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Digital-to-Analogue, POTS, Signalling Interface



  • Full, Global Call Progress Tone Sets
  • CIDCW Alert Signals (CAS Tones)
  • Codec DTMF Encode and Decode to CEPT Specifications
  • Programmable Ringing Signals
  • 12kHz and 16kHz Metering Pulse Generation
  • Modem/Fax: Answer and Originate Tones
  • CCITT R1 and R2 Tone Generation
  • 'Soft-Start' to Final Level Adjustment
  • V.23/Bell 202 FSK Encoder with On-Chip UART
  • Serial Control Interface
  • On-Chip Summing Amplifier


  • ISDN Terminal Adapters
  • FeaturePhones
  • Adjunct Boxes

Supply Requirement:

  • 2.7 to 5.5 V power supply

The CMX605 is an integrated telecom tone generator and DTMF encoder/decoder (codec) designed for ISDN interfaces, Wireless Local Loop and Analogue-to-Digital Phone Conversion systems.

The tone generator covers an extensive range of pre-programmed tones used in analogue phone systems (POTS).
Three outputs are provided: ‘Ringing signals’, ‘In-band tones or FSK data’, and ‘12kHz/16kHz Metering pulses’.

Simple software control facilitates the interface to a wide range of commonly used µCs and SLICs, enabling a comprehensive analogue telephone line presentation.

The DTMF encoder/decoder presents the digital line interface with DTMF dialling information received from the telephone user and generates the appropriate DTMF tones for the POTS interface.

DTMF tone pairs can be encoded along with each tone singly or with other dual tone signals, such as those used in CIDCW systems and ‘On Hook’ signalling systems.

Other tone standards supported are: Fax and Modem ‘answer’ and ‘originate’, ITU (CCITT) ‘R1’ and ‘R2’ signals, and sufficient tones for simple melody generation. Communication to and from the host µController is performed by a ‘C-BUS’ serial interface, which is compatible with the ‘SPI’ interface.

Block Diagram

CMX605: an integrated telecom tone generator and DTMF encoder/decoder (codec) for ISDN interfaces, Wireless Local Loop and Analogue to Digital Phone Conversion systems

Design Support Information



Q. 605 - I see that I can enable and disable transmission with bits in both the SETUP and MODE registers. Is one register recommended over the other for enabling transmission?

A. Bit 6 of the MODE register should be used to actually turn on/off transmissions because this method allows "soft starts and stops" of the transmitted tones.

The enable/disable bits in the SETUP register effectively power-save/power-up the corresponding sections of the chip. Using these bits to turn on/off transmission will create a step response at the Tx output, which will result in discontinuities and undesired spectral content.

Q. 605 - How do I generate caller ID (CID or CLIP)  on the CMX605 and the CMX615?

Q. 605 - How and when should I switch from synchronous mode to asynchronous mode when generating Caller ID on the CMX605 and CMX615?

A. View Diagram

Begin a synchronous transmission of 'channel seize' pattern by loading the Data Buffer with 0x55h. Initially read the Status Register to clear it then enable the FSK transmission using bit 6 of the Mode Register.

The IRQ will go low almost immediately the FSK is enabled because the data buffer becomes available. Ignore this IRQ and IRQN will stay low. After approx 7.5ms (9 bit periods) a Tx Underflow will occur. This will be flagged but no interrupt will occur because the previous one was not serviced. Provided no load of the Data Buffer takes place the 0x55h will re-circulate indefinitely giving the required channel seize pattern.

After the required period, the Data Buffer should be loaded with $FF. The FSK transmission will change to continuous marks. Wait a few bit periods and then select asynchronous mode. The underflow condition will still exist and so the UART will continue to produce continuous marks. When the continuous marks period ends:

1. Load the data buffer with the first byte to be transmitted.

2. Read the Status Register to clear it and re-prime IRQN. Interrupts will now be generated which should be serviced as normal until all the Caller ID data is sent.

Q. 605 - Can I play melodies using the tone generator?

Q. 605 - Can the Tone Generator be re-programmed?

A. The tone generator on the CMX605 and CMX615 can be used to play simple melodies. The ROM tables contain 'notes' covering two octaves, although these are scattered, that can be used to generate simple melodies. It is possible to overwrite the stored values to generate different notes.

An application note entitled Melody Generation on the CMX605, covering both these subjects is available from CML Web-Site.

Q. 605 - How do I get the CMX605 and/or the CMX615 into and out of powersave?

A. Bits 5, 6 and 7 of the Setup Register $D0 enable various sections of the CMX605 and CMX615. Setting any combination of these bits to 1 will bring the CMX605 and CMX615 out of powersave and start the crystal oscillator. Sufficient time for the crystal to settle (typically around 20ms) should be allowed before any of the transmit modes are selected. It is therefore recommended that bit 6 is used to enable the transmit modes when the CMX605 and CMX615 is already running so as not accidentally enter powersave mode. Following a General Reset Bits 5, 6 and 7 of the Setup Register ($D0) will be set low and this will cause powersave mode to be entered.

Q. 605 - What waveshape is the Ringing output of these products?

A. CML decided to make the Ringing signal a squarewave for a number of reasons:

  • Some SLIC interface circuits accept only squarewave signals.
  • A squarewave is easily converted to sine by low pass filtering.
  • A squarewave can be easily converted to other waveshapes such as trapezoidal which delivers more drive current to a bell circuit for equivalent average power.
  • Most SLIC interface circuits use external logic to switch ringing signals and this is more easily accomplished with squarewaves.

Q. 605 - How do I interface the CMX605 / CMX615 to the telephone circuits?

A. The CMX605 and the CMX615 are intended to provide some of the function of the an interface between ISDN and POTS lines. In a typical system the ISDN circuit will connect to a microcontroller via an S-Bus Interface. The POTS line will connect to the microcontroller using a CODEC, for speech, and, via a SLIC, to the CMX605 / CMX615 to handle FSK, DTMF, Call Progress and Special Information Tones, and Ringing Signals.

The microprocessor will pass information relevant signals to the CMX605 or CMX615, via the 'C-BUS', for conversion to the appropriate analogue form.

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Information, or software, viewed or downloaded from the CML FAQ or Application Notes sections of the CML website is provided “as is” without warranty of any kind, either express or implied, including, but not limited to, the implied warranties of fitness for a purpose, or the warranty of non-infringement.
CML reserves the right to make changes to the information available at any time and assumes no liability for applications assistance or customer product design. Reproduction of information from the CML website is permissible only if wholly reproduced and without alteration, and is accompanied by all associated conditions,
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