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PWM GENERATION WITH ST62 AUTO-RELOAD TIMER

Pulse Width Modulation (PWM) Generation

Using the PWM generation capability of the AR-Timer, the microcontroller CPU has only to

start/stop the timer and update the duty cycle. High speed PWM signals in the range of

The duty cycle is defined by the Compare register CP (see Figure 2).

The register TC is incremented from RC to 255d, then reloaded at RC to count again. The

PWM output is set on the overflow of TC and reset when TC=CP.

CP can have any value between RC and 255d. So RC should be minimal and the prescaler

ratio as small as possible to achieve a maximum resolution.

Major formulae for PWM generation are:

Duty cycle: (CP -RC) / (256 - RC)

Resolution: 1 / (256 - RC)

With a 8MHz oscillator and a prescaler ratio of 1, the maximum resolution (1/256) leads to a

Table 1. Prescaler Programming Ratio

Bit 0

Reg.SC1

PS2

PS1

PS0

PRESCALER

Ratio

00

0

0

1

00

0

1

2

00

1

0

4

00

1

1

8

01

0

0

16

01

0

1

32

01

1

0

64

01

1

1

128

10

0

0

3

10

0

1

6

10

1

0

12

10

1

1

24

11

0

0

48

11

0

1

96

11

1

0

192

11

1

1

384

2