John,

Something must be going on here and I think you should try to figure out
what it is(even more carefully than you have) before you throw out the
parts. The reason I say this is that you say that you can program the
device just fine, but if you place it in this other circuit in reset, it
draws a huge amount of current. This would imply (assuming that your
circuit is acting as expected) that the only thing which causes the chip to
switch between acting normally and drawing a huge current is the presence
of the programming lines (and possibly programming voltage).

Here's what I would suggest: take a solderless breadboard and build a new
circuit which JUST has a 5V regulator(even better, a known good 5V power
supply), a capacitor to bypass the Vdd line(as an added protection against
regulator instability) and the 16C55. Attach power, gnd, and MCLR to GND.
See if it misbehaves. If yes, it's probably something REALLY weird with the
chip. If not, then there is something going on with your original circuit.
Where are you measuring the large current? Before or after the regulator?
Could it be that the regulator is going into oscillations under certain
circumstances?

You could also try scoping the power on your original circuit to see if it
is good and stable at 5V.

Sean

At 02:14 PM 5/8/00 +1000, you wrote:
>Hi,
>I definately am not missing a ground. The power supply has correct polarity
>and voltage. Definatley checked out many times. There are only four grounds
>and two 5v connections. If it is latch, up under those conditions would it
>not also latch-up the 'c84 thats in there now? I'm only loading it with a
>CRO and logic probe which have input impedance of several hundred k. Would
>it be safe to say they're throw-away items?
>
>-----Original Message-----
>From: Spehro Pefhany [mailto:speff@INTERLOG.COM]
>Sent: Monday, 8 May 2000 11:36
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: ?? Short ??
>
>
>At 12:28 PM 5/8/00 +1000, you wrote:
>>Hi Spehro,
>>And thanks for your reply. I have a 7805 reg in there thats good for about
>>an amp. It shares the common ground and positive 5V with the '55. The
>>control lines are accounted for at either 0V or 5V. The chip would probably
>>get hot as if I let it go for longer than 2 - 3 seconds. For latch-up to
>>occur, would I not need a bias from somewhere? Apart from porta and control
>>lines, everything else is floating. I don't quite understand.
>
>I've seen it happen with a multimeter test probe (when it is first
>connected).
>
>I've also seen it with power supply connection problems (like a missing
>ground wire).
>
>Could be something else, of course, but it sounds like the classic symptoms.
>
>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
>Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
>=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>=
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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