/* defines */

#define F_CPU 1200000   /* for ATtiny13, running at 1.2MHz  */

#define DATAPORT PORTB
#define DATAPINS PINB
#define CSPIN    2      /* PB2 (Pin#7) */
#define MCPIN    1      /* PB1 (Pin#6) */
#define MDIPIN   0      /* PB0 (Pin#5) */

/* macros */

#define CS_low()   DATAPORT &= ~_BV(CSPIN)
#define CS_high()  DATAPORT |=  _BV(CSPIN)
#define MDI_low()  DATAPORT &= ~_BV(MDIPIN)
#define MDI_high() DATAPORT |=  _BV(MDIPIN)
#define MC_tick()  { DATAPORT |= _BV(MCPIN); _delay_us(1); DATAPORT &= ~_BV(MCPIN); }

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

void write_register(uint8_t reg, uint8_t value)
{
    uint16_t word= (reg << 8) | value;

    CS_low();
    for (uint8_t i=0; i<16; i++)
    {
        if (word & 0x8000)
            MDI_high();
        else
            MDI_low();
        MC_tick();
        word <<= 1;
    }
    CS_high();
}


int main(void)
{
    /* hardware init*/
    /* all pins input, except CS, MC, MDI */
    DATAPINS=(_BV(CSPIN)|_BV(MCPIN)|_BV(MDIPIN));
    /* all pins H (pullups enabled), except MC */
    DATAPORT=~_BV(MCPIN);

    /* write DAC register(s) */
    write_register(18, 0x50);

    /* endless loop */
    while (1) ;
}