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Altera_Forum
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11 years ago

Simulation problem with simple edge detector

I have a very simple edge detector and I'm confused as to why I get this very small blip (pulse) on my output when it detects an edge. I ran into this in a larger design so I simplified it down to a...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
11 years ago

That is due to what is called delta delay.

Your logic says:

elsif(rising_edge(CLK)) then

input_latch <= INPUT;

end if;

OUTPUT <= INPUT and (not input_latch);

thus input_latch acquires input at clock edhe but after delta delay. That very very small delay is seen in the comb logic of:

OUTPUT <= INPUT and (not input_latch);

set INPUT to '1' at 191 ns instead of 190 ns and see it disappear