[sdiy] In praise of the ATM STM32F303

[Simon Brouwer]

Hi Neil, 

So maybe they improved it (the application note that the errata links to does
not say whether the F446 is among the types in which they did). 

The application note also is unclear about how much performance is lost when the
recommendations are followed (i.e. disabling accelerator functions). Maybe for
reading out pot voltages, or even for audio sampling, it may be more efficient
to keep the processor running at maximum speed and spending some of the
conserved MIPs on averaging ADC values. 

Best regards
Simon

> Op 14 februari 2016 om 13:44 schreef Neil Johnson <neil.johnson71 at gmail.com>:
> 
> 
> Checking the errata sheet for the F446 part I see that it also has ADC
> noise issues  (section 2.1.6).
> 
> Neil
> On 14 Feb 2016 11:17, "Simon Brouwer" <simon.o at brousant.nl> wrote:
> 
> > Hi Eric,
> >
> > If it is this errata sheet:
> >
> > http://www.st.com/web/en/resource/technical/document/errata_sheet/DM00037591.pdf
> >
> > It applies to specific revisions of the F40x and F41x families.
> >
> > So it is possible that in newer revisions and other parts such as the F446
> > they
> > fixed it.
> >
> > Anyway, thanks for the heads up, it is good to know that this problem may
> > exist
> > and may need a workaround.
> >
> > Best regards
> > Simon
> >
> > > Op 13 februari 2016 om 16:17 schreef Eric Brombaugh <ebrombaugh1 at cox.net
> > >:
> > >
> > >
> > > I'm fairly certain that it's mostly down to the crosstalk from digital
> > > circuits on the same die. The F4 parts errata sheets specifically
> > > mention that the flash cache system interferes with the ADC and they
> > > have a number of workarounds listed which unfortunately don't really
> > > seem to help much. The best approach with the F4 ADCs is to do a lot of
> > > filtering and hysteresis / guard-banding of the digitized values. This
> > > is fine for slowly varying CVs and pots, but does tend to limit their
> > > application for audio-rate modulation.
> > >
> > > While the F3 CPU which runs slower requires a less aggressive cache
> > > system and hence has less on-chip interference, there may be other
> > > circuit-level differences in the analog section - on the F303 there is a
> > > high-speed SAR ADC which performs pretty well and as Olivier mentioned,
> > > the F373 SD ADCs exhibit very good ENOB. I've got an F373 prototype
> > > running that does stereo audio input/output without an external codec
> > > and sounds quite good.
> > >
> > > Note that I've done some research on the F302 parts and the ADC in that
> > > family seems to have issues that I've not seen on other F3xx parts. I
> > > suspect that there may be some compromises on those lower-cost members
> > > of the F3 series.
> > >
> > > Eric
> > >
> > > On 02/13/2016 04:05 AM, Simon Brouwer wrote:
> > > > Hi Eric,
> > > >
> > > > Any idea whether that noise is due to worse (differential) linearity
> > or to
> > > > worse
> > > > crosstalk from digital circuits?
> > > >
> > > > I am starting out with the STM32F446 for a one-off semimodular
> > monophonic
> > > > synth,
> > > > and was thinking of using one of its built-in DACs for VCO control
> > voltage.
> > > > My
> > > > plan is to deal with its nonlinearities by calibrating its output
> > voltage at
> > > > each semitone, then using a lookup table with correction values.
> > > > But if the DAC output voltage contains LSB's of uncorrelated noise, I
> > may be
> > > > better off using an external SPI DAC.
> > > >
> > > > Best regards
> > > > Simon
> > > >
> > > >> Op 13 februari 2016 om 4:37 schreef Eric Brombaugh <
> > ebrombaugh1 at cox.net>:
> > > >>
> > > >>
> > > >> A couple guesses why:
> > > >>
> > > >> 1) for high-volume production such as Waldorf usually does, every cent
> > > >> makes a
> > > >> difference. In large quantity F3 parts like the 303 are a few bucks
> > cheaper
> > > >> than F4 parts, so that adds up.
> > > >>
> > > >> 2) I've used both F3 and F4 parts in products so I've got some
> > experience
> > > >> with
> > > >> them. The F3 parts analog sections are considerably higher quality
> > than the
> > > >> F4. Noise on F3 ADCs and DACs are a few lsbs lower amplitude than F4
> > parts.
> > > >>
> > > >> Eric
> > >
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