#define F_CPU 8000000UL

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

#define SCK         3
#define MOSI        1
#define CSN         2

uint8_t data = 0;
uint8_t reg = 0;


int main(void)
{
    PRR = 0; // turn off power reduction
    REMAP |= (1<<SPIMAP); // remap SPI pins
    // set SCK, MOSI, CSN as outputs:
    DDRA = (1<<SCK) | (1<<MOSI) | (1<<CSN);
    // enable SPI, set as master, use f_clkIO/16 (500kHz):
    SPCR = (1<<SPE) | (1<<MSTR) | (1<<SPR0);
    reg = SPSR;
    reg = SPDR;

    while(1)
    {
        PORTA &= ~(1 << CSN); // clear CSN
        SPDR = data++; // send byte
        while( !(SPSR & (1<<SPIF)) ); // wait for flag to set
        PORTA |= (1 << CSN); // set CSN
        _delay_ms(10);
    }

    return 0;
}