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(Serial Peripheral Interface)

Initialize spi:

ldx PCADDR,#DDR                    ;Set MOSI and SCK to output, MISO to input, SCK to output
ldx PCCTL,#1110_0111b
ldx PCADDR,#AF                     ;Set alternate SPI functions
ldx PCCTL,#0011_1100b              ;MOSI,MISO,SCK,/SS

ldx SPICTL,#000__11__011b          ;SPI mode Master, transmit on falling edge, SCK idle HIGH
ldx SPIMODE,#00_0_000_1_1b         ;8 bits per char, SS is output and set to 1
ldx SPIBRH,#0                      ;spi_frequency = clock_frequency / (SPIBR[15..0]*2)
ldx SPIBRL,#2                      ;the highest spi rate: SPIBRH = 0000h → clock/2

Some handy macro's:

spi_start: macro 
    andx SPIMODE,#~1

spi_stop: macro
    orx SPIMODE,#1
endmac spi_stop

wait_spi_rdy: macro 
    ldx R0,SPISTAT
    btjnz 1,R0,$B
endmac wait_spi_rdy

To transmit:

    ;.... sending bytes
    ld R1,#5
    call spi_transmit
    ld R1,#10
    call spi_transmit
    ;.... first 5 is sent, then 10 is sent

    ldx SPIDATA,R1

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