Do AVR registers and ports need to be initialized to zero?

Question

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

clr    r0  ; will always stay zero

and:

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?

Answer 1

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:

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

Answer 2

Neither registers nor SRAM is initialized upon reset, only some of the peripheral registers. You should initialize things you use.

Answer 3

• Port initialisation is ALWAYS a good idea, regardless of what the data sheet says.

• If the datasheet says nothing then it is an utterly vital idea.

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You only need to define the port data contents if you care about what will happen when your program runs.

If you don't care about the result you don't have to set the port data bits :-).

If manufacturers say explicitly in the data sheets that port data bits are set or cleared then they may be BUT it is STILL a really really good idea to initialise them yourself anyway. "Boundary conditions" are where most things go wrong - eg start of a loop, end of a loop, circular buffer wrap round point, ... . Processor startup is a hardware equivalent. In a real world with noise and glitches and people in it, being in charge of your programs destiny as much as you can is a really good idea. Port initialisation is an easy part of this.