This seems to happen in the real world often enough without damage. I'm also sure the data sheet gives limitations to the voltages imposed on outputs and the currents drawn. Did you check the data sheet? Am I wrong, there is no spec on this?
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My experience of inadvertently shorting pins is that while it may not damage the micro controller if only done for a short time, the controller cannot be expected to work properly while it's happening.
The controller may have been saved by the current limit in the regulator in my case.
Just this past week, I had two designs that both used the same microcontrol ler - an 8051 variant. Relying heavily on previous experience with similar parts in that family, I really thought I could get away with relying on th e weak internal pull-ups on one of the ports. Nope!
I know it's not your part, but maybe that's something to look at. Maybe the port pins are not holding the values you set, or perhaps reads on the port pins are indeterminate?
I have seen it happen and it did cause damage. Bus contention and then one chip declared itself the winner. In the old days you could fry graphics cards that way, IOW destroy hardware via code. While most micro controllers would survive I never used NXP controllers so I don't know how they fare. Also, some can be safely pulled to GND but not to VDD.
There isn't. Only input voltage limits which do not pertain to a port pins declared as an non-tri-stated output, and current delivery capabilities for outputs. The latter gives an indication of the Rdson of the output devices in there but not about how small their geometries are.
That's just it, when you hardwire an output to the opposite side.
The data on page 35 is ambiguous. For example, VOH at 20mA is VDD-0.4V worst case which indicates an Rdson of 20ohms. That's incredibly low for a uC. Taking process tolerances into account this could be as low as
12ohms. At 3.3V and shorted to GND that would be 275mA for a pin that is asserted high. More than whatever they call short circuit current ... tzzzt .. PHUT. There also is no short-to-VDD current stated.
I am sure the programmers did it correctly, declaring the respective pins as outputs and not relying on pull-ups. What I am trying to do is goosing a complex device connected to it where, due to very cramped layout, I cannot get access to some of the connections. It's that connected device we are suspecting of misbehavior.
There are different types of outputs. The one he is referring to is a "Hig h-drive output pin". This section shows a much higher current drive for a high level output, 20 mA at Vdd-0.4 compared to 4 mA for the standard drive output pins. The low level drive seems to be the same on these pins. So it is true that the data sheet does not seem to show a short circuit curren t for the high level drive when outputting a high level. One would expect that current to be higher than 50 mA.
I suppose he could try asking the maker. I often get responses by contacti ng them.
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Experience, never short a micro output to Vdd or Vss (or even higher voltages). I did it for a split second by accident with a raspberry Pi GPIO output and it not only crashed, but several pins on that GPIO are no longer playing. I now use that one as router, does not need the I/O.
When I am writing software and building test circuits I often just add some series resistors, especially when driving bidirectional busses, LCDs are an example. When all is finished in the final design those can go, scoping for bus conflicts is often easy.
Here you can see those resistors that are not in the circuit diagram, but are in the test setup:
scroll down to last picture from bottom. More relaxed coding.
I don't see anything in the limiting values that would even apply to this o ther than ground or supply current which clearly is not an issue for a sing le pin. The I/O latch up current only applies when the parasitic diodes ar e conducting, not normal current through the output transistors.
So even without the specific spec for short circuit current I don't see whe re it would be expected to cause latch up.
What am I missing?
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