Re: [avr-gcc-list] AT90s1200-12 latched short circuit

[Keith Gudger]

You have a diode between the chip and ground?  That means when you push
the buttons, which go to ground, that you force a voltage below chip
ground on the input pins?  That could be your problem.

Keith

On Tue, 3 Feb 2004, Richard Urwin wrote:

> On Monday 02 Feb 2004 11:33 pm, you wrote:
> > It is not clear to me if this "hot" state occurred once, or if it is
> > reproducable...
> 
> I only noticed it once...
> 
> > Without a schematic for the circuit (indicating at least what you
> > *think* you built! :-) one has to assume that your circuit is OK. 
> 
> The circuit is really simple:
> a handful of decoupling capacitors: 10nF ceramic.
> the (AVR042) standard power-on reset circuitry. (a resistor and diode to 
> Vcc, a capacitor to ground)
> four switches from PD3-6 to ground, diode OR'd to INT0. (No external 
> pull-ups, internal pull-ups enabled.)
> one pizzo between PD0 and ground.
> a 12MHz crystal and loading capacitors.
> two diodes (1N4001) in series with the power feed, one on Vcc, the other 
> on ground.
> 4 AA batteries.
> 
> > Assuming that it is, there are at least a couple of possibilities:
> >
> > (1) You managed to get the chip to "latch up".  The AT90S1200 is a
> > pure CMOS design and therefore static discharge, or any other sort of
> > unexpected pulse on an I/O (or power/ground) pin can induce
> > "latch-up". This means that literally every single transistor on the
> > microcontroller's die has gone into an "on" state.  Therefore every
> > single CMOS transistor on the die is conducting power from Vcc (i.e.,
> > +6V) to ground.  The only way to clear the "latch up" is to remove
> > power.  Usually, the chip never recovers from this, unless you detect
> > it early.  If "latch-up" was to blame, I'd assume that your "trying
> > it again" caused this event because the chip did survive, and thus
> > this was an isolated event.
> 
> I think this must have been caused when I first tried to measure the 
> voltage of the batteries. Maybe I momentarily shorted it out or 
> something. If this had been going on for several days  (a) the 
> batteries would have died, and (b) the chip would have died. Added to 
> which it was inside a closed plastic box with no external conductive 
> contacts and no power switch.
> 
> > (2) You have one of the I/O pins "fighting" an immovable electrical
> > force, such as trying to drive a grounded I/O pin high.
> 
> I am almost certain that this is impossible with the circuit. The only 
> thing that might cause it is a program error trying to drive a button 
> input high when the button is pressed. I'll have another look, but I 
> don't think that's going to happen. On the other hand it seems there 
> must be a bug somewhere to have caused the original failure.
> 
> > (In fact, I 
> > have seen this induce "latch-up" in AVR parts in the past.)  Normally
> > though, the chip resets and re-runs the program over again and then
> > does exactly the same thing.  This can consume a *lot* of power and
> > heat up the chip. Also, your program will look like it's "not
> > working".
> 
> Thanks for that. I'll keep it in mind.
> 
> >
> > > The only routes to either power line with no button pressed is via
> > > the pizzo transducer, or the reset pin (standard power-on reset
> > > circuit.) There is no limiting resistor on the pizzo, but even with
> > > the chip driving it hard I wouldn't expect it to develop much more
> > > than 20mA.
> >
> > Why?
> 
> Because an output pin is only supposed to be able to drive that much 
> current.
> 
> >
> > > That would not make the 1N4001s hot enough to burn my finger.
> >
> > Ah...this is significant.  Follow the current!  Show us the circuit
> > and tell us what you think you are doing with the I/O lines connected
> > to these diodes!  Are they programmed as inputs.  How are you
> > "pulling up" PD3-6? How is the INT0 pin onfigured?  Show us the code!
> 
> The 1N4001s are in series with the power supply, to drop a few fractions 
> of a volt for safety. They would heat up with any significant 
> short-circuit anywhere on the board.
> 
> > > Has anyone seen this behaviour? Might 0.15V above maximum voltage
> > > cause this to happen?
> >
> > Unlikely.
> 
> Thanks, that's what I thought.
> 
> Thanks for your input. I'm reasonably convinced that I have a possible 
> programming bug and the latch-up was caused during the testing process.
> 
> -- 
> Richard Urwin
> 
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