LEDpin and bootloader

This might be a stupid question, but is pin 13 supposed to go high for about 0.1 seconds right after power up or when the reset button is pushed?

I’m trying to find out if it’s one of the libraries I’m using or the bootloader that’s causing it, but I’m trying to send out very short (in the microseconds) high current pulses on that pin and it’s giving me a a lot of trouble.

If it is the bootloader, it’s not a huge problem since when my code is finished I’m planning to use an AVR-ISP to program the chip without the bootloader anyways, but it will be nice to know.


I just tried putting a LED on pin 13 of my old Arduino NG board, and just plugging the USB into the computer lights the LED for a short moment. This happens almost the instant the USB is connected, in my eyes that makes it more likely that it is the bootloader.

Exactly, it happens right away when I apply power or press the reset button which is why I suspected the bootloader when I started this thread. It seems like a strange thing for the bootloader to do though.

I find it very surprising nobody’s commented on this before.

I’m using a Diecimilia.

I just tried again. Watching very carefully, it looks like it’s actually flashing the LED 3 times very very fast.

I noticed this when I was designing my car keypad. I was going to have pin 13 close a solid state relay on a certain command, but I moved it to another pin when I noticed this.

Yes, pin 13 is toggled by the bootloader. It’s the status LED and should also provide enough information to determine which bootloader you’re running.

Any other pin should be fine, or if you must use 13, you can use another pin to enable a latch/buffer or transistor after the sketch starts. Or you could burn a bootloader without the LED code.