595-blink

#define __spi_clock 52      // SCK - hardware SPI
#define __spi_data_in 50    // MISO - hardware SPI (unused)
#define __spi_data 51       // MOSI - hardware SPI
#define __spi_latch 53
#define __LATCH_LOW PORTC &= ~_BV(PC0)
#define __LATCH_HIGH PORTC |= _BV(PC0)

#define __display_enable 49
#define __DISPLAY_ON PORTD &= ~_BV(PD6)
#define __DISPLAY_OFF PORTD |= _BV(PD6)

#include <avr/io.h>
#include <avr/interrupt.h>
#include <stdint.h>

void setup(void)
{
    pinMode(__spi_clock, OUTPUT);
    pinMode(__spi_latch, OUTPUT);
    pinMode(__spi_data, OUTPUT);
    pinMode(__spi_data_in, INPUT);
    pinMode(__display_enable, OUTPUT);
    digitalWrite(__spi_latch, LOW);
    digitalWrite(__spi_data, LOW);
    digitalWrite(__spi_clock, LOW);
    setup_hardware_spi();
}

void loop(void)
{
    __DISPLAY_OFF;
    __LATCH_LOW;
    spi_transfer(0xFF);
    spi_transfer(0xFF);
    spi_transfer(0xFF);
    spi_transfer(0xFF);
    __LATCH_HIGH;
        __DISPLAY_ON;

    delay(1000);

    __DISPLAY_OFF;
    __LATCH_LOW;
    spi_transfer(0x00);
    spi_transfer(0x00);
    spi_transfer(0x00);
    spi_transfer(0x00);
    __LATCH_HIGH;
        __DISPLAY_ON;

    delay(1000);
}

void setup_hardware_spi(void)
{
    uint8_t clr;
    // spi prescaler:
    //
    // SPCR: SPR1 SPR0
    // SPSR: SPI2X
    //
    // SPI2X SPR1 SPR0
    //   0     0     0    fosc/4
    //   0     0     1    fosc/16
    //   0     1     0    fosc/64
    //   0     1     1    fosc/128
    //   1     0     0    fosc/2
    //   1     0     1    fosc/8
    //   1     1     0    fosc/32
    //   1     1     1    fosc/64

    /* enable SPI as master */
    SPCR |= (_BV(SPE) | _BV(MSTR));
    /* clear registers */
    clr = SPSR;
    clr = SPDR;
    /* set prescaler to fosc/2 */
    SPCR &= ~(_BV(SPR1) | _BV(SPR0));
    SPSR |= _BV(SPI2X);
}

uint8_t spi_transfer(uint8_t data)
{
    SPDR = data;        // Start the transmission
    while (!(SPSR & _BV(SPIF))) // Wait the end of the transmission
    {
    };
    return SPDR;        // return the received byte. (we don't need that here)
}