PIC16F716 physically exploding

How does something "intermittently explode" ?!?

What is connected to it ?

-jg

A schematic might give us a clue? Or is it too confidential?

All PICs have schottky diodes on all I/O pins. Wire -12 volts strong enough to drive several amps onto an I/O pin and it may blow like you are describing.

I/O drivers are current limited. You should not destroy PIC by driving an output against output or voltage source between 0-Vdd.

Your description is not enough, this is just a hint.

-- Roman Ziak

If you have a power supply capable of very high current, it could easily be latchup, but you'd have to be severely abusing the part to get it to latch up.

Best regards, Spehro Pefhany

My power supply is 4xAAs. No pin is seeing more than 6v. There is a diode in series with the batts, but other than that, no current limiting.

What kind of thing causes latchup?

Have any of you personally seen one of these chips blow? The tops literally come off, leaving a 1/4" diameter pock-mark.

Fred

In the bad old days it was by turning on a parasitic SCR when you apply a voltage above Vdd to an I/O pin. Most parts have protection diodes to prevent that from happening.

I've not seen that particular one blow up, but I've seen others do it. I remember one 14-pin DIP op-amp that had supply and ground pins centered on each side. If you put the chip in upside down it would quite reliably blow a chunk out of the package leaving a nice crater. If you took a drinking straw and held one end against the part when you flipped the power switch you could shoot a chunk of epoxy a fair distance with some accuracy.

Add some current limiting, or better, a LDO regulator.

Current injection into the parasitic SCR all CMOS devices have. can be -ve or +ve spikes, of short duration - just need to be long enough, and of enough magnitude, to fire the SCR. Once fired, it attempts to crowbar the Supply

- in this case, loosing :)

What is connected to this chip ?

-jg

The only time I have seen top off and crater is when the part joined the 110 club. 220 or 240 in other countries

I have seen some very hot parts from latch up but never one to explode.

w..

Fred wrote:

Where is this device used? We have controllers in remote areas that are subject to lightning strikes. The spark gaps and input protection usually handle it, but sometimes we get controllers back with missing chips and lots of epoxy powder.

Mark Walsh Rogue Engineering Inc.

In the bad old days of Unibus and wirewrap boards, a wireman had a pin on a Biploar PROM that was not connected, so he 'helpfully' connected it to a spare pin on the edge connector. Shame it was -15V.

When the system was turned on and the board was tested in an open rig the complete top of the PROM was blown across the lab.

Mind you that engineer always had the misfortune of having new boards catching fire, blowing up, smoking, sparking and many other disasters.

They don't prevent it, only increase the current required to make it happen. A few hundred mA for a fraction of a microsecond ought to do it at room temperature, less if the part is hot.

Best regards, Spehro Pefhany

NiCd cells by any chance? They can supply extremely high fault currents.

Best regards, Spehro Pefhany

Not Schottky.

Or a brief pulse (eg. from static discharge or lightning).

Not instantly anyhow, if the ambient is reasonable.

Best regards, Spehro Pefhany

Yes, it's amusing when chips blow up ... except when they're your own :-( ...

Here's a schematic of the Exploding PIC (part U10):

Some notes:

• there are 3 PIC16F716's on the board (it's improperly labeled as a

• Vdd is 4xAA alkaline run through a Schotty diode, so it could draw ample amounts of current.

• the chip is wont to blow up even when nothing's plugged into the motor outputs or sensor inputs.

• 6 of about 50 units have had the chip blow. in an earlier production run (last year), none exhibited any problem.

The only thing I can imagine that I'm doing differently with this PIC (versus the other 2 on this board, and all of my previous designs) is that I'm driving its oscillator-in (pin 16) with the oscillator-out of one of the other PICs.

Can this possibly explain anything?

Thanks all for your suggestions!

Fred

Hi Spehro, can you elaborate on this? Thank you. Also, it has blown with nothing plugged in.

Fred

What clamps the motor inductve energy ? With no regulator, and battery/diode, there is nothing to stop high short-term voltages, and that WILL fry chips. They may be stressed, and fail some time later.

There are no series ESD resistors in the sensor lines - also bad practice.

-jg

Probably a layout issue (causing latchup)-- with those motors it just requires a lack of consideration of the flow of ground return currents.

Best regards, Spehro Pefhany

If current (particularly current that is high and/or changes quickly) goes through a path with enough inductance, a voltage will appear when the current changes quickly. If that voltage is higher than a volt or so, depending on the layout, and flows through a "ground" trace, it can cause current to flow into or out of an I/O pin because two "grounds" are not at the same potential. If there's enough current available, latchup can occur. That's not necessarily fatal for the chip, but if the power supply (Vdd for the chip) can supply enough current, it delivers the coup de grâce.

Oh, and be sure you actually have the L293D and not the L293.

Best regards, Spehro Pefhany

Fred,

Do you ever use /MCLR ?

Grounding the pin using even a short track can cause latchup due to undershoot transients. You need to resistor in series with the /MCLR pin so the cct should be VDD - 1K - 100R - /MCLR and the reset switch is connected at the junction of the 1K and the 100R, NOT directly to the PIC. We normally use VDD-10K-1K0-PIC with a 1nF to ground at the PIC to ensure there are no ugly transients.

Robert