[time-nuts] Automatic switching of a µC osc. to external 10MHz reference as clock?

Chris Albertson albertson.chris at gmail.com
Wed Sep 12 15:37:50 UTC 2012


On Tue, Sep 11, 2012 at 6:31 PM, Kevin Rosenberg <kevin at rosenberg.net>wrote:

> On Sep 11, 2012, at 4:42 PM, Peter Krengel <krengeldatec at gmx.de> wrote:
> > I'm looking for a possibility to make an automatic switching
> > between the internal µC oscillator (Atmel µC) to the 10MHz output
> > of my thunderbolt to use it as a precise µC clock. Has anybody an idea
> how to do best
> > without disturbing the accuracy of the ref. signal?
>
> Hi Peter,
>
> I'm likely misinterpreting what you want to do. From my reading your
> question, the answer is: no, you can't the change the source of an AVR's
> system clock bus at runtime. You can only change the CLKSEL fuses only
> during programming.
>

I don't think he was talking about fuses.  The way I'd do this is to have a
"quality" 10MHz oscillator supply the uC clock pin.  This 10MHz oscillator
is phased locked with a reasonably long time constant to either a local
10MHz TTL can oscillator or to a BNC jack.  When the jack is connected to
the t-bolt the local oscillator will lock to it otherwise it falls back to
the TTL can.   The long TC on the PPL is so there to prevent a transient
when the switch is flipped.   I assume one would use a toggle switch but it
could be automated by detecting power through a narrow bandpass filter from
the BNC jack.

Chris Albertson
Redondo Beach, California



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