Hi Declan, > 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.
Maybe you are but you don't see it? > 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. Longer wires? Different pcb? Different voltages? PAL's are often very sensitive for the timing of signals on the inputs, where race- conditions can occure, leading to wrong states that will not give your pulse. All depends on the logic of the programming. I would check the circuits and timing that connect to the PAL's inputs. > 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. A 20MHz scope will not catch this. I have a 400 MHz tektronix that could do this. But I would still use my logic analyser (175 MHz, Philips). Can't you borrow a logic analyser somewhere? They really are very handy for catching one-time pulses, they are made for that. And you can see all pins at the same time. > 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? 40 ns is very short, you could use a set-reset flipflop/latch 74<whatever>, preferrably something fast (ACT?), to see if the pulse is seen. You need short wires and good decoupling! For the scope: A 40 ns pulse has a base frequency of 1/40e-9 = 25 MHz. To "see" a reasonable pulse on a scope you need as rule 10* this, so 250 MHz bandwidth. Even double this if you assume the pulse to be the half of a sinewave. Regards, Pieter ______________________________________________ Hoeben Electronics Phone: +31 6 51590081 Ronkert 44 Fax: +31 13 5096025 5094 EW Lage Mierde Private: +31 13 5096200 The Netherlands E-mail: [EMAIL PROTECTED] http://www.hoeben.com ______________________________________________ -- Author: Pieter Hoeben INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
