[time-nuts] GPS DO Alternatives
hmurray at megapathdsl.net
Sun Dec 9 22:00:40 UTC 2012
lists at rtty.us said:
> The flip flop would have a metastable region. It would be quite narrow, but
> it would be there. As soon as you try to take the square wave into the
> processor, you are right back to the same problems we've discussed before.
> The flip flop, run this way does not improve that part of the situation.
That's not the issue I was trying to bring up.
The problem is that set and reset on most FFs are asynchronous. You have to
look at the fine print in the data sheet to see what happens if they overlap.
Consider the 74x74 that I looked at. It has true and compliment output
pins. If set and reset are both active, both outputs go high. So if the
reset signal comes first, you might expect the FF to get turned off, but with
that chip, the FF will be high.
(A few FFs have synchronous set and reset, but since there was no mention of
a clock, I assume we are talking about asynchronous inputs.)
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