This circuit outlines two different methods of DTMF generation using a COP820C/840C microcontroller clocked with a 3.58 MHz crystal in the divide by 10 mode. This yields an instruction cycle time of 2.79 ms. The application note also provides a low true row/column decoder for the DTMF keyboard. The first method of DTMF generation provides two PWM (Pulse Width Modulation) outputs on pins G3 and G2 of the G port for 100 ms. These two PWM outputs represent the selected high band and low band frequencies respectively, and must be combined externally with an LM324 op amp or equivalent feed back circuit to produce the DTMF signal. The second method of DTMF generation uses ROM lookup tables to simulate the two selected DTMF frequencies.
|
|||||