spi.h

#include <avr/io.h>

#define SS 4
#define MOSI 5
#define MISO 6
#define SCK 7
#define SS_Enable PORTB &= ~(1<<SS)
#define SS_Disable PORTB |= (1<<SS)

void SPI_Master_Init()
{
    DDRB |= (1<<MOSI)|(1<<SCK)|(1<<SS);
    DDRB &= ~(1<<MISO);
    SS_Disable;
    SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
    SPSR &= ~(1<<SPI2X);
}

void SPI_Slave_Init()
{
    DDRB &= ~((1<<MOSI)|(1<<SCK)|(1<<SS));
    DDRB |= (1<<MISO);
    SPCR = (1<<SPE);
}

void SPI_Master_Write(char data)
{
    char flush_buffer;
    SPDR = data;
    while(!(SPSR & (1<<SPIF)));
    flush_buffer = SPDR;
}

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

char SPI_Master_Read()
{
    SPDR = 0xFF;
    while(!(SPSR & (1<<SPIF)));
    return(SPDR);
}

char SPI_Slave_Receive()
{
    while(!(SPSR & (1<<SPIF)));
    return(SPDR);
}