RFC: osmo-clk-gen v2

Müller, Marcus (CEL) mueller at kit.edu
Tue May 21 11:48:49 UTC 2019


Hi Sylvain & Harald,

On Tue, 2019-05-21 at 11:37 +0200, Harald Welte wrote:
> Hi Sylvain,
> 
> On Mon, May 20, 2019 at 05:13:42PM +0200, Sylvain Munaut wrote:
> > > 2) allow different output voltages for two of the four banks of the Silabs chip
> > >    https://osmocom.org/issues/3905
> > 
> > I'd use the DAC for software voltage tuning.
> > Use 0R to select which bank uses which (default rail or programmable
> > one, with just default to have 1/2 bank being the programmable vio).
> 
> Our solution so far was to have two banks fixed and two banks either
> fixed or variable by means of (mechanical, tht) jumpers.  I'm not sure
> we can fit our of them, or if we can fit 0R SMD resistors at lest. @Martin?

Knowing that 2.54 mm jumpers can be huge – can't one just add a
resistor in series with the DAC output and connect both that and the
external Vref input to the tracking input of the regulator? (I didn't
even know there was tracking regulators, that's awesome! I'd just have
gone for an SC-70 opamp voltage follower; considering the 5.6 mA max
bank I_DDOx current, that would suffice, I think?)

That way, with no external voltage fed in to these jumpers, the DAC is
"alone" and assuming no significant current into the tracking input,
the resistor doesn't skew the voltage. With an external source
connected, one could just set the DAC output to be a high-Z input. The
resistor then just fulfills the role of avoiding high currents going
between DAC and external voltage source if one forgets to float the
output first or the external voltage is significantly higher than the
MCU's VCC. (the question would be "how does the MCU know there's an
external voltage reference connected", but if the tracking input's
impedance is high enough, one could use a very weak resistor, so that a
"competing" DAC wouldn't even affect the external reference, and it
wouldn't have to know.)

> > > 3) should we keep the VCTCXO?
> > 
> > No.  As you explained I think it's better to split functions and not
> > overload this board.
> 
> happy to see we're in agreement here.
> 
> > > 4) Use SAMD XOSC / PLL / GCLK to allow lower reference frequencies
> > >    https://osmocom.org/issues/3857
> > >    Where we'd actually use one of the SAMD GCLK outputs as one of the
> > >    intputs to the Si5351C, and expose a GCLK input of the SAMD on an
> > >    external header.  This way, much lower frequencies can be used to
> > >    driver the Si5351C.  Or one could even go for deriving them from the
> > >    SAMD RTC XTAL.
> > 
> > Does that mean the SAMD core would be running from the reference clock
> > ? (and so wouldn't run if there is no reference connected)
> 
> The SAMD can run from its internal oscillator, even while communicating over
> USB where it derives/recovers the USB clock from the SOF.  So while one could
> run it from an external reference, it is not mandatory/required.

I think the XOSC input pads of the SAM D11/21 can also function as
normal GPIOs, so one could comfortably use these to detect the presence
of a roughly suitable clock signal, and only then configure one of the
internal clock buses (don't know – maybe even one used by nothing else)
to source the clock from that, and enable the output of that internal
clock to the respective GCLK to the GCLK_IO pin.

Regards,
Marcus
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