Accessing AVR microcontroller ports with AVR GCC

All AVR ports have Read-modify-write functionality when used for general I/O functionality. Direction of each separate port pin can be changed. Each pin buffer has symmetric capability with ability to drive and sink source. Pin driver is strong enough to drive LED directly, but it is not recommended to drive even small loads without using proper driver circuit such as transistors. All port pins have internal selectable pull-up resistors. And finally all pins have protection diodes to both VCC and GND rails.

Each port of AVR consists of three registers DDRx, PORTx and PINx (where x means port letter). DDRx register selects direction of port pins. If logic one is written to DDRx then port is configured to be as output. Zero means that port is configured as input. If DDRx is written zero and PORTx is written logic “1” then port is configured as input with internal pull-up resistor. Otherwise if PORTx is written to zero, then port is configured as input but pins are set to tri-state and you might need to connect external pull-up resistors.

If PORTx is written to logic “1” and DDRx is set to “1”, then port pin is driven high. And if PORTx=0, then port is driven low.

How we can control AVR port pins using C language? When using AVR GCC first we need to set up proper library where PORT register names have their addresses defined. Main library where general purpose registers are defined is io.h located in avr directory of AVR GCC installation location.

#include <avr/io.h>

Now we can use port names instead of their addresses. For instance if we want to set all pins of PORTD as output we simply write:

DDRD=0xFF; //set port D pins as outputs

Now we can output a number to port D:

PORTD=0x0F; //port pins will be set to 00001111

if we have 8 bit variable i we can assign this variable to port register like this:

uint8_t i=0x54;


Lets read port D pins:

DDRD=0; //set all port D pins as input

i=PIND; //read all 8 pin bits of port Dand store to variable i

There is ability to access separate port pins. So all eight port pins can be used for multiple purposes.

Some of the pins may be configured as outputs and some as inputs and performs different functions.

Lets say we need 0,2,4,6 pins to be as input and 1,3,5,7 as output. Then we do like this:

DDRD=0; //reset all bits to zero

DDRD |=(1<<1)|(1<<3)|(1<<5)|(1<<7); //using bit shift “<<”operation and logical OR to set bits 1,3,5,7 to “1”

So we can output values to 1,3,5 and 7 pins

PORTD |=(1<<1)|(1<<3)|(1<<5)|(1<<7);

Or clear them

PORTD &=~((1<<1)|(1<<3)|(1<<5)|(1<<7));

Reading of port pins is easy. Set any pin(s) for input like this:

DDRD &=~((1<<1)|(1<<3)); //This clears bits 1 and 3 of port direction register

i=PIND; //reads all 8 pins of port D

You can read 1 and 3 bits by using masks or simply shift i value by 1 ar 3 positions to compare LSB with 1. Of course there are some functions in sfr_defs.h library like bit_is_set() or bit_is_clear() to check particular bits and make these tasks little easier.

Following example should clarify some issues:

#include "avr\io.h"
#include "avr\iom8.h"
int main(void) {
  DDRD&=~_BV(0);//set PORTD pin0 to zero as input
  PORTD|=_BV(0);//Enable pull up
  PORTD|=_BV(1);//led OFF
  DDRD|=_BV(1);//set PORTD pin1 to one as output
  while(1) {
   if (bit_is_clear(PIND, 0))//if button is pressed
			PORTD&=~_BV(1);//led ON
			loop_until_bit_is_set(PIND, 0);//LED ON while Button is pressd
			PORTD|=_BV(1);//led OFF



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