You should be able to trap such pulses with a 74x121/123 monostable.
Choose a time constant of several hundred ms and connect an LED to
it or choose a time constant of some us and use your scope. The
minimum trigger pulse length for the 74HC123 is 14-26ns.
I once had a problem with a PAL (or was it a GAL) not doing what it
was supposed to do and it was caused by bad filtering of the 5V supply
voltage. I had to add an additional bypass capacitor close to the chip
then it worked.
Uwe.
> I have a fault in my test equipment here centred around AD767 DACs. What
> you need to know is that the AD767s latch an input with a short negative
> pulse (as short as 40nS) on the /CS line. I'm not getting this action.
> The pulse appears to come from a Pal. I sent the Pals to the
> manufacturer who tried them in another unit and they worked all day.
> I feel I am looking for a voltage level or timing problem.
> I have a 20Mhz scope here and can't see the pulse, or trigger on it.
> My thoughts turned to building a latch trap - some latch circuit that
> would come up low and latch high if the pulse appeared. I could then
> tack this onto the chip, and look for the pulse.
> Such a latch could be made from a 74F<something>. Any thoughts on how to
> check the level (e.g. does it go below 0.8V) or dwell (does it last
> 40+nS) with this or something similar?
> --
> With best Regards,
> Declan Moriarty.
--
Author: Uwe Zimmermann
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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