Remove spi_word and replace with spi_deselect

Since the goal is to have the MAX3421E working, which uses multiple 8
bit transfers, we should support multiple 8 bit transfers instead of
trying to use 16 bit transfers.

When using spi_byte(), the device is automatically selected. After you
have made as many transfers as you want, you must deselect the device
with spi_deselect().

sw/spi.h

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

sw/spi.s

@@ -1,7 +1,7 @@
 .include "io.inc65"
 
 .export _spi_byte
-.export _spi_word
+.export _spi_deselect
 
 .importzp	sp, sreg, regsave, regbank
 .importzp	tmp1, tmp2, tmp3, tmp4, ptr1, ptr2, ptr3, ptr4
@@ -47,61 +47,16 @@ _spi_byte:
         lsr                 ; Select next bit
         tax
         bne @loop           ; Stop when mask is 0
-        lda #SPI_SSn        ; Raise Slave Select
-        sta BB_SPI_BASE
         tya                 ; Get read value from y
         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
-
+;   Deselect the spi device.
+;   spi device is automatically selected during read/write.
+_spi_deselect:
+        pha
         lda #SPI_SSn        ; Raise Slave Select
         sta BB_SPI_BASE
-        tya                 ; Get read value from y
-        ldx tmp3
-        ply
-        rts                 ; Return
+        pla
+        rts

sw/tests/test_main.c

@@ -23,13 +23,7 @@ int main(void)
     }
     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);
+    printf("Starting spi_deselect test...\n");
+    spi_deselect();
     return 0;
 }