i am working with an atmel at89c4051 Uc ..but there seems to be a problem..evrything was fine until a voltage regulator failed momentarily and 10 volts was applied to the vcc of the micro.....since then the thing is not working..all the pins show a value between 3 and 5 except reset..i thought the controller must have burnt out so replaced it with a at89c2051 which is the same as my previous model but surprisngly even with this micro the moment i apply vcc all the pins are showing a high value around 5V...this time there was no power surge i powered the micro using a fixed 5v dc supply...i am at a complete loss to understand whether the micro has burnt out again or what is happening???? can anyone help? when can a micro give a high value in al its pins>?

 

No micro that I know of since the RCA 1802 (COSMAC) can withstand high voltage on its pins. In all probability there were other chips on the board that died as well. All of this is rank speculation however without a schematic -- I presume you can provide one. In a manufacturing environment any board subjected to that kind of stress is deemed permanently unreliable and becomes a frisbee or a chicklet depending on your mood.

 

I put 13V across a pic12F675 input pins once. I was surprised to find that when it had cooled down enough to touch that it worked fine.

It's possible that the pins are configured as inputs - a voltmeter with a high input impedence could detect almost anything if the pins are floating.

 

well actually i would have put up the schematic but the circuit does work... the program is fine...ive tried it before and it was all working...even today it was workng until that momentary lapse...just tell me one thing..when is it posible for all the pins of the micro to give a high value? if a uc does burn out then are the pins shorted to the vcc or something? ive tried looking for a circuit schematic for the pin arrangement on a micro but couldnt find any...i read somewhere there are some diodes on the pins which get shorted when a high voltage is applied..is this correct?im working on a breadboard..

 

The typical protection circuit is a diode to Vcc and a diode to ground. As long as you don't draw large amounts of current you are OK. Current preceeds heat and heat preceeds destruction.

The purpose of the diodes is to clamp the input to the range of -0.7V to Vcc+0.7V. A low impedance voltage source will supply any amount of current at a fixed voltage. It is the current in the protection diodes that will destroy the chip by generating heat.