printk.c

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

void printk(const char *fmt, ...) {
  char str[64];
  uint8_t pos = 0;
  va_list args;

  va_start(args, fmt);
  vsprintf(str, fmt, args);

  while(str[pos] && pos < 64) {
    uart_transmit(str[pos]);
    pos++;
  }

  va_end(args);
}

spi_test_master.c

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

uint8_t spi_transceive(uint8_t data) {
  SPDR = data;
  while(!(SPSR & (1<<SPIF)));
  return SPDR;
}

int main() {
  uart_init();

  DDRB=(1<<PB5)|(1<<PB3)|(1<<PB2);
  PORTB |= (1<<PB2);

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

  _delay_ms(2000);

  printk("Start master program\r\n");
  uint8_t index = 0;
  uint8_t data = 0;

  while (1) {
    PORTB &= ~(1<<PB2);
    data = spi_transceive(index);
    _delay_ms(1);
    PORTB |= (1<<PB2);
    printk("data %d\r\n", data);
    index = (index + 1) % 10;
    _delay_ms(1000);
  }
}

spi_test_slave.c

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

uint8_t spi_transceive(uint8_t data) {
  SPDR = data;
  while(!(SPSR & (1<<SPIF)));
  return SPDR;
}

int main() {
  uart_init();

  DDRB = (1<<PB4);   //MISO as OUTPUT
  SPCR = (1<<SPE);   //Enable SPI

  printk("Start slave program\r\n");
  uint8_t index = 0;
  uint8_t data = 0;
  while (1) {
    data = spi_transceive(index);
    printk("data %d\r\n", data);
    index = (index + 1) % 10;
  }
}

uart.h

#include <avr/io.h>

#define BAUD 38400
#define BAUDRATE ((F_CPU)/(BAUD*16UL)-1)

void uart_init();
void uart_transmit(uint8_t data);
uint8_t uart_recieve();