How to check if a certain PIN on PORTx is pressed

Question

So I have an assignment that wants me to perform certain code when a button on PD2 is presed.

The problem i'am having right now is i don't really know what to check for neither do i understand the underlying logic.

So this is the code i've come up with thus far.

 DDRD=0x00;   //PORTD pin 0 as input
    PORTD=0x00;
    
    DDRB=0xFF;   //PORTB as output
    PORTB=0x00;
    
    
    while(1){
        if (PIND & (1<<PD2)==1) // check if PD2 is pressed on PIND2
        {
            // modify bits on PORTB here
        }
        
        }

I'm using Atmega328 and running this on AtmelStudio 7

Answer 1

unsigned char input_byte;
while(1){
    __no_operation(); /* using nop for synchronization */
    input_byte = PIND; /* read entire PIND */
}

This way you should be able to read your inputs every program cycle. But I've not tested this. It's important to use __no_operation() before reading again PIND.

Answer 2

To set PD2 to an input, run this C/C++ code, which clears bit 2 in register DDRD. There is no need to modify entire registers:

DDRD &= ~(1 << 2);

(You could skip this because every pin will be an input by default after the AVR resets.)

I don't know how you wired your button. If the button is wired between the pin and GND, and there is no external pull-up resistor to pull your pin high, then you should enable the internal pull-up after the pin is an input by running this code, which sets bit 2 in register PORTD:

PORTD |= 1 << 2;

Now to read the state of the button, you can use this C/C++ expression, which will evaluate to 0 if the pin is low or non-zero if the pin is high:

PIND & (1 << 2)

The expression below also works. It has the advantage of always evaluating to 0 or 1:

PIND >> 2 & 1

Here's some (untested) code that ties it all together, reading from PD2 and writing the value of PD2 to an output on PB3:

#include <avr/io.h>

int main()
{
  // Make PD2 an input, pulled high.
  DDRD &= ~(1 << 2);
  PORTD |= 1 << 2;

  // Make PB3 an output.
  DDRB |= 1 << 3;

  while (1)
  {
    if (PIND >> 2 & 1)
    {
      // PD2 is high, so drive PB3 high.
      PORTB |= 1 << 3;
    }
    else
    {
      // PD2 is low, so drive PB3 low.
      PORTB &= ~(1 << 3);
    } 
  }
}

You might have to adapt the part of the code that deals with the output. I don't actually know what pin your output is on and what kind of behavior you want, so I just did something simple.

This code is full of the C bitwise operators. It is important for you to get a good book about C and learn exactly what those operators do, and this answer would be too long if I attempted to teach you all of them. Here is a list of operators for you to learn: <<, >>, &, &=, |, |=, ~.