MT8880C
DTMF Generator
Data Sheet
The DTMF transmitter employed in the MT8880C is capable of generating all sixteen standard DTMF tone pairs
with low distortion and high accuracy. All frequencies are derived from an external 3.579545 MHz crystal. The
sinusoidal waveforms for the individual tones are digitally synthesized using row and column programmable
dividers and switched capacitor D/A converters. The row and column tones are mixed and filtered providing a
DTMF signal with low total harmonic distortion and high accuracy. To specify a DTMF signal, data conforming to the
encoding format shown in Figure 7 must be written to the transmit Data Register. Note that this is the same as the
receiver output code. The individual tones which are generated (f
LOW
and f
HIGH
) are referred to as Low Group and
High Group tones. As seen from the table, the low group frequencies are 697, 770, 852 and 941 Hz. The high group
frequencies are 1209, 1336, 1477 and 1633 Hz. Typically, the high group to low group amplitude ratio (pre-
emphasis) is 2 dB to compensate for high group attenuation on long loops.
F
LOW
F
HIGH
DIGIT
D
3
D
2
D
1
D
0
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
941
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
1633
1
2
3
4
5
6
7
8
9
0
*
#
A
B
C
D
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
0= LOGIC LOW, 1= LOGIC HIGH
Figure 7 - Functional Encode/Decode Table
7
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
