Hz)
4
5
6
B
R2 (770 Hz)
7
8
9
C
R3 (852 Hz)
0
#
D
R4 (941 Hz)
Figure 63 DTMF Keypad and Frequencies
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