[STM32H7] Why is there no spread spectrum functionality?

[MaxMax]

Hi,

as there is apparently no more spread spectrum functionality in the STM32H7 to reduce EMI, is there some counterpart? I heard that the PLL fractional divider might be used for that purpose, but the amount of processor overhead to change the three PLLs say once every millisecond sounds like a lot. Is there a better way to have a similar functionality? Does anyone use the fractional divider in such a way?

Best regards,

Max

[berendi]

I haven't actually used it that way, but it should be possible according to the reference manual.

If the application intends to tune the PLLx frequency on-the-fly (possible only in

fractional mode), then:

a) PLLxFRACEN must be set to ‘0’,

When PLLxFRACEN = ‘0’, the Sigma-Delta modulator is still operating with the

value latched into SH_REG.

b) A new value must be uploaded into PLLxFRACR (FracValue(n)).

c) PLLxFRACEN must be set to ‘1’, in order to latch the content of PLLxFRACR into

its shadow register.

A few lines of code in the probably already existing 1kHz system tick interrupt handler is not a big deal for a MCU running at maybe 480 MHz.

uint32_t pllcfg = RCC->PLLCFGR;
RCC->PLLCFGR = pllcfg & ~ RCC_PLLCFGR_PLL1FRACEN;
RCC->PLL1FRACR = whatever;
RCC->PLLCFGR = pllcfg;

[MaxMax]

Well, then I will just give it a try. I was not so happy about changing all three PLLs every ms, but you are right, it should not be that much overhead.

Thank you!

[t.poncot]

Hi MaxMax,

Did you give it a try as you said ? If yes, was it successful ? Thank you for your answer.

Regards,