41 lines
974 B
ArmAsm
41 lines
974 B
ArmAsm
.include "io.inc65"
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.export _spi_write_byte
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.importzp sp, sreg, regsave, regbank
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.importzp tmp1, tmp2, tmp3, tmp4, ptr1, ptr2, ptr3, ptr4
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.code
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SPI_SCLK = $01
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SPI_SSn = $02
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SPI_MOSI = $04
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; Write a single byte to the SPI device
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; @in A The byte to write
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_spi_write_byte:
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phx ; Save regs
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phy
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sta tmp1 ; Save value into tmp1
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lda #$80
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tax
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@loop: bit tmp1 ; Check if high bit set
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beq @1
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lda #SPI_MOSI ; Bit not set.
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bra @1
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@1: lda #$00 ; Bit set
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sta BB_SPI_BASE ; Write data
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adc #SPI_SCLK
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sta BB_SPI_BASE ; Write clock
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txa
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lsr ; Select next bit
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tax
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bne @loop ; Stop when mask is 0
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lda #SPI_SSn ; Raise Slave Select
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sta BB_SPI_BASE
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ply ; Restore regs
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plx
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rts ; Return
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