I have a RasPi B+ running Raspbian and have connected an LED to GPIO 16 (pin 36) and another LED to GPIO 12 (pin 32).
It is my understanding that the GPIO ports are all off (low) by default and return to that state after a power reset. So, as expected, the LED on GPIO 12 is off when the device boots and only comes on when I pragmatically control it.
The LED on GPIO 16, however, comes on immediately when the Pi is powered on and stays on. During shutdown (running
sudo poweroff), the LED begins to blink, then turns off for a short period before turning back on once the system is halted.
I can change the state of the LED pragmatically as well, but it seems something on the Pi itself is using that pin -- but I can't tell what or for what purpose.
Can anyone explain why GPIO 16 appears to be on by default and what is using the pin, causing it to change state during shutdown?