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[avr-gcc-list] AT90s1200-12 latched short circuit
From: |
Richard Urwin |
Subject: |
[avr-gcc-list] AT90s1200-12 latched short circuit |
Date: |
Mon, 2 Feb 2004 22:22:34 +0000 |
User-agent: |
KMail/1.5.3 |
I have a AT90s1200-12 with four switches (PD3-6 to earth) and a
pizzo-electric transducer (PD0 also to earth.) The switches are
diode-OR'd to INT0 (PD2.)
The program spends most of it's time in power-down, until it gets a
button press.
I powered this from 4 AAA cells. Since new 1.5V cells give over 1.5V I
included a couple of diodes (1N4001) in series to pull the voltage down
a bit. I measured it to be about 6.15V and decided that was close
enough. (Maximum voltage for a AT90s1200-12 is 6V.) I don't have good
current measuring capability, but I did what I could to check that the
current was about 10-20mA active and < 1uA inactive.
The project worked for several days and then lay idle for a week or two.
On trying it again it was not working (pressing the buttons - no
sound.) Now, several days after noticing this, I just opened it and
burned myself. The chip was too hot to touch, and the diodes were
hotter. The voltage was 5V. The open circuit voltage was 6V. So it was
sinking a lot of power. On putting the processor back in, it works OK
and there's no indication of it failing again.
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.
That would not make the 1N4001s hot enough to burn my finger.
(The 6502 had a Halt-And-Catch-Fire instruction. It seems I may have
found one for the AVR. ;-)
Has anyone seen this behaviour? Might 0.15V above maximum voltage cause
this to happen? Or am I looking for something else?
TIA for any suggestions.
--
Richard Urwin
- [avr-gcc-list] AT90s1200-12 latched short circuit,
Richard Urwin <=