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AD8804AN

12 Channel, 8-Bit TrimDACs with Power Shutdown

Analog Devices 

AD8802/AD8804

;

; 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

DAC_VALUE

DATA

40H

DAC_ADDR

DATA

41H

SHIFT1

DATA

042H

SHIFT2

DATA

043H

SHIFT_COUNT

DATA

44H

;

ORG

100H

DO_8802:

CLR

SCON.7

CLR

SCON.6

CLR

SCON.5

CLR

SCON.1

ORL

PORT1.1,#00001110B

CLR

PORT1.1

MOV

SHIFT1,DAC_ADDR

ACALL

BYTESWAP

MOV

SBUF,SHIFT2

ADDR_WAIT:

JNB

SCON.1,ADDR_WAIT

CLR

SCON.1

MOV

SHIFT1,DAC_VALUE

ACALL

BYTESWAP

MOV

SBUF,SHIFT2

VALU_WAIT:

JNB

SCON.1,VALU_WAIT

CLR

SCON.1

SETB

PORT1.1

RET

;

BYTESWAP:

MOV

SHIFT_COUNT,#8

SWAP_LOOP:

MOV

A,SHIFT1

RLC

A

MOV

SHIFT1,A

MOV

A,SHIFT2

RRC

A

MOV

SHIFT2,A

DJNZ

SHIFT_COUNT,SWAP_LOOP

RET

END

;SFR register for port 1

;DAC Value

;DAC Address

;high byte of 16-bit answer

;low byte of answer

;

;arbitrary start

;set serial

;data mode 0

;clr transmit flag

;/RS, /SHDN, /CS high

;set the /CS low

;put DAC value in shift register

;

;send the address byte

;wait until 8 bits are sent

;clear the serial transmit flag

;send the DAC value

;

;

;wait again

;clear serial flag

;/CS high, latch data

; into AD8801

;Shift 8 bits

;Get source byte

;Rotate MSB to carry

;Save new source byte

;Get destination byte

;Move carry to MSB

;Save

;Done?

Listing 1. Software for the 8051 to AD8802/AD8804 Serial Port Interface

8051 µC

PORT 1 1.7 1.6 1.5 1.4

+5V

P1.7

P1.6

P1.5

P1.4

VDD VREFH

AD8804

SDI

O1

CLK

CS

O12

SHDN

GND VREFL

Figure 25. An AD8802/AD8804-8051 µC 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.

–10–

REV. 0

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