AD8801 查看數據表（PDF） - Analog Devices

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AD8801

Octal 8-Bit TrimDAC with Power Shutdown

Analog Devices 

AD8801/AD8803

The subroutine begins by setting appropriate bits in the Serial

Control register to configure the serial port for Mode 0 opera-

tion. Next the DAC’s Chip Select input is set low to enable the

AD8801/AD8803. The DAC address is obtained from memory

location DAC_ADDR, adjusted to compensate for the 8051’s

serial data format, and moved to the serial buffer register. At

this point, serial data transmission begins automatically. When

all 8 bits have been sent, the Transmit Interrupt bit is set, and

the subroutine then proceeds to send the DAC value stored at

location DAC_VALUE. Finally the Chip Select input is re-

turned high, causing the appropriate AD8801/AD8803 output

voltage to change, and the subroutine ends.

The 8051 sends data out of its shift register LSB first, while the

AD8801/AD8803 require data MSB first. The subroutine there-

fore includes a BYTESWAP subroutine to reformat the data.

This routine transfers the MSB-first byte at location SHIFT1 to

an LSB-first byte at location SHIFT2. The routine rotates the

MSB of the first byte into the carry with a Rotate Left Carry in-

struction, then rotates the carry into the MSB of the second byte

with a Rotate Right Carry instruction. After 8 loops, SHIFT2

contains the data in the proper format.

The BYTESWAP routine in Listing 1 is convenient because the

DAC data can be calculated in normal LSB form. For example,

producing a ramp voltage on a DAC is simply a matter of re-

peatedly incrementing the DAC_VALUE location and calling

the LD_8801 subroutine.

If the µC’s hardware serial port is being used for other purposes,

the AD8801/AD8803 can be loaded by using the parallel port.

A typical parallel interface is shown in Figure 26. The serial data

is transmitted to the DAC via the 8051’s Port1.7 output, while

Port1.6 acts as the serial clock.

Software for the interface of Figure 26 is contained in Listing 2. The

subroutine will send the value stored at location DAC_VALUE to

the AD8801/AD8803 DAC addressed by location DAC_ADDR.

The program begins by setting the AD8801/AD8803’s Serial

Clock and Chip Select inputs high, then setting Chip Select low

to start the serial interface process. The DAC address is loaded

into the accumulator and three Rotate Right shifts are per-

formed. This places the DAC address in the 3 MSBs of the ac-

cumulator. The address is then sent to the AD8801/AD8803 via

the SEND_SERIAL subroutine. Next, the DAC value is loaded

into the accumulator and sent to the AD8801/AD8803. Finally,

the Chip Select input is set high to complete the data transfer.

; This 8051 µC subroutine loads an AD8801 or AD8803 DAC with an 8-bit value,

; using the 8051’s parallel port #1.

; The DAC value is stored at location DAC_VALUE

; The DAC address is stored at location DAC_ADDR

;

; Variable declarations

PORT1

DATA

90H

DAC_VALUE

DATA

40H

DAC_ADDR

DATA

41H

LOOPCOUNT

DATA

43H

;

ORG

100H

LD_8803:

ORL

PORT1,#11110000B

CLR

PORT1.5

MOV

LOOPCOUNT,#3

MOV

A,DAC_ADDR

RR

A

RR

A

RR

A

ACALL

SEND_SERIAL

MOV

LOOPCOUNT,#8

MOV

A,DAC_VALUE

ACALL

SEND_SERIAL

SETB

PORT1.5

RET

;SFR register for port 1

;DAC Value

;DAC Address (0 through 7)

;COUNT LOOPS

;arbitrary start

;set CLK, /CS and /SHDN high,

;Set Chip Select low

;Address is 3 bits

; Get DAC address

; Rotate the DAC

;address to the Most

;Significant Bits (MSBs)

;Send the address

;Do 8 bits of data

;Send the data

;Set /CS high

;DONE

SEND_SERIAL:

RLC

MOV

CLR

SETB

DJNZ

RET;

END

A

PORT1.7,C

PORT1.6

PORT1.6

LOOPCOUNT,SEND_SERIAL

;Move next bit to carry

;Move data to SDI

;Pulse the

; CLK input

;Loop if not done

Listing 2. Software for the 8051 to AD8801/AD8803 Parallel Port Interface

REV. A

–11–

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