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Forum: µC & Digital Electronics Shorted reset button?


Author: Friedemann Masur (sungod3k)
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Im having some trouble with an Sanguinololu board after I shorted the 
chip. The chip is replaced and works but I still have the board 100% 
working.

One issue is that the firmware directly after start up says it got an 
"external reset" which indicates a short somewhere.

So I traced the layout and tested continuity from GND to all the pins.

Im getting beeps on pins 11 and 31 which are ground, also on 10 and 30 
which are VCC and AVCC (from what I read those have powersupply 
function)

BUT most interestingly I also get continuity on pin 9 which is reset and 
that should only be the case if the button is pressed, shouldnt it?

That could explain some of the unresponsive behavior of the board, the 
reset button has no other connected function and I can just remove it 
and hope the the board isnt reset permanently anymore, right?

Im thankful for any tips.

: Moved by Admin
Author: 6A66 (Guest)
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Friedemann Masur wrote:
> BUT most interestingly I also get continuity on pin 9 which is reset and
> that should only be the case if the button is pressed, shouldnt it?

Hi Friedemann,

did you inspect the pushbutton or measure it? This would be the easiest 
way to find out a fault there.

rgds

Author: Guest (Guest)
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R12 is to strong. Take 4k7 up to 10K.

Author: stefanus (Guest)
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The microcontroller would not run at all if the push button would have a 
short circuit. So that cannot be the problem cause.

I would also suggest to change R12. The wires on the reset line might 
receive electromagenic waves. By using 10k instead of 100k, you put more 
load on the wire, so it does not react so easily on weak signals.

Another solution might be to put a small capaciator (10-100nF) parallel 
to the push button. The the capaciator short-cuts high frequency 
signals, while the DC signal from the button still works.

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