General Description

The 75T204 is a complete Dual-Tone Multifrequency (DTMF) receiver that detects 16 standard digits. No front-end pre-filtering is needed. The only external components required are an inexpensive 3.58-MHz television "colorburst" crystal for frequency reference and a bias resistor. An Alternate Time Base (ATB) is provided to permit operation of up to 10 75T204's from a single crystal. The 75T204 employs state-of-the-art "switched-capacitor" filter technology, resulting in approximately 40 poles of filtering, and digital circuitry on the same CMOS chip. The analog input signal is pre-processed by 60-Hz reject and band split filters and then zero-cross detected to provide AGC. Eight bandpass filters detect the individual tones. Digital processing is used to measure the tone and pause durations and to provide output timing and decoding. The outputs interface directly to standard CMOS circuitry and are three-state enabled to facilitate bus-oriented architectures.

Features

  • Intended for applications with less requirements than the 75T202/75T203
  • 14-pin plastic DIP or 16-pin SO package for high system density
  • NO front-end band-splitting filters required
  • Single low-tolerance 5-volt supply
  • Detects all 16 standard DTMF digits.
  • Uses an inexpensive 3.579545-MHz crystal
  • Excellent speech immunity
  • Output in 4-bit hexadecimal code
  • Three-state outputs for microprocessor interface
    75T204