rpdom wrote: ↑Tue Jan 29, 2019 2:29 pmThere is a very good chance that the rest of the chip will be damaged as the damage will slowly spread across the chip even after the bad voltage has been removed.
Latched Up Rpi Post Mortem
Ah, I always thought that Rpi was like the human body - if one of the 10 fingers is burnt, all the rest 9 fingers will still work happily. There is a automatically set up hard border between the bad finger and the good fingers, in other words, damage would be confined and controlled.
It was only last year when one of my Rpi's passed away prematurely, then I realized that Rpi is so stupid, compared to the intelligent humans.
Let me tell you the sad story of the miserable, probably latched up Rpi. I was experimenting with different things, including over voltage and current, inverted power polarity, over current drive (source and sink), over cpu frequency drive etc. Those were the days until one morning I woke up and found my Rpi seemed not behaving as usual - one weird thing I still vividly remember is the following:
I used sudo or gui command to hardware shut down the Rpi, it always did not hardware shut down, but only software reboot. I changed SD cards but the symptom did not go away.
At that time, I thought perhaps the boot ROM was corrupted, so did not load the OS correctly. But after perhaps a couple of weeks, my Rpi passed away suddenly, without any warning (I did not make any hardware or software changes in the previous couple of days).
Looking back, I guess the Rpi already fell ill long before resting in peace, though its risk seeking master did not notice anything.
My verdict is that the cause is latching up, and the damage spread over a couple of weeks, ...
So I now think all the risk seeking Rpi newbies should seriously study the latch up thing before just wanting to have fun.
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Update 2019feb03hkt2157 - Latch Up References
Latch-up - Wikipedia
A latch-up is a type of short circuit which can occur in an integrated circuit . More specifically it is the inadvertent creation of a low-impedance path between the power supply rails of a MOSFET circuit, triggering a parasitic structure which disrupts proper functioning of the part, possibly even leading to its destruction due to overcurrent.
The parasitic structure is usually a PNPN structure which acts as a PNP and an NPN transistor stacked next to each other. During a latch-up when one of the transistors is conducting, the other one begins conducting too. They both keep each other in saturation for as long as the structure is forward-biased and some current flows through it.
The latch-up does not have to happen between the power rails - it can happen at any place where the required parasitic structure exists.
A common cause of latch-up is a positive or negative voltage spike on an input
or output pin of a digital chip that exceeds the rail voltage by more than a diode drop.
Another cause is the supply voltage exceeding the absolute maximum rating, often from a transient spike in the power supply.
It leads to a breakdown of an internal junction.
Parasitic structure - Wikipedia
In a semiconductor device, a parasitic structure is a portion of the device that resembles in structure some other, simpler semiconductor device, and causes the device to enter an unintended mode of operation when subjected to conditions outside of its normal range.
For example, the internal structure of an NPN bipolar transistor resembles two PN junction diodes connected together by a common anode. If a sufficient forward bias is placed on this junction it will form a parasitic diode structure, and current will flow from base to collector.
A common parasitic structure is that of an SCR. Once triggered, an SCR conducts for as long as there is a current, necessitating a complete power-down to reset the behavior of the device. This condition is known as latchup.