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

Hal Murray hmurray at megapathdsl.net
Tue Sep 11 23:13:29 UTC 2012


krengeldatec at gmx.de said:
> 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? 

It's a bit tricky, but doesn't take much logic after you get it.

Xilinx has that feature in at least one of their FPGAs.  The circuit may be 
in a data sheet or app-note.  I'm pretty sure the details were discussed on 
the usenet FPGA group.  That was several/many years ago.

The problem is that you have to avoid runt clock pulses and metastability.  
It also has to switch to a clock when the other clock has stopped.


Another approach might be to PLL the local clock to an external reference.




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