// Button  PB2 (pin 11).
// LED     PA2 (pin 6).
#include <avr/io.h>

#define output(directions,pin) (directions |= pin)     // macro used to set port direction for output (set to '1')
#define input(directions,pin) (directions &= (~pin))  // macro used to set port direction for input (set to '0')
#define set(port,pin) (port |= pin)           // macro used to set port pin
#define clear(port,pin) (port &= (~pin))        // macro used to clear port pin
#define pin_test(pins,pin) (pins & pin)         // macro used to test for port pin
#define bit_test(byte,bit) (byte & (1 << bit))      // macro used to test for bit set
//
//  The following definitions are specific to the particular configuration of LEDs and buttons on pins.  
//  These are the only things that need to change when you move to other pins or add devices.
//
#define input_port PORTB   // The button is on port A
#define input_direction DDRB  //  DDRA defines input/output direction for port A
#define input_pin (1 << PB2)  //  The button is on pin 2 of port A
#define input_pins PINB     //  PINA is the register that is read to detect input high or low.
#define output_port PORTA   //  port A will be used for the LED
#define output_direction DDRA //  DDRA defines input/output direction for port A
#define output_pin (1 << PA2) //  The LED is on pin 7 of port A



int main(void) {              //  This is the main function
  //
    // initialize pins
    //
   output(output_direction, output_pin);  // sets the proper bit for output to the LED  
   input(input_direction, input_pin);   // sets the appropriate bit to enable input on the input pin.
   set(input_port, input_pin);        // turn on pull-up resistor on the input (button) pin
   //
   // main loop
   //
   while (1) {                // loops forever
      if (pin_test(input_pins,input_pin)) // test to detect button push.  Result is true if button is up.
        clear(output_port,output_pin);    // set output low if button is up.
    else                  // otherwise... 
      set(output_port,output_pin);    // set output high.
      }
   }