[sdiy] Clock signal detector

[Martin Naef]

Is there any particular reason not to use a microcontroller for this? 
It's an absolutely trivial single chip solution I'd personally do with 
an AVR ATtiny25 (DIP8) and its internal counter module:

- Reset counter on positive edge on pin (interrupt), set clock detect 
output high
- On counter overflow (or compare match), set clock detect output low.

Clocking (internal) and compare value of the counter can be set freely 
to tweak the timeout value. Power consumption will be minimal as the 
controller will spend most of its time in sleep mode and can be clocked 
low. It's maybe 20 lines of C code...

Martin

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