Can LPC812 pins (NXP ARM uC) be shorted briefly?

Is it ok to short a port pin on LPC821 micro controllers to GND and maybe also to VDD for a second without frying it? Only one pin at a time.

I'll be receiving systems with weird field failures tomorrow and I won't have a chance to change a port pin status in the code.

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Regards, Joerg 

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Reply to
Joerg
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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?

Rick C.

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Reply to
gnuarm.deletethisbit

tirsdag den 15. januar 2019 kl. 00.57.12 UTC+1 skrev Joerg:

I assume you mean lpc812, the datasheet says that high/low short circuit current is 45/50mA and allowed as long as it doesn't exceed maximum device current, which I assume is the 100mA per supply pin

Reply to
Lasse Langwadt Christensen

Ground is certainly safe, and Vdd probably OK. The n-fets typically turn on harder than the p-fets.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

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.

Sylvia.

Reply to
Sylvia Else

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?

Reply to
mpm

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.

Don't let its battery get too cold :-)

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Reply to
Joerg

That's just it, when you hardwire an output to the opposite side.

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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.
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Regards, Joerg 

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Reply to
Joerg

Yes but this device has unusually low Rdson when it is pulling up. They state a 400mV drop at 20mA worst case so typically even better.

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Reply to
Joerg

Ugh. That would also not be cool because then whatever tests I do may not be valid.

That would sure cause its MCU to misfire.

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Regards, Joerg 

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Reply to
Joerg

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.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

So the current would probably level off below 100 mA. That should be OK for a second.

To reduce dissipation, pull the pin up or down through a diode.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

the output might become current limited rather than just simple Rdson above at higher currents?

page 34 lists, VOH = 0 V and VOL = VDD as max 45/50mA

Reply to
Lasse Langwadt Christensen

Hi

It is NOT ok to short a pin. The resulting current is above abs max and can trigger all device latch up frying the entire device. Alternatively it wil l fry the port or internal metal layers

We have just resently done tests on another company?s micro, and sh orting fried it so it continuously drew to much current and no way to conne ct to it

Cheers

Klaus

Reply to
Klaus Kragelund

r
s

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.

Rick C.

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Reply to
gnuarm.deletethisbit

an trigger all device latch up frying the entire device. Alternatively it w ill fry the port or internal metal layers

shorting fried it so it continuously drew to much current and no way to con nect to it

Which absolute max does this violate? I can't find it.

Rick C.

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Reply to
gnuarm.deletethisbit

Page 31 abs max ratings

But, page 34 lists:

LOW-level short-circuit output current

Never seen this before for a microcontroller

So seems it?s okay

Reply to
Klaus Kragelund

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:

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scroll down to last picture from bottom. More relaxed coding.

Reply to
<698839253X6D445TD

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?

Rick C.

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Reply to
gnuarm.deletethisbit

How about running the thing on a lower voltage and/or choosing a forcing resistor that gets you to within 0.4v of the rail you want to force to.

That should be a little less stressful on the output you are fighting whilst still being a good logic level for the other device input.

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Regards, 
Martin Brown
Reply to
Martin Brown

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