Does anyone have experiencing seeing a PIC16F716 physically blow its lid (in other words, explode)? I have a design that uses three SO pkg of the F716 and one of them is intermittently exploding. It's always the same one, and never either of the other two.
I have never seen this before and I don't know what would be causing it.
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.
Grant Edwards grante Yow! Pardon me, but do you
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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
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.
Yes, it's amusing when chips blow up ... except when they're your own :-( ...
Here's a schematic of the Exploding PIC (part U10):
there are 3 PIC16F716's on the board (it's improperly labeled as a
715). This is the only one that's blowing.
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.
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.
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.
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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.