October
2004, Issue 171
Telephone
Message Watchdog
An
Intelligent Call-Forwarding System
by
Jingxi Zhang, Yang Zhang, & Huifang Ni
DTMF
GENERATION
The
DTMF tone generator is implemented using a D/A converter
and sine wave look-up table, which is a 50-entry byte
array representing a complete precalculated sine wave.
Two 16-bit timer units drive D/A converters with accurate
clock rates. When a DTMF tone is requested, the two
basic frequencies are selected. The clock rate for each
DAC is determined with the following equation:
For
example, 38,500 Hz = 770 Hz × 50. The timer units are
set to generate the required clock rate for each frequency.
Two channels of DAC output are mixed at the op-amp in
the telephone line interface circuit.
