On Fri, 16 Nov 2001, James Williams wrote:

> 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.

What are the rise/fall times of the two signals?  And how precisely
matched are the leading/training edges?  Do you want to detect when both
chnge at EXACTLY the same time (how excat?) or both change within X number
of -- what?  Microseconds? Nano? Pico?

> 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.

Well, that and it would also trigger on this:
A ____________________/---------------
B -----------------\__________________

Using one edge to trigger somethng is common.  I don't see anything that
leaps out at me for digitally triggering on two edges simultaneously...
perhaps one of our more linearly inclined bretheren could suggest some
creative op-amp or comparator solution.

Dale

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