Hi Anand,

My guess would be that the output was actually acting as a very weak
pull-up (perhaps it was set to be open-drain and we are looking at
diode leakage) so that when the meter is put between the pin and Vcc,
there is no current flow at all through the meter, so it reads 0
volts. When the meter is placed between the pin and ground, the input
impedance of the meter draws enough current to cause a 1 V drop across
whatever is pulling the pin high.

Sean


On 4/2/07, Anand Gadiyar <gadiyar@ieee.org> wrote:
> Hi all,
>
>     I noticed something interesting while playing around with a
> GAL16V8 device. I finally figured it out, but it's an interesting
> puzzle and I thought it worth sharing.
>
>    Using a voltmeter, the Vcc pin shows 5 volts with respect to GND.
> And a output pin at logic high shows 4 volts with respect to GND. The
> same pin shows 0 volts with respect to the Vcc pin. The same is
> observed when i used a CRO to look at the waveforms.
>
>    Also tried this on a PALCE22V10 device and it behaves similarly.
>
>    See if you can figure it out. I'm pretty sure you guys will throw
> more light on the puzzle, else answers in two days.
>
> Warm regards,
> Anand
>
> __________________________
> Anand Gadiyar
> Indian Institute of Science,
> Bangalore - 560 012
> __________________________
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist