The CMX868 Modem

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The CMX868 Modem

• Presented by David Falp

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CMX868 - Low power V22bis modem

• Extends the CMX624/CMX644A family
• Advanced DSP based implementation
• V21, V22, V23, V22bis supported
• Bell 103, Bell 202, Bell 212A supported
• Caller ID decode both DTMF and FSK based
• Call Progress programmable tone detectors
• On chip hybrid for simple line interfacing
• Powersave mode ring detect still functions at µA
  current
• 2.7 to 5.5 volt - 7mA power requirements
• 3 device packages incl. 28 pin SSOP
• Smallest, lowest power available

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V series and Bell format summary
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The CMX868 - Block Diagram
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Tone transmission detection

• DTMF transmit detect
• Single tone transmit, 2100Hz 2225Hz tone detect
• Call progress signal detect
• User programmed tone/tone pair transmit detect
• Disabled (Power save)

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C-BUS Interface

• Handles the transfer of data control/status
  between CMX868 and µC
• Compatible with most common µC serial interfaces
• Received and transmitted data transferred via
  C-Bus
• Up to 5Mb/s transfer rates
• When line reversal detected IRQ can be generated
  to wake up µC
• Hook switch relay drive pin

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Tx USART(Universal Synchronous/Asynchronous
Receiver Transmitter)

• Programmable for continuous patterns, Start-Stop
  characters or synchronous data
• Generates parity, start stop bit
• Data is loaded into the Tx Data register then
  into Tx data buffer before sending
• Bits are added as necessary dependant on
  operating mode
• In Sync mode, data from the buffer is re-sent if
  no further data presented to Tx register
• In Start-Stop mode a continuous Stop signal (1)
  is sent if no data presented
• Meets V.14 requirements for QAM and DPSK

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Rx USART

• Programmable Synchronous / Start-Stop operation
• In Sync mode received bits fed into Rx data
  buffer then copied into C-BUS Rx register every 8
  bits
• In Start-Stop mode USART detects each character
  and feeds only the required number of bits into
  Rx buffer
• When data is copied to C-BUS Rx register, a flag
  is set to prompt the µC to read data

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FSK Modulator

• Serial data from USART fed (via optional
  scrambler) into FSK modulator or QAM/DPSK
  modulator depending on mode selected
• FSK modulator generates two frequencies

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DPSK Modulator (V.22)

• DPSK modulator generates 1200Hz carrier
  (Low-band, calling) or 2400Hz (High band
  Answering) modulated at 600symbols/s
• 600bps V.22 transmitted as a phase change (90º
  0, 270 º 1)
• For V.22 1200bps DPSK Tx data stream divided into
  groups of two consecutive bits encoded as carrier
  phase change

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QAM Modulator (V.22 bis)

• For V.22bis 2400bps QAM Tx data stream divided
  into groups of four consecutive bits encoded as
  phase quadrant change and define four elements
  within a quadrant
• Baud rate of 600 symbols/second transmits 2400
  bits per second data as modulation is 4 level

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QAM Modulation
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Tx Filter and Equaliser

• Modulator output signal fed through Tx block
• Includes a fixed line equaliser automatically set
  for modulation types and frequency band
• Equalisation compensates for relative amplitude
  and delay distortion over frequency band of PSTN
  improving bit error rate

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Rx Filter and Equaliser

• Bandpass filter to attenuate unwanted signals
• Provides line equalisation for all modes
• Bandpass and equaliser characteristics determined
  by the chosen Rx modem type and frequency band
• Output of block is measured by an Energy detector
  and compared to a threshold value

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Tone generation

• DTMF tone generator
• In QAM and DPSK modes this is used to generate
  guard tone
• Calling and answer tone/tone pair generation
• Used to establish handshake
• Programmable tone/tone pair

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Tone detection

• Standard DTMF tone detector
• Answer tone detector
• Measures amplitude and frequency of received
  signal and sets status register when valid 2225Hz
  or 2100Hz detected
• Call progress detector
• Programmable tone pair detector
• Measures signal amplitude at output of bandpass
  filter
• Sets bits in status register when tones are
  detected

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868 Line interfacing

• Line connection
• 2 or 4 wire circuits supported
• Internal compensation for transmitted signal in
  Rx path
• Reduced component count
• Improved Trans-Hybrid Loss (THD)

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EV8680
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EV8680 Evaluation kit

• Single PCB containing
• Line interface components for matching and
  isolation
• PIC µC with serial com port for PC interfacing
• AT command control
• PSU provides two voltage levels for 5V and 2.7V
  testing
• On-board xtal or operation from external clock
• Test points/header pin for signal monitoring

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CMX868 - Low power V22bis modem
Yes with second IC DAA
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CMX868 - Low power V22bis modem

• Applications
• Telephone Telemetry Systems
• Remote Utility Meter Reading
• Security Systems
• Payphones
• Cable-TV Set-Top Boxes
• Industrial Control Systems
• Electronic Cash Systems
• Vending Machines

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CMX868 Summary

• Inherent flexibility
• Multiple schemes offered
• Low power operation
• Easy interfacing
• Available now under concession with bug list
• Product release Q4/00
• Versatile product with great potential