ST7FOXU0 datasheet(71/123 Pages) STMICROELECTRONICS

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ST7FOXU0

On-chip peripherals

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PWM Frequency and Duty Cycle

The PWM signal frequency (fPWM) is controlled by the counter period and the ATR register

value.

fPWM = fCOUNTER / (4096 - ATR)

Following the above formula, if fCPU is 8 MHz, the maximum value of fPWM is 4 Mhz (ATR

Note:

The maximum value of ATR is 4094 because it must be lower than the DCR value which

must be 4095 in this case.

At reset, the counter starts counting from 0.

Software must write the duty cycle value in the DCR0H and DCR0L preload registers. The

DCR0H register must be written first. See caution below.

When a upcounter overflow occurs (OVF event), the ATR value is loaded in the upcounter,

the preloaded Duty cycle value is transferred to the Duty Cycle register and the PWM0

signal is set to a high level. When the upcounter matches the DCRx value the PWM0 signals

is set to a low level. To obtain a signal on the PWM0 pin, the contents of the DCR0 register

must be greater than the contents of the ATR register.

The polarity bit can be used to invert the output signal.

The maximum available resolution for the PWM0 duty cycle is:

Resolution = 1 / (4096 - ATR)

Note:

To get the maximum resolution (1/4096), the ATR register must be 0. With this maximum

resolution and assuming that DCR=ATR, a 0% or 100% duty cycle can be obtained by

changing the polarity.

Caution:

As soon as the DCR0H is written, the compare function is disabled and will start only when

the DCR0L value is written. If the DCR0H write occurs just before the compare event, the

signal on the PWM output may not be set to a low level. In this case, the DCRx register

should be updated just after an OVF event. If the DCR and ATR values are close, then the

DCRx register should be updated just before an OVF event, in order not to miss a compare

event and to have the right signal applied on the PWM output.

Figure 33.

PWM function

DUTY CYCLE

AUTO-RELOAD

4095

000

WITH OE0=1

AND OP0=0

(ATR)

(DCR0)

WITH OE0=1

AND OP0=1

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