The following is a re-post of something I read in sci.electronics. I believe that T.H. Tsim is the author.

   Author : T.H. Tsim
   Version: 0.3 (draft, pre-release :-)
   Date : April 12, 1994
   E-mail :
    1. Short introduction to DTMF
    2. List of DTMF-receiver chips
   Hi! After reading sci.electronics for a couple of years, I've
   discovered that many people are interested in decoding DTMF-signals
   for various applications. A frequently asked question (FAQ) concerning
   this subject is: "What IC should I use in order to decode DTMF
   1) Short introduction to DTMF
   DTMF means: Dual Tone Multiplexed Frequency, the tones you might have
   heard while pressing the keys on your telephone. Each key produces a
   slightly different signal, which is a mixture of two frequencies, i.e.
   pressing '1' will send a tone made of 1209 Hz and 697 Hz to the other
   end of the line.

          1209 Hz 1336 Hz 1477 Hz 1633 Hz

                   ABC     DEF
697 Hz      1       2       3       A

           GHI     JKL     MNO
770 Hz      4       5       6       B

           PRS     TUV     WXY
852 Hz      7       8       9       C

941 Hz      *       0       #       D

   There are many ways to detect and decode these DTMF tones. One idea
   could be an eight sharp-tuned filter combination with detection
   circuits. Needless to say, this is very impractical, considering the
   various ICs (Integrated Circuits or 'chips') made by different
   manufacturers all over the world.
   Most of these ICs do not require more than one (inexpensive) 3.58 MHz
   x-tal or resonator and the power circuitry. Usually the output is
   4-bit binary + 1 strobe.

           l              l- d3
           l     DTMF     l- d2
signal in -l    Decoder   l- d1     4-bit binary out
           l     chip     l- d0
           l              l- strobe

   Note: This figure is merely for decoration, it does not imply package
   Nice text to read about DTMF and telephony can be found in:
          "Understanding Telephone Electronics"
          Howard W.Sams & Co
          ISBN: 0-672-27018-18
   2) List of DTMF-receiver chips
   The companies are listed in a random order:
   Silicon Systems Inc.
          SSI202 DIP-18
          SSI203 DIP-18
          SSI204 DIP-14
   All req. 3.58 MHz (3.579545 MHz) crystal
   A neat and well documented design + PCB layout by Tom Miller KA1JQW,
   can be found in "The ARRL handbook for radio amateurs", ISBN:
   Chrystal Semiconductor Corp.
          CS20x family
          MSM6843 DIP-28, 3.58, 4b1s
   Application note in OKI Telecom LSI data book
   (From Arkady Horak-Systems)
   I spent about the last 14 months of my life working on an integrated
   DTMF receiver here at Motolora and I would like to have you add the
   part to the DTMF-list. The receiver is part of a 6805 8-bit
   microcontroller and is called the MC68HC05F5. Here is a quick feature
     * MC68HC05 based core micro
     * Memory mapped I/O registers
     * 5632 bytes of user ROM
     * 224 bytes of RAM
     * 15-bit multi-function timer
     * Power saving STOP, WAIT and Data-Retention modes
     * Mask optional COP (computer operating properly) or watchdog timer
     * Self check ROM
     * Fully static operation with no minimum clock speed
     * 30 bidirectional I/O pins
     * On chip oscillator for use with external crystal or ceramic
       resonator 2 or 4 MHz operation only while DTMF rec. is functioning
     * DTMF receiver
     * 40 pin plastic dip or 44 pin plastic leaded chip carrier packages
   Being an avid electronics hobbyist I feel this part will see a lot of
   service in hobby applications. This part is, to the best of my
   knowledge the first general purpose microcontroller with an integrated
   DTMF receiver. Currently only a ROM based version is available but as
   sales grow a 705 or EPROM version may be manufactured. Also, this part
   is compatible with the 6805 development tools and software.
   Arkady Horak 
   Motorola CSIC Microcontroller Division
   Austin, TX
          M957-0x DIP-22 or SOIC-24, 4b1s, 5 and or 12 Volt (read below)
   CAUTION: M957-02 and M957-01 differ in operating voltage!, M957-01 can
   operate from 5 - 12V, M957-02 operates 5V only (not suitable for 12 V
   A ready to build schematic by R.G. Krijgsman based on this Teltone
   M957-2 chip can be found in: "Het Elektuur telefoonboek" ISBN
   90-70160-94-3, (Dutch language)
          KT3170 DIP-18, minimal ext. parts ?
   UMC, United Microelectronics Corp.
          UM9203 DIP-18 4b1s and 2of8
          UM9204 DIP-14
          UM92870A/B/C DIP-18 (three variants)
   Examples of line-interfaces and app-notes in the UMC communications
   ICs databook.
   Mitel Corp
   Note: Teltone seem to carry this type-number as well.
          TC35301AP DIP-28, 4b1s
          CD22202 DIP-18, 4b1s
          CD22203 DIP-18 4b1s and 2of8
          CD22204 DIP-14 or SOIC-24, 4b1s
   3.58 = an inexpensive 3.58 MHz needed
   4b1s = 4-bit 1 strobe output (Hexoutput)
   2of8 = 2-of-8 code

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