[time-nuts] Re: The amazing $5 timestamper, part 2: discovering a bug in my signal generator

[Lux, Jim]

On 5/5/21 12:57 PM, Poul-Henning Kamp wrote:
> --------
> Pluess, Tobias writes:
>
>> In my very first GPSDO I built, I used a STM32F303. This one had a quite
>> bad PLL stability, the frequency was varying over time in a sawtooth like
>> manner for some reason.
> It's called "spread-spectrum" and is done deliberately to game the EMI
> criteria for various certifications.
>
> By sweeping the frequency through a range, the peak energy of any one
> frequency, as averaged over a second, drops correspondingly.
>
> In many cases you can actually disable it, but you may have to punk the
> manufacturer quite hard to find out what bit to set or clear.
>
https://www.st.com/resource/en/application_note/dm00281138-stm32-mcus-spreadspectrum-clock-generation-principles-properties-and-implementation-stmicroelectronics.pdf

Says the STM32F2, F4, F7, but not the F3. But it might be similar.

You could look at the register map of the F3 for MODPER, INCSTEP, 
SPREADSEL, SSCGEN ?

A casual grep didn't find it.

There's also a scheme in another ap note about going in and changing the 
PLL fractional divider every millisecond using software. (I guess that's 
if you fail your radiated emissions test, and you're desperate?)