SPI

https://learn.sparkfun.com/tutorials/raspberry-pi-spi-and-i2c-tutorial/all


wiringPi I/O library for C/C++
http://wiringpi.com/





//set up SPI
		clr		r16
		ldi		r16,(1<<SPE)|(0<<DORD)|(1<<MSTR)	;SPI Enable, MSB of the data word is transmitted first, Master
		out		SPCR,r16							;Save settings for SPI
		clr		r16
		ldi		r16,(1<<SPI2X)						;Set the speed setting
		out		SPSR,r16							;Save speed settings for SPI



; --------------------------------------------------------------
		; -- DRIVER - transmits data to the BCD-display
		; -- Uses: r16,r17
		; -- Gets data from stack (4 bytes)
DRIV_TRANSMIT:
		//push all registers used.
        push    r16
        push    r17
        push    ZH
        push    ZL

		//Load the Z pointer with stack pointer and add 7 to get down the the data.
        in      ZH,SPH
        in      ZL,SPL
        adiw    Z,7

		//Load the loop register to 4 (one for each of the data we need to send)
		ldi		r16,4
DISPLAY_TO_LED_TRANSMIT:
		ld      r17,Z+									;Get the data from the stack
		out		SPDR, r17								;Send the data
DISPLAY_TO_LED_WAIT_TRANSMIT:
		sbis	SPSR,SPIF								;Check if transmission is completed
		rjmp	DISPLAY_TO_LED_WAIT_TRANSMIT			;If not, go back
		dec		r16
		brne	DISPLAY_TO_LED_TRANSMIT

		//pop all registers used.
        pop     ZL
        pop     ZH
        pop     r17
        pop     r16

		ret