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Written by Amol Shah

DTMF is as acronym for Dual Tone Multi-frequency Signaling it is used in telecommunication signaling basically it is a signal that is sent to the switching center (phone company) when the phones keys are pressed.

DTMF is also called as Multi Frequency Signaling because for each key you press two tones of specific frequencies are generated. This is done so that a voice cannot imitate the tones. One tone is generated from a high frequency group and the other from a low frequency group. In all there are 16 DTMF tones but currently only 12 tones are being used in our phones so we will only study those over here.

1209 Hz
1336 Hz 1477 Hz
697 Hz 1 2 3
770Hz 4 5 6
852 Hz 7 8 0
941 Hz * 0 #

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In the above table you can see the row is representing a low frequency and the columns represent the high frequency. So when a key is pressed a sinusoidal signal containing corresponding low & high frequency is sent. E.g. if key ‘5’ is pressed the sinusoidal signal will consist of two frequencies 770Hz & 1336Hz. These tones are then decoded at the switching center to determine which key was pressed.

A number of companies make chips which decode these DTMF signal one such IC is MT 8870. MT 8870 is a complete DTMF receiver so all you have to do is to is give it a supply and an oscillator (crystal) and it will decode the received tones pairs it into a 4 bit code.

MT8870 Pin Diagram
Figure 1: 8870 Pin Diagram
The device after detecting a valid tone-pair makes one of the pins high (StD) for a short duration and the output latch is updated (Q1-Q4) according to the received tone-pair.
MT8870 Decode Table
Figure 2: 8870 Decode Table
Figure 2 shows the output latch status according to the tone pair received.
Basic Circuit
Figure 3:8870 Basic Circuit
Interfacing MT8870 to 8051 Microcontroller
Figure 4: 8870 Interfacing to 8051 Microcontroller

Figure 3 shows the basic circuit for 8870. Figure 4 shows how to connect the 8870 to a Microcontroller.

Now let’s write a simple code to interface 8870 to 8051 Microcontroller.


Make pin P2.5 = 1 when key ‘1’ is pressed and make P2.5=0 when key ‘2’ is pressed.

ORG 0000h
MOV P2, #0FFH                                                //Configure P2 as Input
JNB P2.0, $                                          //Wait till a High Pulse is received from StD
MOV A, P2                                          //Take data in from the output Latch
ANL A, #1EH                                       //Mask the unwanted bits
CJNE A, #01H, C1_MAIN                               //CHECK IF KEY PRESSED IS ‘1’
SETB P2.5                                             //IF YES P2.5=1
JMP Main
CJNE A, #01H, MAIN                       //CHECK IF KEY PRESSED IS ‘2’
CLR P2.5                                               //IF YES P2.5=0
JMP Main

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