Update spi.s

Use names instead of magic values.

sw/spi.s

@@ -7,6 +7,10 @@
 
 .code
 
+SPI_SCLK = $01
+SPI_SSn  = $02
+SPI_MOSI = $04
+
 
 ;   Write a single byte to the SPI device
 ;   @in A The byte to write 
@@ -19,16 +23,18 @@ _spi_write_byte:
         tax
 @loop:  bit tmp1            ; Check if high bit set
         beq @1
-        lda #$04            ; Bit not set.
+        lda #SPI_MOSI            ; Bit not set.
         bra @1
 @1:     lda #$00            ; Bit set
         sta BB_SPI_BASE     ; Write data
-        adc #$01
+        adc #SPI_SCLK
         sta BB_SPI_BASE     ; Write clock
         txa
         lsr                 ; Select next bit
         tax
         bne @loop           ; Stop when mask is 0
+        lda #SPI_SSn        ; Raise Slave Select
+        sta BB_SPI_BASE
         ply                 ; Restore regs
         plx
         rts                 ; Return