DTMF †DECODER (MT 8870):
It is used for decoding the mobile signal. It gets DTMF tone from the mobile headsetís speaker pins (here we used pin 1) at its Pin 2 & 3 and decodes the DTMF tone into 4-bit digital signal which is fed to the 4:16 DECODER via Pins 11,12,13,14 & 15(StD).
The DTMF Decoder is operated with a 3.579MHz crystal (XTAL2) applied between Pin 7 and 8.
In the DTMF receiver MT8870 (IC3), capacitor C12 is used to filter the noise and the resistors R6 and R7 help to amplify the input signal using the internal amplifier.
Pin 16 connected to resistor R5 provides the early steering output. It goes high immediately when the digital diagram detects a valid tone pair. Any momentary loss of signal condition causes ESt to return to low state.
Pin 17 connected to capacitor C11 is bidirectional, acting as steering input/ guard time output (St/Gt). A voltage greater than threshold of the steering logic VTSt†† detected at St causes the device to register the detected tone pair. The guard time output resets the external steering time constant, and its state is a function of ESt and the voltage at St.
Pin 10 is kept high by Vcc. Pins 5,6,9 are grounded. Pin 1 & 4 is shorted.