spread spectrum on STM32MP1

[MArdi]

Hi,

I have to use the spread spectrum on PLL4 on STM32MP1

I have set the pll4 in tfa and u-boot as follows

tfa

  pll4:st,pll@3 {

    cfg = < 1 49 11 10 1 3>;

csg = < 80 80 SSCG_MODE_CENTER_SPREAD >;

  };

u-boot

  pll4:st,pll@3 {

    cfg = < 1 49 11 10 1 7>;

    csg = < 16000 16000 SSCG_MODE_CENTER_SPREAD >;

    u-boot,dm-pre-reloc;

}

When I check the register (on linux) RCC_PPLL4CSGR the settings seems ok, but when I write SSCG_CTRL=1 on RCC_PLL4CR nothing happened.

Where I am wrong?

[PatrickF]

Hello,

could you please elaborate a bit on 'nothing happened' ?

Spread spectrum is not/barely visible using an oscilloscope on an output signal. Need to have a spectrum analyzer to confirm the 'spread'.

Btw, for which purposes are you using Spread Spectrum ?

[MArdi]

Hi,

we have an anechoic chamber and we checked.

When I said "nothing happened" I mean that with the anechoic chamber I can't see the "spread spectrum".

We need the spread spectrum for the LCD (parallel interface), the clock is connected to pll4.

Thanks!

[PatrickF]

You already got answers using another channel, but for the sake of community view, I confirm the SSCG is not available for PLL3 and PLL4 (documentations will be updated to reflect this).

[MArdi]

thanks