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PWM GENERATION WITH THE ST62 16-BIT AUTO-RELOAD TIMER

36 cycles are enough to load the CMP register after the overflow.

This program dials a telephone number, for example 012345678 and loops when it is finished.

To manage the timing (40ms for the break and 96ms for the tone), the standard 8-bit timer in-

terrupts the sinus calculation. The input clock pin of the standard 8-bit timer is connected to

the ARTimer OVF output pin which is set in toggle mode to give a frequency of:

5319/2=2659.5Hz. (toggle mode divide the 5319Hz by 2).

The TCR register of the standard 8-bit timer is loaded with:

40ms*2659.5 = 6Ah for the break

96ms*2659.5 = FFh for the tone

To be connected to a telephone line, a 2-order analog 500Hz/1700Hz Tchebycheff bandpass

filter is enough (see Figure 3.).

To respect the maximum shift frequency of +/- 10Hz for each frequency the clock frequency

precision must be better than:

10/1633=0.61% (1633 is the maximum DTMF frequency used).

Figure 3. Connection between modules

OVF

TIMER

500/1700Hz

PSC=4

ARTIMER

PSC=4

TIMER

2659.5Hz

Telephone line interface

PWM