At 08:51 AM 2/27/2004 -0800, you wrote:
>>is the source of the FET in the Micron chip referenced to the PIC 5V? If
>>not
>>then the results will not be what you expect.
>
>
>Now this might be a clue.  The PIC is running off of a 3.3V supply, while
>the FET source is tied to a different (totally different rail) 5V.  In other
>words, the 3.3V is from one DC/DC while the source is another totally
>different DC/DC, not even the same input source.  Of course the grounds are
>all common.

Ahah!

The MOSFET gate has to get within 200mV or so of the 5V rail to make sure
it is turned off. Otherwise it could burn up, especially when the
ambient temperature is high.

(see the top left graph on page 4)
http://www.micrel.com/_PDF/mic94052-53.pdf

Suggest you use the one open-drain pin (RA4) and perhaps parallel the
internal pullup with another resistor to the 5V rail that is much lower,
something in the low k-ohms.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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