Simple demonstration of SPI for atmega16 as master

atmega16_spi_master.c

#ifndef F_CPU
#define F_CPU 1000000UL
#endif

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

#define ACK 0xF8

void spi_init_master(void);
uint8_t spi_tranceiver(uint8_t data);
void led_blink(uint16_t i);

//Initialize SPI Master Device
void spi_init_master(void)
{
    DDRB = (1<<PB7)|(1<<PB5)|(1<<PB4);      //Set MOSI, SCK, SS as Output
    SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);    //Enable SPI, Set as Master
                                            //Prescaler: Fosc/16, Enable Interrupts
}

//Function to send and receive data
uint8_t spi_tranceiver(uint8_t data)
{
    SPDR = data;                        //Load data into the buffer
    while(!(SPSR & (1<<SPIF) ));        //Wait until transmission complete
    return(SPDR);                       //Return received data
}

//Function to blink LED
void led_blink(uint16_t i)
{
    //Blink LED "i" number of times
    for (; i>0; --i)
    {
        PORTA|=(1<<0);
        _delay_ms(100);
        PORTA=(0<<0);
        _delay_ms(100);
    }
}

int main(void)
{
    spi_init_master();                   //Initialize SPI Master
    DDRA |= 0x01;                        //PA0 as Output
    
    uint8_t data;                        //Received data stored here
    uint8_t x = 0x00;                    //Index value of led to light
    
    while(1)
    {
        data = 0x00;                     //Reset ACK in "data"
        data = spi_tranceiver(1<<x++);   //Send number, receive ACK in "data"
        
        //if index is on the last position, return to the first LED
        if(x>=8) x=0;
        
        //If received data is not the same as ACK, blink LED 3 times
        if(data != ACK) led_blink(3);
        
        _delay_ms(500);
    }
}