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.h

@@ -3,6 +3,6 @@
 
 #include <stdint.h>
 
-void spi_write_byte(uint8_t);
+uint8_t spi_byte(uint8_t);
 
 #endif

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
@@ -10,14 +10,17 @@
 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
@@ -29,12 +32,23 @@ _spi_write_byte:
         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

sw/tests/test_main.c

@@ -1,10 +1,26 @@
 #include <stdio.h>
+#include <stdint.h>
+
 #include <spi.h>
 
+char retval;
+
 int main(void)
 {
-    printf("Starting spi_write_byte test...\n");
-    spi_write_byte(0xa5);
-    printf("Done!\n");
+    printf("Setting SPI location to 0x02\n");
+    *(uint8_t*)0x7ff0 = 2;
+    if (!(*(uint8_t*)0x7ff0 == 2)) {
+        printf("Expected 0x02 at 0x7ff0\n");
+        return 1;
+    }
+    printf("Done!\n\n");
+
+    printf("Starting spi_byte test...\n");
+    retval = spi_byte(0xa5);
+    if (retval != 0) {
+        printf("Expected 0 return value from spi_byte\n");
+        return 1;
+    }
+    printf("Done!   %x\n", retval);
     return 0;
 }