Application Note

1 of 12

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REV 1.1 11/12/02

AN 114

Interfacing the X9408/X9418 XDCP to 8051 Microcontrollers

by Applications Staff,

This application note describes the routines for the

control of an X9408 or X9418 digitally controllable

potentiometer. The X9408/X9418 devices have a variety

of different instructions that provide flexibility to the

designer. Additionally, the nonvolatile nature of the

device allows for stored wiper positions that can be

retrieved after power cycles.

The following code implements all of the available

X9408/X9418 instructions using a standard bi-direc-

tional bus protocol. Although the subroutines occupy

about 300 bytes of program memory, designers who

won't need to implement all of the instructions can

shorten the code by removing any unnecessary routines.

However, this will necessitate the reassembly of the

code.

For those instructions which program the nonvolatile

data registers (XFR_WCR, GXFR_WCR, &

WRITE_DR), acknowledge polling has been imple-

mented to determine an early completion of the internal

write cycle. Although this is automatically handled by

the routines, a word or two regarding the procedure

should be informative. After issuing a start condition, the

master sends a slave address and receives an acknowl-

edge. It then issues an instruction byte to the X9408/

X9418 and again receives an acknowledge. If necessary,

it now transmits the data byte and receives a final

acknowledge. The master must then initiate a stop condi-

tion which will cause the X9408/X9418 to begin an

internal write cycle. The X9408/X9418 pins go to high

impedance until this internal cycle is complete. The

Figure 1. Connecting the X9408 to an 80C51 microcontroller

+5V

-5V

U1

80C51

21

22

23

24

25

26

27

28

17

16

29

30

11

10

31

19

18

9

39

38

37

36

35

34

33

32

1

2

3

4

5

6

7

8

12

13

14

15

P2.0

P2.1

P2.2

P2.3

P2.4

P2.5

P2.6

P2.7

RD

WR

PSEN

ALE/P

TXD

RXD

EA/VP

X1

X2

RST

P0.0

P0.1

P0.2

P0.3

P0.4

P0.5

P0.6

P0.7

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

INT0

INT1

T0

T1

U2

X940

8

19

15

14

16

22

21

23

1

24

17

7

18

5

8

20

3

4

2

10

11

9

13

VSS

VH2

VW2

VL2

VH3

VW3

VL3

VCC

V+

SCL

SDA

A3

A2

A1

A0

VH0

VW0

VL0

VH1

VW1

VL1

V-

R1

10K

R2

10K