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

David davidwhess at gmail.com
Wed Sep 12 16:12:44 UTC 2012


On Wed, 12 Sep 2012 00:42:48 +0200, "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?

Which Atmel microcontroller?  Is this an application where you are
modifying an existing design and only have access to the clock signal?

Many microcontrollers support selecting from multiple clock sources
and include the logic necessary to do so without glitching.  They do
not usually have more than one high speed clock input though.  One way
to get around that is to switch from the high speed external clock to
an internal clock or a low speed external clock, swap the high speed
clock source, and then switch back.

There are limits on the minimum positive and negative pulse widths in
the clock so any transition needs to be glitch free.  A little bit of
external logic can handle that or a VCO and PLL like Chris suggests.

http://www.eetimes.com/electronics-news/4138692/Techniques-to-make-clock-switching-glitch-free



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