#include <util/delay.h>

Написал вот такой код:
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

#define UBRRVAL 47

#define SPI_PORT PORTB
#define SPI_DDR DDRB

#define MOSI_P PB3
#define MISO_P PB4
#define SCK_P PB5
#define SS_P PB2

#define SELECT() SPI_PORT &= ~(1 << SS_P)
#define DESELECT() SPI_PORT |= (1 << SS_P)

unsigned char command,statbuf,buf;

void delay (unsigned int d,char s) {
    unsigned int i;
    if (s=='u') {
        for(i=0;i<d;i++) _delay_us(1);
    }
        if (s=='m') {
        for(i=0;i<d;i++) _delay_ms(1);
    }
}

void USART_init(void) {
    
    UBRRL=UBRRVAL;            
    UBRRH=(UBRRVAL>>8);    
    UCSRC=(1<<URSEL)|(0<<UMSEL)|(0<<UPM1)|(0<<UPM0)|(0<<USBS)|(0<<UCSZ2)|(1<<UCSZ1)|(1<<UCSZ0);    
    UCSRB=(1<<RXEN)|(1<<TXEN)|(1<<RXCIE);
}
void USART_Transmit(unsigned char data) {

    while ( !( UCSRA & (1<<UDRE)) ) {};
    UDR = data;
}

void SPI_init(void) {

    SPI_DDR = (1 << MOSI_P)|(1 << SCK_P)|(1 << SS_P);
    SPI_PORT = (1 << SS_P);
    SPCR = (1 << SPE)|(1 << MSTR)|(1 << SPR1)|(1 << SPR0); // F_CPU/128
    
}
/*
unsigned char spi_shift_byte(unsigned char data) {
    
    SPDR = data;
    while(!(SPSR & (1 << SPIF)));    
    return SPDR;
}

unsigned char RREG(unsigned char start_address) {
    unsigned char buffer;
    SELECT();
    spi_shift_byte(0x10 | (start_address)); // 0001 rrrr
    spi_shift_byte(0x01); // xxxx nnnn, почему "минус 1" - написано в документации
    _delay_loop_2(250);
    buffer = spi_shift_byte(0xAA); // Читаем count регистров в массив buffer
    DESELECT();
    return buffer;
}

void WREG(unsigned char address, unsigned char value) {
    SELECT();
    spi_shift_byte(0x50 | (address)); // 0101 rrrr
    spi_shift_byte(0x00); // xxxx nnn, почему "минус 1" - написано в документации
    spi_shift_byte(value); // Передаем count значений из массива buffer
    DESELECT();
}
*/
ISR (USART_RXC_vect) {
    
    while ( !(UCSRA & (1<<RXC)) );
    buf=UDR;
    statbuf=0x01;
}

void processing (void) {

    unsigned char SPI_buf,SPI_buf1;
    
    if ((statbuf==0x01) && (command==0x00)) {
            command=buf;
            buf=0x00;
            statbuf=0x00;
    }
    if  (command!=0x00) {
        switch (command) {
                case 0x31: {
                            USART_Transmit(0x01);
                            command=0x00;
                            break;    
                }
                case 0x32: {
                            
                                                
                                                
                                                /*
                                                SELECT();
                                                    SPDR = 0x50;
                                                    while(!(SPSR & (1 << SPIF)));
                                                    delay(5,'u');
                                                    SPDR = 0x01;
                                                    while(!(SPSR & (1 << SPIF)));
                                                    delay(5,'u');
                                                    SPDR = 0x03;
                                                    while(!(SPSR & (1 << SPIF)));
                                                    delay(5,'u');
                                                    SPDR = 0x03;
                                                    while(!(SPSR & (1 << SPIF)));
                                                    delay(5,'u');
                                                DESELECT();
                                                */
                                                
                                                
                                                SELECT();
                                                
                                                SPDR = 0x10;
                                                while(!(SPSR & (1 << SPIF)));
                                                delay(5,'u');
                                                SPDR = 0x01;
                                                while(!(SPSR & (1 << SPIF)));
                                                delay(250,'u');
                                                SPDR = 0xAB;
                                                while(!(SPSR & (1 << SPIF)));
                                                SPI_buf=SPDR;
                                                delay(5,'u');
                                                SPDR = 0xAB;
                                                while(!(SPSR & (1 << SPIF)));
                                                SPI_buf1=SPDR;
                                                delay(5,'u');
                                                
                                                
                                                DESELECT();
                                                
                                                
                                                USART_Transmit(SPI_buf);
                                                USART_Transmit(SPI_buf1);
                                                SPI_buf=0x00;    
                                                SPI_buf1=0x00;
                                                buf=0x00;
                                                statbuf=0x00;
                                                command=0x00;
                            break;                    
                }
                
                default: {
                           command=0x00;
                           break;
                          }
        }                  
    
    }
}


int main (void) {

    USART_init();
    SPI_init();
    sei();

    while (1) {
        processing ();

    }
}