Hi It is not unusual to see MCU PLL’s up in the “nanosecond” range in terms of jitter. Finding one that is below 200 ps was pretty much impossible last time I dug into them.
Bob > On May 6, 2021, at 8:39 AM, Pluess, Tobias <[email protected]> wrote: > > Good to know that they offer the Spread Spectrum functionality. > But as I see, the function is switched off by default. > And it still does not explain why the F303 had the issue with the frequency > modulated CPU clock. > But as far as I understand, I can use the PLL without any issues, as long > as the spread spectrum feature is switched off. > It would still be interesting to see how much jitter the PLL adds to the > OCXO's one. > > Tobias > HB9FSX > > > On Thu, May 6, 2021 at 12:45 PM Detlef Schuecker via time-nuts < > [email protected]> wrote: > >> Hi, >> >> yes, the STM32F4xx series has the spread spectrum feature, the STM32F3xx >> does not. >> See the RCC_SSCGR (reset and control clock spread spectrum clock >> generation register), Bit 31 in >> dm00031020-stm32f405-415-stm32f407-417-stm32f427-437-and-stm32f429-439-advanced-arm-based-32-bit-mcus-stmicroelectronics.pdf, >> >> catchy names :)). >> >> But the STM32 are real fine machines imho.. >> >> Cheers >> Detlef >> >> >> >> >> Von: "Lux, Jim" <[email protected]> >> An: [email protected] >> Datum: 05.05.2021 22:21 >> Betreff: [time-nuts] Re: The amazing $5 timestamper, part 2: >> discovering a bug in my signal generator >> >> >> >> 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?) >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send >> an email to [email protected] >> To unsubscribe, go to and follow the instructions there. >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send >> an email to [email protected] >> To unsubscribe, go to and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
