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I was driving a relay from Arduino Uno R3 Pin 13. I observed that every time I upload some code the led on the board designating pin 13 would blink. I'm curious why this is so. I understand that the serial LED blinks cause there's a serial transmission but why an IO pin?

asked Oct 14, 2015 at 5:10

3 Answers 3

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See What happens when code is uploaded using the bootloader? - the flashing is by design.

Optiboot flashing pin 13

Probably the reason is to confirm that the bootloader has started.

answered Oct 14, 2015 at 6:10
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It's driven by the bootloader, which is performing the flashing (I suppose you are flashing through the USB port).

answered Oct 14, 2015 at 5:19
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To show that the bootloader is functioning satisfactorily to the hobbyist. Boot loader helps to upload new sketch on the chip.

Butzke
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answered Oct 15, 2015 at 17:44

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