Simple demonstration of SPI for atmega16 as slave

atmega16_spi_slave.c

#ifndef F_CPU
#define F_CPU 1000000UL
#endif

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

#define ACK 0xF8

void spi_init_slave(void);
uint8_t spi_tranceiver(uint8_t data);

void spi_init_slave(void)
{
    DDRB = (1<<PB6);                            //MISO as OUTPUT
    SPCR = (1<<SPE);                            //Enable SPI
}

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

int main(void)
{
    uint8_t data;
    
    spi_init_slave();                            //Initialize slave SPI
    DDRA  = 0xFF;                                //Initialize PORTA as OUTPUT
    
    while(1)
    {
        data = spi_tranceiver(ACK);               //Receive data, send ACK
        PORTA = data;                             //Display data on PORTA
        _delay_ms(20);
    }
}