AVR SPI Master in C: Driving 74HC595 Shift Registers with SPCR and SPDR

#define F_CPU 16000000UL

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

#define LATCH_PIN PB2              /* also the SPI SS pin */

static void spi_master_init(void)
{
    /* MOSI, SCK and SS as outputs; MISO stays input */
    DDRB |= (1 << DDB3) | (1 << DDB5) | (1 << DDB2);
    PORTB &= ~(1 << LATCH_PIN);

    /* Enable SPI, master, mode 0, MSB first, f/16 = 1 MHz */
    SPCR = (1 << SPE) | (1 << MSTR) | (1 << SPR0);
    SPSR = 0;                      /* SPI2X off */
}

/* Send one byte and return the byte received at the same time */
static uint8_t spi_transfer(uint8_t data)
{
    SPDR = data;                   /* start transmission */
    while (!(SPSR & (1 << SPIF)))  /* wait for completion */
        ;
    return SPDR;                   /* reading SPDR clears SPIF */
}

/* Write 16 bits to two chained 74HC595s */
static void shift_out16(uint16_t value)
{
    spi_transfer(value >> 8);      /* goes to the far chip (chip 2) */
    spi_transfer(value & 0xFF);    /* stays in the near chip (chip 1) */
    PORTB |= (1 << LATCH_PIN);     /* rising edge copies to outputs */
    PORTB &= ~(1 << LATCH_PIN);
}

int main(void)
{
    spi_master_init();

    while (1) {
        for (uint8_t i = 0; i < 16; i++) {      /* move right to left */
            shift_out16((uint16_t)1 << i);
            _delay_ms(60);
        }
        for (int8_t i = 14; i > 0; i--) {       /* and back */
            shift_out16((uint16_t)1 << i);
            _delay_ms(60);
        }
    }
}