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

[Bob Camp]

Hi

Some chips (dspic33's for example) have "oscillator fail" protection. You
blow the magic fuse bits and when the TBolt isn't giving you a clock the
micro drops back to the internal R/C oscillator. It's magic....

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Wednesday, September 12, 2012 11:38 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts]Automatic switching of a µC osc. to external 10MHz
reference as clock?

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
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.