ST72C171 datasheet(71/152 Pages) STMICROELECTRONICS

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ST72C171

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PWM AUTO-RELOAD TIMER (Cont’d)

Independent PWM signal generation

This mode allows up to four Pulse Width Modulat-

ed signals to be generated on the PWMx output

pins with minimum core processing overhead.

This function is stopped during HALT mode.

Each PWMx output signal can be selected inde-

pendently using the corresponding OEx bit in the

PWM Control register (PWMCR). When this bit is

set, the corresponding I/O pin is configured as out-

put push-pull alternate function.

The PWM signals all have the same frequency

which is controlled by the counter period and the

ARR register value.

fPWM = fCOUNTER / (256 - ARR)

When a counter overflow occurs, the PWMx pin

level is changed depending on the corresponding

OPx (output polarity) bit in the PWMCR register.

When the counter reaches the value contained in

one of the output compare register (OCRx) the

corresponding PWMx pin level is restored.

It should be noted that the reload values will also

affect the value and the resolution of the duty cycle

of the PWM output signal. To obtain a signal on a

PWMx pin, the contents of the OCRx register must

be greater than the contents of the ARR register.

The maximum available resolution for the PWMx

duty cycle is:

Resolution = 1 / (256 - ARR)

Note: To get the maximum resolution (1/256), the

ARR register must be 0. With this maximum reso-

lution, 0% and 100% can be obtained by changing

the polarity.

Figure 43. PWM Auto-reload Timer Function

Figure 44. PWM Signal from 0% to 100% Duty Cycle

DUTY CYCLE

AUTO-RELOAD

255

000

WITH OEx=1

AND OPx=0

(ARR)

(DCRx)

WITH OEx=1

AND OPx=1

COUNTER

FDh

FEh

FFh

FDh

FEh

FFh

FDh

FEh

OCRx=FCh

OCRx=FDh

OCRx=FEh

OCRx=FFh

ARR=FDh

fCOUNTER

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