MegaCore ATmega2560 PB7 activation randomly at boot

Question

I uploaded in my custom board (ATmega2560 with an external oscillator 16MHz) the MegaCore bootloader using the AVR pinout. I found that when I switch on the board the pin PB7 is activated 2 short times. This happens even with a blank Arduino sketch.

I found out that the problem was the reset pin connection. When the reset pin (for uploading a new sketch) is connected to the board I got the previous behaviour.

How can the reset pin affect only the pin PB7?

PS I found this issue even in the Arduino Mega standard bootloader.

Answer 1

PB7 is pin 13. That's the traditional LED pin.

The bootloader blinks that LED to indicate it's active.

If you want other behaviour then you will have to compile your own version of the bootloader that doesn't blink the LED.