June 2000, Issue 107

Low-Cost Software
Bell-202 Modem

DTMF AND FSK GENERATION

Digital data received from the PC through the RS-232 interface and software UART is converted to analog signals for transmission over the telephone network using PWM techniques. To minimize I/O pin usage, a single PWM output pin is used. This arrangement is possible even for the two DTMF tones because the SX28AC sums the DTMF signals internally to create a single signal.

To ensure the smooth frequency shifts that are required by the Bell-202 specification, all the FSK shifts are phase-coherent. Although this requirement is easily accomplished in hardware, it requires a level of processing power only available in the SX series to work in software.

The PWM output of the MCU is applied to the op-amp, where its DC offset is moved to accommodate the needs of the telephone network. The result is applied to the acoustic transformer and on to the network.

Even though it provides all the functionality of a complete 1200-bps modem, this design uses only nine of the 20 I/O pins available with the SX28AC MCU.

It also doesn’t use all of the device’s 50 MIPS of processing power. This setup makes it easy to add features like an LCD interface, motor controller, or I2C-to-EPROM interface by loading the appropriate virtual peripherals into the on-chip memory and using the extra I/O pins.