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DTMF Codec/FSK Combo



  • V.23 1200/75, 1200/1200, 75, 1200 bps FSK
  • Bell 202 1200/150, 1200/1200, 150, 1200 bps FSK
  • V.21 or Bell 103 300/300 bps FSK
  • Low Voice Falsing DTMF Decoder
  • DTMF/Tones Transmit and Receive
  • Low Power - High Performance
  • 'Powersave' Standby Mode
  • Can achieve simple voice record and playback actions


  • Wireless Local Loop
  • Fixed Wireless Terminals (FCT/FWT)
  • SMS Phones
  • Security Systems
  • Remote Utility Meter Reading
  • Industrial Control Systems
  • Pay-Phones
  • Set-Top Boxes

Supply Requirement:

  • 3.0 to 3.6 V power supply

The CMX865A is a multi-standard modem for use in Wireless Local Loop, including Fixed Wireless Terminals and Fixed Wireless Phone installations and Short Message Service telephone systems.

Flexible line driver, hybrid and receiver circuits are integrated on chip, requiring only passive external components to build a 2 or 4-wire line interface.

POTS Signalling

A high-quality DTMF decoder with excellent immunity to falsing-on-voice, and a standard DTMF encoder are included. Alternatively, these blocks can be used to transmit and detect user-specific: programmed single and dual-tone signals, simple melodies, call progess signals or modem calling and answering tones.

Host control and data transfer is via a high-speed serial bus that operates in 'normal' and 'powersave' modes, and which is compatible with most simple types of µC serial interface.

An embedded USART allows multi-format asynchronous data and unformatted synchronous data to be received or transmitted as 8-bit bytes.

Block Diagram

 CMX865A is a multi-standard modem for use in Wireless Local Loop, including Fixed Wireless Terminals and Fixed Wireless Phone installations and Short Message Service systems

Design Support Information


CMX865 and CMX865A FAQ

Q. CMX865A: The DTMF de-response is given as 30ms max but I measure more than this on some DTMF digits under certain conditions.

A. The de-response time of the DTMF decoder is correct.
The programmer cannot see the DTMF response/de-response directly. What the programmer sees is b5 which loosely tracks the DTMF decoder response.

When a DTMF digit ends, the DTMF decoder will de-respond and some time later b5 will signal this by going low.
To allow for polling, the minimum time that b5 is high is 14ms. To ensure that short DTMF digits with short gaps between can be detected in the code, b5 will de-respond when energy from the following tone is detected. Under these conditions the maximum high time for b5 is 30ms. This maximum high time is called the DTMF de-response time.
Note that with longer DTMF tones and longer gaps, b5 can be high for more than 30ms.

This downloadable graphic shows the response of b5 in a number of different receive scenarios.

Q. Can you detail the differences of the CMX865A with respect to the CMX865?

A. The listings below should help you:

CMX865A Advantages over CMX865

  • Smaller package, less pins = smaller footprint
  • Pin count reduced from 24 to 16
  • V.21 function added
  • No MOQ

CMX865A Functional Difference from the CMX865

  1. The RT and RD pins are removed. The ring detector function is not required in Wireless Local Loops and most alarm systems.
    This function is easily provided by a GPI/O pin on the host micrprocessor.
    Schematics are available from CML Tech Support.
  2. The hookswitch pin is not present. This is a simple output function that is more cost-effective when moved to the host processor.
  3. The receiver interface has been simplified by making it single-ended.
    The internal hybrid function eliminates the need for a differential receiver.
  4. V.21 transceive capability has been instated for alarm panels and other niche applications
  5. 6.144MHz operation only

Development Support

  • A series of Application Notes to assist with design-ins to the target markets
  • Can be evaluated in the EV8600 Modem Evaluation Kit
  • An adapter board, HB865A, is available to allow device evaluation in a dual-in-line footprint


  • The CMX865A is designed to interface easily to a SLIC.
    All of the transmit signals are summed within the device and there is a register for software setting the output signal level (the same as the CMX865). The output can be single-ended or differential.
  • The outputs tri-state automatically when the modem is not being used thus providing a high impedance to the line. This allows simple mixing of other signals, such as voice.
  • The input is single-ended with a gain-setting, feedback path for convenience.
    Like the CMX865, the CMX865A also has a software controlled attenuator.
  • The CMX865A will provide exactly the same signalling as the CMX865 and is directly register compatible
  • The operating voltage is the same as the CMX865 but the operating current is expected to be slightly lower
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