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

[Pluess, Tobias]

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 <
time-nuts at lists.febo.com> 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" <jim at luxfamily.com>
> An:     time-nuts at lists.febo.com
> 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?)
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