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PWM GENERATION WITH ST62 AUTO-RELOAD TIMER

Figure 2. PWM Timer Operation

Example 1 :

Target: Generate a 12kHz PWM signal with a duty cycle of 37%, using an oscillator frequency

of 4MHz:

We want to generate a periodic signal at a frequency of 12 kHz and a duty cycle of 37% .

The CPU frequency (quartz frequency) is 4 MHz.

Let’s try with a prescaler ratio of 3:

12 kHz = 1333.33 kHz / (256d - RC)

gives RC = 144.889, rounded to 145

Resolution = 1 /(256 - 145) = 1 / 111

Duty cycle desired: 37%

0.37 = (CP - 145) / (256 - 145) yields CP = 186.07, rounded to 186.

Summary: prescaler = 3, RC = 145d, CP = 186d: these numbers yield a PWM frequency of

12.012 kHz, a duty cycle of 36.94 % and a resolution of 0.9 %, which is very close to the initial

goal.