Cirrus Logic AN74 - Manual (Page 11)

AN74
AN74Rev2 11
;****************************************************************
;* Routine - TRANSFER_BYTE
;* Input
- byte to be transferred
;* Output - None
;* Description - This subroutine transfers 1 byte to the CS5525/6/9
;
It transfers the byte by first placing a bit in PORT1 BIT 1.
; It then pulses the SCLK to advance the A/D’s serial
; output shift register to the next bit.
; It does this eight times to transmit one complete byte.
; The function prototype is: void TRANSFER_BYTE(char);
;Note: This routine can be used three time consecutively to transfer all 24 bits
; from the 80C51 to the internal registers of the CS5525/6/9.
;****************************************************************
$DEBUG
USING 0
; Use register bank 0
TCOD SEGMENT CODE ; Make TCOD a segment of code
TDAT SEGMENT DATA ; Make TDAT a segment of data
PUBLIC TRANSFER_BYTE ; Make subroutine global
PUBLIC ?TRANSFER_BYTE?BYTE ; Make subroutine global
RSEG TDAT ; Make code relocatable
?TRANSFER_BYTE?BYTE:
VAR: DS 1
; Define a storage location
RSEG TCOD ; Make code relocatable
TRANSFER_BYTE:
MOV A,VAR ; Move byte to be transmitted to ACC
MOV R1,#08 ; Set count to 8 to transmit byte
CLR P1.3 ; Clear SCLK
loop: ; Send Byte
RLC A ; Rotate Accumulator, send MSB 1st
MOV P1.1,C ; Transmit MSB first through C bit
SETB P1.3 ; Set SCLK
CLR P1.3 ; Clear SCLK
DJNZ R1,loop ; Decrement byte, repeat loop if not zero
CLR P1.1 ; Reset SDI to zero when not transmitting
RET ; Exit subroutine
END