HD404618 Datasheet, PDF (62/89 Pages) Hitachi Semiconductor – 4-Bit Single-Chip Microcomputer

HD404618 Series
DTMF Generation Circuit
The MCU has a dual-tone multifrequency (DTMF) generation circuit.
The DTMF signal consists of two sine waves to access the switching system.
Figure 39 shows the DTMF keypad and frequencies. Pressing a key generates a tone corresponding to its
frequency. Figure 40 shows a block diagram of the DTMF circuit.
The MCU uses an oscillation frequency reduced to 400 kHz, an eighth of the conventionally used
frequency, for low-power consumption. This, however, causes a potential frequency deviation. The MCU
provides transformed programmable dividers in addition to sine wave counters and a control register to
reduce frequency deviation.
The DTMF generation circuit is controlled by the following three registers.
1
2
3
A
R1 (697 Hz)
4
5
6
B
7
8
9
C
*
0
#
D
R2 (770 Hz)
R3 (852 Hz)
R4 (941 Hz)
Figure 39 DTMF Keypad and Frequencies
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