Product description

The CMX631A is a low-power, system-selectable Subscriber Pulse Metering (SPM) detector to indicate the presence, on a telephone line, of both 12kHz and 16kHz telephone call-charge tone frequencies.

Deriving its input directly from the telephone line, input amplitude/sensitivities are component adjustable to the user’s national ‘Must/Must-Not Decode’ specifications via an on-chip input amplifier, whilst the 12kHz and 16kHz frequency limits are accurately defined by the use of an external 3.579545MHz telephone-system Xtal or clock-pulse input.

The CMX631A, which demonstrates high 12kHz and 16kHz performance in the presence of both voice and noise, can operate from either a single or differential analogue signal input from which it will produce two individual logic outputs.

This system (12kHz/16kHz) selectable microcircuit, which may be line-powered, is available in: 24-pin SSOP, 16-pin SOIC and 16-pin plastic DIP packages.

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Devices are available through our distributors below:


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Block diagram


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  • Low Power Operation
  • Detects 12kHz and 16kHz SPM Frequencies
  • High Speechband Rejection Properties
  • Tone Follower and Packet-mode Outputs


  • Complex and/or Simple Telephone Systems
  • Hotel and Office Call-Charge/Logging Systems
  • Billing Systems
  • Public Call Monitoring
  • Payphone Systems
  • PCMCIA Fax and Data Modems

Power supply requirement

  • 3.0 to 5.5 V

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Design resources


Q. 631 – What is the difference between the FX631 and the CMX631A?

A. The CMX631A is a development of the FX631 to give lower voltage operation. The FX631 is not recommended for new designs. The CMX631A is intended as a pin and function equivalent part. New designs should use the CMX631A.

The FX631 is a 3.3V-to-5V part whereas the CMX631A will operate between 3.0V and 5.0V. It is also possible to operate the CMX631A down to 2.7V over a reduced temperature range although there may be some small parametric differences.

FX631 Security and Telephone SPM Detector

This Application Note describes how two FX631 Low-Voltage (Subscriber Pulse Metering) SPM Detector microcircuits can be used, with additional external filter components, to detect both the 12kHz SPM and the 18kHz security tones employed in the French payphone/cardphone system.

FX631 FX631xxX clocking information

Application Note to describing the various Xtal/clock set-ups available to these products.

CMX631A/FX631 External Component Calculation - A Worked Example

Worked calculations to enable selection of the correct component values to obtain required 'must' and 'must-not' decode frequencies when using these products.

Package: D4 (DW)

16-pin SOIC

Package: P3 (P)

16-pin PDIP

Package: D5

24-pin SSOP

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