You should always make it a point for sensitive circuits like these
to have a good ground.  In this case since you have one in a metal
case that's even better.  But not tying it to the board ground causes
a problem. When you have an arc the case has a difference of potential
compared to the board.  So more than likely some energy tranferred to
the board.  At least if it was ground you might still have a problem but
not as bad.  That and the next part is the PIC board itself.  

 You should make it a default design to put a decoupling capacitor from
Vdd to ground as close to the PIC as possible and one on Vdd to 
ground on any chip on the board.  

 Getting up out of your chair generates large amounts of static electricity.
But now that you can pretty much duplicate the test anyway you can then
go to trying to ground things, putting in decoupling capacitors and may have
to resort to ESD protection devices as an extra caution.  The decoupling
capacitors if nothing else will reduce noise on the chip but might also act
to absorb some of the energy.  

 If nothing else grounding and shielding with a metal case (tied to ground of
course) will go a long ways.

Michael

----- Original Message ----
> From: trossin <ted_rossin@yahoo.com>
> To: piclist@mit.edu
> Sent: Thursday, March 27, 2008 1:55:43 PM
> Subject: [PIC] very slight ESD events wake up my sleeping PICs
> 
> 
> I've built a few "executive" (I'm just a grunt geek) toys and two of them
> have the same problem. That is when I get up from my chair, they turn on. 
> Both of these toys have push buttons to activate them. When the button is
> pushed they do their thing then go to sleep. I enable wake up on port B
> change and use the internal pull up resistor. On my latest toy I added a 1K
> pull up to try to stop this problem from happening (it reduces the problem
> but does not eliminate it). One toy uses the 16LF628A and the other uses a
> 16LF873A. One runs on two AA cells while the other runs on 3 AA cells.
> 
> On the toy with the 1K pull up (the 16LF873A), I put the PIC in a metal case
> but did not ground the case. The case is just "floating". On this device I
> once had an arc to the case and it required me to pop out the battery to
> reboot it. I think that grounding the case should solve the problem but I'm
> not sure.
> 
> The toy with the 16LF628A is just an exposed PC board but has a 5 inch
> twisted pair wire for the switch.
> 
> Has anyone else had problems like this? It only is a problem for me with
> projects that use the wake from sleep on interrupt feature. Any ideas on
> how to fix it. I guess I could use at 220 Ohm resistor but the lower I go
> the more power that is wasted when the button is pushed.
> -- 
> View this message in context: 
> http://www.nabble.com/very-slight-ESD-events-wake-up-my-sleeping-PICs-tp16331156p16331156.html
> Sent from the PIC - [PIC] mailing list archive at Nabble.com.


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