68.6.2.8 Spread Spectrum Counter

The PWM macrocell includes a spread spectrum counter allowing the generation of a constantly varying duty cycle on the output PWM waveform (only for the channel 0). This feature may be useful to minimize electromagnetic interference or to reduce the acoustic noise of a PWM driven motor.

This is achieved by varying the effective period in a range defined by a spread spectrum value which is programmed by the field SPRD in the PWM Spread Spectrum Register (PWM_SSPR). The effective period of the output waveform is the value of the spread spectrum counter added to the programmed waveform period CPRD in the PWM Channel Period Register (PWM_CPRD0).

It will cause the effective period to vary from CPRD-SPRD to CPRD+SPRD. This leads to a constantly varying duty cycle on the PWM output waveform because the duty cycle value programmed is unchanged.

The value of the spread spectrum counter can change in two ways depending on the bit SPRDM in PWM_SSPR.

If SPRDM = 0, the Triangular mode is selected. The spread spectrum counter starts to count from -SPRD when the channel 0 is enabled or after reset and counts upwards at each period of the channel counter. When it reaches SPRD, it restarts to count from -SPRD again.

If SPRDM = 1, the Random mode is selected. A new random value is assigned to the spread spectrum counter at each period of the channel counter. This random value is between -SPRD and +SPRD and is uniformly distributed.