1. Configure I/O Digital Pins

Usually there are 8 I/O pins in each port in an Atmel AVR microcontroller. These pins can be set as input or output from the Data Direction Register (DDRx), and can be set in HIGH, LOW, or tri state from Pin Output Register (PORTx).

 

Example 1: set PIN0 - PIN4 in PORTA to output HIGH, and PIN5-PIN7 in port A to output LOW

 

DDRA = 0b11111111; //Set all pins in PORTA as outputs

PORTA = 0b00011111; //Set PIN0-PIN4 to HIGH , while the rest of pins stay LOW

 

Alternatively, the code in Example 1 can be written as:

 

DDRA = 0xFF; //Set all pins in PORTA as outputs

PORTA = 0x1F; //Set PIN0-PIN4 to HIGH , while the rest of pins stay LOW

 

In many cases, only a specific pin needs to be changed while other pins remain the same. The code below then becomes more efficient with an OR operation.

 

Example 2: set PIN 4 in PORTA to output HIGH, while the rest of pins stay at the original stage

 

DDRA |= 0b00010000; //Set PIN4 in PORTA as an output

PORTA |= 0b00010000; //Set PIN4 in PORTA to HIGH

 

Alternatively, the code in Example 2 can be written as:

 

DDRA |= 1 << 4; //Set PIN4 in PORTA as an output

PORTA |= 1 << 4; //Set PIN4 in PORTA to HIGH

 

There is no direct way to set 0 to a register bit. The best way is to use NOT + AND operation.

 

Example 3: Set PIN4 in PORTA to input and make the pin in tri state

 

DDRA &= ~(1 << 4); //Set PIN4 in PORTA as an input

PORTA &= ~(1 << 4); //Set PIN4 in PORTA to tri state

 

When the pin is tri-stated, the internal pull-up resistor is disabled, so the pin needs to connect to an external pull-up or pull-down resistor.

 

Example 4: Write an if statement to do something when the input pin senses a HIGH signal

 

DDRA &= ~(1 << 4); //Set PIN4 in PORTA as an input

PORTA &= ~(1 << 4); //Set PIN4 in PORTA to tri state, the pin needs to connect to a pull-down resistor

if (bit_is_set (PINA, 4))

{

     Do something

}

 

Example 5: Write an if statement to do something when the input pin senses a LOW signal

 

DDRA &= ~(1 << 4); //Set PIN4 in PORTA as an input

PORTA |= 1 << 4; //Set PIN4 in PORTA to HIGH or enable the pull-up

if (bit_is_clear (PINA, 4))

{

     Do something

}