[time-nuts] Switching oscillators

[David]

On Sun, 09 Dec 2012 15:49:04 -0800, Hal Murray
<hmurray at megapathdsl.net> wrote:

>lists at rtty.us said:
>> 1) Convert them both to logic levels and use a gate.
>> 2) Use a relay. 
>> Both would need some sort of timer to drive them. Both would disrupt the
>> instrument when the switch took place. I know of no "cheap / easy" ways to
>> take care of the switching glitch. There are fancy / expensive ways. 
>
>Switching clocks is really hard if they are driving digital logic that you 
>want to keep working correctly.  You have to avoid runt pulses.  Is that what 
>you mean by "switching glitch"?

I linked this article last time someone asked about glitchless clock
switching:

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

The Racal Dana 1992 automatically switches clocks when an external
reference is applied but all of the details are hidden.  I suspect its
processor qualifies the external clock somehow before switching.

I have done the something similar on microcontrollers where instead of
glitchless external clock switching, I switched to the internal clock
or auxiliary 32KHz oscillator, swapped the external clock, and then
switched back to the external clock.