The CM8870PI provides full DTMF receiver capability by integrating both the band split filter and digital decoder functions into a single 18-pin DIP. The filter section uses a switched capacitor technique for both high and low group filters and dial tone rejection. The CM8870 decoder uses digital counting techniques for the detection and decoding of all 16 DTMF tone pairs into a 4-bit code. This DTMF receiver minimizes external component count by providing an on-chip differential input amplifier, clock generator, and a latched three state interface bus. The on-chip clock generator requires only a low cost TV crystal or ceramic resonator as an external component.
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