General Description
‘C-BUS’ Serial Interface
This block provides for the transfer of data and control or status information between the CMX644A’s internal registers and the µC over the ‘C-BUS’ serial bus. Each 'C-BUS' transaction consists of a single Register Address byte sent from the µC, as illustrated in Figure 3, which may be followed by either of:

1. A single data byte sent from the µC to be written into one of the CMX644A’s Write Only Registers, as illustrated in Figure 4.
2. A single byte of data read out from one of the CMX644A’s Read Only Registers, as illustrated in Figure 5.

Data sent from the µC on the COMMAND DATA line is clocked into the CMX644A on the rising edge of the SERIAL CLOCK input. REPLY DATA sent from the CMX644A to the µC is valid when the SERIAL CLOCK is high. The interface is compatible with the most common µC serial interfaces such as SCI, SPI and Microwire, and may be easily implemented with general purpose µC I/O pins controlled by a simple software routine. See Figure 15 for detailed ‘C-BUS’ timing requirements

FEATUREs
• Bell 212A and V.22 Compatible Modem
• 1200bps Full Duplex Operation (2 or 4 Wire)
• UART for Start/Parity/Stop bit processing
• Non-UART Operating Mode
• Software Adjustable Tx and Rx Levels
• Programmable Group Delay Equalizer
• Answer/Originate Tone Generator/Detector
• Call Progress Tone Detection
• Integrated DTMF Encoder
• Line Reversal and Ring Detector
• Hook Switch Relay Driver
• Flexible Xtal/Master Clock Selection
• Simple Serial Control Interface
• Zero-Power Standby Mode
• 3.0V to 5.0V Operation

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