Add spi_word()

sw/spi.h

@@ -4,5 +4,6 @@
 #include <stdint.h>
 
 uint8_t spi_byte(uint8_t);
+uint16_t spi_word(uint16_t);
 
 #endif

sw/spi.s

@@ -1,6 +1,7 @@
 .include "io.inc65"
 
 .export _spi_byte
+.export _spi_word
 
 .importzp	sp, sreg, regsave, regbank
 .importzp	tmp1, tmp2, tmp3, tmp4, ptr1, ptr2, ptr3, ptr4
@@ -13,7 +14,7 @@ SPI_MOSI = $04
 SPI_MISO = $08
 
 
-;   Write a single byte to the SPI device
+;   Read and write a single byte from the SPI device
 ;   @in A The byte to write 
 ;   @out A The read byte
 
@@ -52,3 +53,55 @@ _spi_byte:
         ply
         plx
         rts                 ; Return
+
+
+;   Read and write 16 bits from the SPI device
+;   @in AX The word to write 
+;   @out AX The read word
+
+_spi_word:
+        phy
+        ldy #$00
+        sta tmp1            ; Save value into tmp1
+        stx tmp2
+        lda #$02
+        sta tmp4
+
+@byte:  lda #$80
+        tax
+@loop:  bit tmp1            ; Check if high bit set
+        beq @1
+        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
+        tya
+        asl
+        inc
+        bra @3
+@2:     tya                 ; Add current value
+        asl
+@3:     tay                 ; Move read value back to y
+        txa
+        lsr                 ; Select next bit
+        tax
+        bne @loop           ; Stop when mask is 0
+
+        lda tmp2            ; Switch to second byte
+        sta tmp1
+        sty tmp3            ; Store read data in tmp3
+        dec tmp4
+        bne @byte
+
+        lda #SPI_SSn        ; Raise Slave Select
+        sta BB_SPI_BASE
+        tya                 ; Get read value from y
+        ldx tmp3
+        ply
+        rts                 ; Return

sw/tests/test_main.c

@@ -3,7 +3,7 @@
 
 #include <spi.h>
 
-char retval;
+uint16_t retval;
 
 int main(void)
 {
@@ -21,6 +21,15 @@ int main(void)
         printf("Expected 0 return value from spi_byte\n");
         return 1;
     }
+    printf("Done!   %x\n\n", retval);
+
+    printf("Starting spi_word test...\n");
+    retval = spi_word(0xa5a5);
+    if (retval != 0) {
+        printf("Expected 0 return value from spi_word.\n");
+        printf("Got: %x\n\n", retval);
+        return 1;
+    }
     printf("Done!   %x\n", retval);
     return 0;
 }