Spikes in digital logic outputs on startup? - Page 2

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Re: Spikes in digital logic outputs on startup?
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If its just one output line a gate is often the simplest option: just
one tr, R and C. It allows conduction after the RC has reached enough
V for tr to conduct. For digital lines where you cant afford the
Vdrops of a bjt, use a fet.


Regards, NT

Re: Spikes in digital logic outputs on startup?
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One example where i've seen a problem like this, well not necessarily
digital logic, but close enough was a 555 timer. The problem was that
after a power outage, the 555 timer would fire once the power got
switched back on, and momentarily unlock an electric lock. You
wouldn't want this happening at midnight where no one was in a
building. I proposed a solution to just use a cheap microcontroller.
That way, you just needed to add a pulldown resistor to the port to
initialize in a safe state, and the micro would initialize as an input
anyways, so it wouldn't drive the relay to unlock the system. A cheap
8-pin micro is not many more cents than a logic gate in cost too.

Re: Spikes in digital logic outputs on startup?
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All logic states are UNDEFINED when the power is off.
At some later time, after all the transients have settled
down and rippled through the system, states can be considered
as defined (not to be confused with the way you WANT it to be).
Since the only way to get from one place to another
is through some continuous time, you can guarantee that there
WILL be turn-on transients along the path.  It's UNavoidable.

However, there are techniques that can be used to make the probability
very low.  For an example, look at the software hoops you have to jump
thru to write flash ram.  That's done to minimize the possibility
of trashing the contents on power up/down.
mike

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