Datasheet

AD8802/AD8804
REV. 0
–10–
;
; This subroutine loads an AD8802/AD8804 DAC from an 8051 microcomputer,
; using the 8051’s serial port in MODE 0 (Shift Register Mode).
; The DAC value is stored at location DAC_VAL
; The DAC address is stored at location DAC_ADDR
;
; Variable declarations
;
PORT1 DATA 90H ;SFR register for port 1
DAC_VALUE DATA 40H ;DAC Value
DAC_ADDR DATA 41H ;DAC Address
SHIFT1 DATA 042H ;high byte of 16-bit answer
SHIFT2 DATA 043H ;low byte of answer
SHIFT_COUNT DATA 44H ;
;
ORG 100H ;arbitrary start
DO_8802: CLR SCON.7 ;set serial
CLR SCON.6 ;data mode 0
CLR SCON.5
CLR SCON.1 ;clr transmit flag
ORL PORT1.1,#00001110B ;/RS, /SHDN, /CS high
CLR PORT1.1 ;set the /CS low
MOV SHIFT1,DAC_ADDR ;put DAC value in shift register
ACALL BYTESWAP ;
MOV SBUF,SHIFT2 ;send the address byte
ADDR_WAIT: JNB SCON.1,ADDR_WAIT ;wait until 8 bits are sent
CLR SCON.1 ;clear the serial transmit flag
MOV SHIFT1,DAC_VALUE ;send the DAC value
ACALL BYTESWAP ;
MOV SBUF,SHIFT2 ;
VALU_WAIT: JNB SCON.1,VALU_WAIT ;wait again
CLR SCON.1 ;clear serial flag
SETB PORT1.1 ;/CS high, latch data
RET ; into AD8801
;
BYTESWAP: MOV SHIFT_COUNT,#8 ;Shift 8 bits
SWAP_LOOP: MOV A,SHIFT1 ;Get source byte
RLC A ;Rotate MSB to carry
MOV SHIFT1,A ;Save new source byte
MOV A,SHIFT2 ;Get destination byte
RRC A ;Move carry to MSB
MOV SHIFT2,A ;Save
DJNZ SHIFT_COUNT,SWAP_LOOP ;Done?
RET
END
Listing 1. Software for the 8051 to AD8802/AD8804 Serial Port Interface
+5V
P1.7
P1.6
P1.5
P1.4
1.51.61.7
PORT 1
8051 µC
1.4
CLK
V
REFL
SDI
O1
O12
CS
SHDN
GND
AD8804
V
DD
V
REFH
Figure 25. An AD8802/AD8804-8051 µ C Interface Using
Parallel Port 1
to start the serial interface process. The DAC address is loaded
into the accumulator and four Rotate Right shifts are per-
formed. This places the DAC address in the 4 MSBs of the ac-
cumulator. The address is then sent to the AD8802/AD8804 via
the SEND_SERIAL subroutine. Next, the DAC value is loaded
into the accumulator and sent to the AD8802/AD8804. Finally,
the Chip Select input is set high to complete the data transfer
Unlike the serial port interface of Figure 24, the parallel port in-
terface only transmits 12 bits to the AD8802/AD8804. Also, the
BYTESWAP subroutine is not required for the parallel inter-
face, because data can be shifted out MSB first. However, the
results of the two interface methods are exactly identical. In
most cases, the decision on which method to use will be deter-
mined by whether or not the serial data port is available for
communication with the AD8802/AD8804.