Electronic – Do AVR registers and ports need to be initialized to zero


During the initialization routine of my code I use to do such things as:

clr    r0  ; will always stay zero


out    PORTA, r0; initialize ports
out    DDRA, r0
out    PORTB, r0

Is this actually necessary? Or can I be sure this is automatically done upon reset? Especially, can I rely on all ports to be set as inputs by default so there is no problem with external votages when no code is executed?

Best Answer

The I/O ports of an AVR are set to INPUT / Tri-State / Hi-Z (DDRx = 0x00) upon reset. Most microcontrollers (if not all?) have this behavior. It's the safest state for a pin to be in. So yes, you can rely on the ports to be set automatically as inputs.

Some excerpts an the ATmega16 show exactly that:

enter image description here

The Port C pins are tri-stated when a reset condition becomes active, even if the clock is not running.