mysterious failure on pin 11

hey all,

I was running my arduino uno for a couple of hours switching a few led’s on and off. nothing special, flickering them on for around 10millis then off for around 5 seconds. pins 8,9,10,11,12,13 are all wired the same way:-

pin → led → resistor (270ohm) → gnd

all are wired seperately (i.e. 6 led’s and 6 resistors).

this has been working just fine for a few weeks, I use it as a test board. then, suddenly, pin 11 stopped working. nothing unusual, it was sat on my desk merrily blinking away, then suddenly pin 11 stopped blinking.

I’ve measured the output from the pin, the others all read 4.3v and pin11 reads 0.25v, and if I PWM it then it does change, it just maxes out at 0.25v… as though this pin has been “turned down” compared to the others.

I should probably note that I measured it at the pins directly on the chip, so I know it’s definately the chip, not a bad connection on the board.

I suspect the pin is “broken”, is this the case or am I missing something? Is it usual for a broken pin to still be responsive in this way? (namely, still responding and scaling with PWM, but at a much lower voltage).

if so, can i just pop out the chip and pop in a new one?

– edit –
forgot to say which model of arduino i have, it’s the UNO :slight_smile:

aaaaand updated with the right resistor value, because my blinky eyes don’t seem to be working today!

10 ohm?
Sure about that?
Can you read the bands on the resistor?

I double-checked, 270ohm, thank god for multimeters, I mis-read the value in the bad light.

While it is unusual for an output pin to fail it is not impossible.
You were only sourcing about 72mA in total (assuming they were red LEDs ) so you were not exceeding any limits.

Had the board been subject to some stress in the past? Like running an LED without a resistor or feeding a voltage higher than 5V or lower than 0V? That could increase the chance of failure.

Otherwise it is just one of those things, it is called "infant mortality" or early life failure in the trade.

yeap, they were just plain old red leds. nothing special, just flickering them on and off. I've looked after the board fairly well, only had it a couple of weeks. I'm a developer by trade and figured this would be a cool way to go from software to hardware.

I guess i'm just unlucky enough to be the one in a million that got a dodgy chip. :frowning:

in this case, what's the best way to replace the chip? I know that in theory I can use the arduino to program another avr chip, but that seems to rely on pin11 operating as the MOSI pin... and that's the pin that appears to be broken.

I think the easiest thing is to buy another 328 chip with a boot loader ready burnt in. They are quite readily available.

Don't throw the old chip away, it is still good for building permanently into a project, not every project needs pin 11.

yeah, that's what I figured. I just checked and I can grab a pre-burned chip from farnell for a couple of quid, so it's not going to be a huge problem to replace it.

I figure I can use this chip for a while, at least until my shiny new ones arrive. Like you say, not everything needs every pin.

Thanks for the quick help guys, nice to see an active community, and even though it wasn't good news, at least it's just down to dumb luck and not anything more serious.


Damaged components do not always completely fail at the time of damage.