[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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