I guess it depends what you mean by 'at the same time' How many
nanoseconds? I don't think you'll ever get it to 'exactly' at the same time.

        If you use 74F series, I think you can go up to 100MHz. e.g. the two
inputs would have to change within 10nsec of each other. Is that close
enough? If so, my first prototype try would be two edge-triggered
flip-flops,  a '109 (positive) and '112 (negative). AND the outputs
together. Make the F.F. reset every clock, so that essentially the inputs
must change within one clock period. I guess you're looking at a (small)
state machine, running at a decent clock speed.


> -----Original Message-----
> From: Brian Aase [mailto:ulcsa@ESKIMO.COM]
> Subject: Re: [PIC]:Does anyone know of some logic circuits that would
> detect the following situation?
>
>
> One rather Rube Goldberg-ish approach might be to use two one-
> shots (74LS121 or whatever) with one triggering on the rising edge
> of A and the other on the falling edge of B.  AND the two outputs
> together, there you are.  This may or may not work, depending on
> your timing requirements.
>
> Brian Aase
>
> > A  _____/^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > B  ^^^^^\________/^^^^^^^\_____________
> >
> > I can't seem to come up with some DISCRETE logic that will
> detect when A
> > goes from low to high at the SAME time as B goes from high to low.
> >
> > Any ideas, without a pic.  Sorry pics are not fast enough.
> I know that I
> > need to store previous state, then latch current state of
> either rising edge
> > of A or falling edge of B, and then compare the two.
> However i am not
> > curtain of how to do it.  I mean if A is used to clock B,
> then B would be at
> > 2.5v when A clks, which gives false indication.
> >
> > Any ideas greately appriciated.
> >
> > Thanks,
> >
> > James
> >
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