Change spi_write_byte to spi_byte

the SPI module reads and writes at the same time. If you don't want to
write a value, then write all zeros or whatever the device you are
communicating with wants.

sw/spi.s

@@ -1,6 +1,6 @@
 .include "io.inc65"
 
-.export _spi_write_byte
+.export _spi_byte
 
 .importzp	sp, sreg, regsave, regbank
 .importzp	tmp1, tmp2, tmp3, tmp4, ptr1, ptr2, ptr3, ptr4
@@ -10,31 +10,45 @@
 SPI_SCLK = $01
 SPI_SSn  = $02
 SPI_MOSI = $04
+SPI_MISO = $08
 
 
 ;   Write a single byte to the SPI device
 ;   @in A The byte to write 
+;   @out A The read byte
 
-_spi_write_byte:
+_spi_byte:
         phx                 ; Save regs
         phy
+        ldy #$00
         sta tmp1            ; Save value into tmp1
         lda #$80
         tax
 @loop:  bit tmp1            ; Check if high bit set
         beq @1
-        lda #SPI_MOSI            ; Bit not set.
+        lda #SPI_MOSI       ; Bit not set.
         bra @1
 @1:     lda #$00            ; Bit set
         sta BB_SPI_BASE     ; Write data
         adc #SPI_SCLK
         sta BB_SPI_BASE     ; Write clock
+        stz tmp2            
+        lda BB_SPI_BASE     ; Check MISO value
+        and #SPI_MISO
+        beq @2
+        inc tmp2
+@2:     clc                 ; Shift previous value left
+        tya                 ; Add current value
+        asl
+        adc tmp2
+        tay                 ; Move read value back to y
         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
+        tya                 ; Get read value from y
+        ply
         plx
         rts                 ; Return