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V.22 bis Modem



  • V.22 bis 2400/2400 bps QAM
  • V.22, Bell 212A 1200/1200 or 600/600 bps DPSK
  • V.23 1200/75, 1200/1200, 75, 1200 bps FSK
  • Bell 202 1200/150, 1200/1200, 150, 1200 bps FSK
  • Bell 103 300/300 bps FSK
  • DTMF/ Tones Transmit and Receive
  • 'Powersave' Standby Mode


  • Telephone Telemetry Systems
  • Remote Utility Meter Reading
  • Security Systems
  • Industrial Control Systems
  • Electronic Cash Terminals
  • Pay-Phones
  • Set-Top Boxes

Supply Requirement:

  • 2.7 to 5.5 V power supply

The CMX868A is a multi-standard modem IC for use in telephone based information and telemetry systems.

Control of the device is via a simple high speed serial bus, compatible with most types of µC serial interface.
The data transmitted and received by the modem is also transferred over the same serial bus.

On-chip programmable Tx and Rx USARTs meeting the requirements of V.14 are provided for use with asynchronous data and allow unformatted synchronous data to be received or transmitted as 8-bit words.

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

The device also features a hook switch relay drive output and a Ring Detector circuit which continues to function when the device is in the Powersave mode, providing an interrupt which can be used to wake up the host µController when line voltage reversal or ringing is detected.

The CMX868 operates from a single 2.7 to 5.5V supply over a temperature range of -40°C to +85°C and

is available in 24-pin TSSOP, SOIC and DIP packages.

Block Diagram

CMX868A: a multi-standard modem for use telephone based information and telemetry systems

Design Support Information


CMX868 and CMX868A FAQ

Q. I see that a CMX868 and CMX868A both exist can you tell me what the difference is and whether I would have problems migrating from my existing CMX868 design to a CMX868A based unit.

A. There are three major changes, the DTMF decoder performance has been improved to give greater compatibility with worldwide exchanges and the DTMF encoders supplied by other manufacturers. The dynamic range has been increased to improve the compatibility with normal transmission conditions particularly where lines are encountered. The acceptable twist has been increased to cope with low-cost DAAs and lines. Additionally the algorithm has been extensively modified to provide a high immunity to voice signals.

Secondly the hybrid function has been improved to increase rejection of transmitted signals and harmonics and reduces the number of components in the DAA and simplifies the design of DC-coupled DAAs.

Lastly a transmit output tri-state capability has been added where the outputs of the CMX868A are independently tri-stateable allowing the line and, hence, the receiver
to be biased, without terminating the line. Caller-ID signals flowing through a hook switch bypass capacitor will see a high impedance termination, which gives a better match to the receiver input. This method offers the possibility of improved S/N performance over other low-cost approaches.

A current design that uses the CMX868 can use the CMX868A as it should be considered a drop in replacement, the additional features should appear invisible to existing software and hardware designs.

Q. I see that the CMX868A datasheet / data bulletin has a paragraph that discusses the need for an additional 7dB of "trans-hybrid" loss. What is this talking about, and why do I need it?

A. The trans-hybrid loss refers to attenuation of the Tx output signal that gets fed back into the Rx input path of the same device.

If the 7dB trans-hybrid loss is not provided then the sensitivity in originate mode will be reduced from the minimum "must detect" value of -43dB.

This reduction in sensitivity is due to 2400Hz carrier rejection in the receiver path being around 36dB.

While the addition of 7dB of trans-hybrid loss will result in optimum performance, neglecting the additional 7dB of trans-hybrid loss will not prevent operation and may be acceptable in some applications.

Q. I have been trying to program the CMX868A Programming register, but it does not seem to work. What is the suggested method of writing to this register?

A. Writing to the Programming register is performed via the CML C-BUS. One register address is required for each data word; you must also ensure that the programming flag bit in the Status register is set before writing to the Programming Register

The process is as follows:

  • Address for programming register ($E8) sent on Command Data line
  • First word for the programming register is loaded onto Command Data line
  • Check the Programming Bit Flag in the Status Register - b13 = 1; if true then continue
  • Address for programming register ($E8) sent on Command Data line
  • Second Word for the programming register is loaded onto Command Data line
  • Check the Programming Bit Flag in the Status Register - b13 = 1; if true then continue
  • Address for programming register ($E8) sent on Command Data line
  • Third Word for the programming register is loaded onto Command Data line
  • Continue . . .

If the latency between writes is long, typically greater than 150us it is not required to check the Status Register bit flag (please refer to the data sheet).

Q. Is it possible to use AT commands to control the CMX868A V.22bis modem?

A. The CMX868A interfaces directly to a host controller via a simple serial control bus which permits low level control. If an AT command set is desired then an interpreter will be resident in the host controller.

To control the CMX868A within its Demo Kit, the DE8681, a simple AT command parser is provided in the embedded software. The full 'C' source code for the controller software is available from the CML web-site. See Evaluation Tab.

Q. How do I detect CAS on the CMX860, CMX867 or CMX868?

Q. Can I do Type 2 CID (Caller Identity) on the CMX860, CMX867 or CMX868?

A. The Programmable Tone Pair can detect the CAS tone required for type 2 CID but an unacceptable tendency to false is likely. This is because both filters are likely to ring due to the narrow bandwidth required. The CMX602B provides a very efficient CAS detector that can be interfaced through the CMX868, CMX867 and CMX860. The method described does not require any additional connections to the host microprocessor and very few additional components.

View Informative Diagram.

The CMX602B input signal is derived from the same signal input as the CMX86x. The gain components around the CMX602B may need to be adjusted. See the CMX602B Data Sheet for more information.

The CMX602B Mode input is clamped to Vss so that its mode is selected by the state of the ZP input. When the CMX86x is powersaved or on-hook, the CMX602B will be powersaved. When the CMX86x goes off-hook the CMX602B will come out of powersave and go into CAS detect mode.

The CAS should be detected around 55ms after it starts and DET will go low. This will cause the CMX86x to detect a ring signal. The Ring Detect flag will go low and the CMX86x will generate an interrupt (provided this interrupt is un-masked). This method allows the CAS detector to be polled and the interrupt to be masked. Polling is necessary to measure the length of the CAS to qualify it.

The software must remember to differentiate between a genuine ring detect and a CAS detect.

CAS Detection Method

The CMX86x will be off-hook.

Approximately 55ms after the start of the CAS the CMX86x will signal a ring detect. Ring Detect will go high (Status Register ($E6) b14 = 1) and, if unmasked ((General Control Register ($E0) b5 = 1), an interrupt will be signalled.

The Interrupt should be masked and Ring Detect polled to confirm that the CAS is present for the expected period.

Following the CAS detect a short period of silence occurs during which the near end handset is muted. Any line activity can be confirmed by polling the Energy Detect flag (Status Register ($E6) b10). This is important to help reduce the incidence of false CAS detection.

The Ring Detect interrupt should be masked for the remainder of the CID transaction.

Please note the CML disclaimer notice for applications and FAQ information.

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