spi_test_master.c

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

int main() {
  uint8_t index;

  DDRB=(1<<PB5)|(1<<PB3);

  /* Enable SPI, Master, set clock rate fck/128 */
  SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR1)|(1<<SPR0);

  _delay_ms(2000);
  index = 0;
  while (1) {
    SPDR = index;
    while(!(SPSR & (1<<SPIF)));
    index = (index + 1) % 10;
    _delay_ms(5000);
  }
}

spi_test_slave.c

#include <avr/io.h>
#include <util/delay.h>
#include "uart.h"
#include "printk.h"

int main() {
  uart_init();

  DDRB = (1<<PD6);   //MISO as OUTPUT
  SPCR = (1<<SPE);   //Enable SPI

  printk("Start slave program\r\n");
  uint8_t data = 0;
  while (1) {
    SPDR = 0xff;

    while(!(SPSR & (1<<SPIF)));
    data = SPDR;

    printk("data %d\r\n", data);
  }
}