[time-nuts] FE5680 missing PPS soln

bob grant bobgrant at fastmail.fm
Thu Feb 2 20:46:01 UTC 2012


Some info...

Its tempting to attach an LED to the /LOCK signal on the DB9.
However this signal is very weak and the LED does not seem very bright
and PPS signal does not pulse...Hmm

Internally the /LOCK pin is connected the 74AC240 buffer, but with an
LED helping to keep the signal voltage high (2.3V) a logic low is never
asserted.
This logic low is needed to enable one half of the 74AC240 buffer (pin
1) that gates the PPS signal.

Don't directly drive LEDs from the LOCK signal on the DB9 and, voila,
the PPS signal reappears.

Bob
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  bob grant
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