CM8870 can also be termed as DTMF receiver and it is integrated with both band split filter and decoder function into a single IC 18-pin. The power consumption of the IC is 35mW maximum.
BANDSPLIT FILTER – as we have already seen that a single tone is superimposed with two frequencies, one is low and the other is high frequency, to split this two frequency into high group frequency and low group frequency and then it is decoded.
DECODING – In CM8870, digital counting techniques are used to decode all 16 DTMF tone pairs into a 4 bit code.
For example: if you press a key 3, the corresponding low and high frequency is 697Hz and 1477Hz (refer the DTMF frequency table) these two frequencies superimposed to produce a single tone and this tone code is sent through the microphone and the signal is received by a DTMF receiver (CM8870), then the superimposed frequencies are separated by bandpass filter, then decoded using digital counting techniques and the digital output is 1100 (Q1, Q2, Q3, Q4) pin (11, 12, 13, 14). So it is very simple to interface.
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