#define F_CPU 20000000

#include <avr/io.h>
#include <util/delay.h>

//note division factor of 8 by default in CLKPS register

#define output_low(port,pin) port &= ~(1<<pin)
#define output_high(port,pin) port |= (1<<pin)
#define set_input(portdir,pin) portdir &= ~(1<<pin)
#define set_output(portdir,pin) portdir |= (1<<pin)

#define LED PA7

void delay_ms(uint16_t millis) {
 while ( millis ) {
 _delay_ms(1);
 millis--;
 }
}

int main(void) {

 CLKPR = (1 << CLKPCE); //set clkps register to not divide by 8
 CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0);

 DDRA |= (1<< LED); // PA7 output
 
 while(1) {
   // turn on the LED
   output_high(PORTA, LED);
   delay_ms(1000);
   // now turn off the LED 
   output_low(PORTA, LED);
   delay_ms(1000);
 }
  
 return 0;
}