EVAL-AD421EB【PDF 8/14 页】Loop-Powered 4 mA to 20 mA DAC

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AD421

–

REV. C

WORD "N"

0 0

1 1

CLOCK

DATA

(

LATCH

X X X X X X X

Figure 7. Write Cycle for Programming Alarm Current

Data

MICROPROCESSOR INTERFACING

AD421 – MC68HC11 (SPI BUS) INTERFACE

Figure 8 shows a typical interface between the AD421 and the

Motorola MC68HC11 SPI (Serial Peripheral Interface) bus.

The SCK, MOSI and SS pins of the 68HC11 are respectively

connected to the CLOCK, DATA IN and LATCH pins of the

AD421.

SCK

MOSI

CLOCK

DATA IN

LATCH

AD421*

68HC11

* ADDITIONAL PINS OMITTED FOR CLARITY

Figure 8. AD421 to 68HC11 Interface

A typical routine such as the one shown below begins by initializ-

ing the state of the various SPI data and control registers.

INIT

LDAA #$2F

STAA

LDAA #$38

STAA

LDAA #$50

STAA

;

= 1; SCK = 0; MOSI = 1

;SEND TO SPI OUTPUTS

;

, SCK, MOSI = OUTPUTS

;SEND DATA DIRECTION INFO

;DABL INTRPTS, SPI IS MASTER & ON

;CPOL = 0, CPHA = 0, 1MHZ BAUDRATE

;LOAD ACCUM W/UPPER 8 BITS

SENDAT ;JUMP TO DAC OUTPUT ROUTINE

NEXTPT ;INFINITE LOOP

#$1000

;POINT AT ON-CHIP REGISTERS

$08,Y,$20 ;DRIVE

(LATCH) LOW

SPDR

;SEND MS-BYTE TO SPI DATA REG

LDAA SPSR

;CHECK STATUS OF SPIE

BPL

WAIT1

;POLL FOR END OF X-MISSION

LDAA LSBY

;GET LOW 8 BITS FROM MEMORY

STAA

SPDR

;SEND LS-BYTE TO SPI DATA REG

LDAA SPSR

;CHECK STATUS OF SPIE

BPL

WAIT2;

;POLL FOR END OF X-MISSION

BSET

$08,Y,$20 ;DRIVE

HIGH TO LATCH DATA

RTS

The SPI data port is configured to process data in 8-bit bytes.

The most significant data byte (MSBY) is retrieved from

memory and processed by the SENDAT routine. The

pin is

driven low by indexing into the PORTD data register and clear

Bit 5. The MSBY is then sent to the SPI data register where it is

automatically transferred to the AD421 internal shift resistor.

PORTD

DDRD

SPCR

NEXTPT LDAA MSBY

BSR

JMP

SENDAT LDY

BCLR

STAA

WAIT1

WAIT2

of 11.147 mA. With 16 clock pulses between consecutive latch

signals data written is for normal 4 mA to 20 mA operation.

Table II. Ideal Input/Output Code Table

for 4 mA to 20 mA Operation

Code

Output Current

0000 0000 0000 0000

0000 0000 0000 0001

0000 0000 0000 0010

0100 0000 0000 0000

1000 0000 0000 0000

1100 0000 0000 0000

1111 1111 1111 1101

1111 1111 1111 1110

1111 1111 1111 1111

4 mA

4.000244 mA

4.000488 mA

8 mA

12 mA

16 mA

19.999268 mA

19.999512 mA

19.999756 mA

WORD "N"

WORD "N +1"

1 1

0 0

CLOCK

DATA

(

LATCH

Figure 6. Write Cycle for 4 mA to 20 mA Operation

Alarm Current Coding

Table III shows the ideal input-code-to-output-current relation-

ship for alarm current programming of the AD421. In this case,

the equivalent span is 0 mA to 32 mA but a reliable operating

span is 3.5 mA to 24 mA. The part may give an indeterminate

output for code values outside the range given in the table. As a

result, the user is advised to restrict the code programmed to the

part in alarm current mode to within the range shown in Table

III. Figure 7 shows a timing diagram for loading an alarm cur-

rent of 3.75 mA to the AD421 with an 8-bit microcontroller

using three 8-bit writes.

The output current values shown assume a REF IN voltage of

+2.5 V. With a REF IN of +2.5 V, an ideal 1 LSB = 32 mA/

131,072 = 244 nA.

Table III. Ideal Input/Output Code Table

for Alarm Current Operation

Code

Output Current

0 0011 1000 0000 0000

0 0011 1100 0000 0000

0 0100 0000 0000 0000

0 1000 0000 0000 0000

1 0000 0000 0000 0000

1 0100 0000 0000 0000

1 0110 0000 0000 0000

1 1000 0000 0000 0000

3.5 mA

3.75 mA

4 mA

8 mA

16 mA

20 mA

22 mA

24 mA

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