AD5231_15 datasheet(14/28 Pages) AD

Part #	AD5231

Description	Nonvolatile Memory, 1024-Position Digital Potentiometer

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AD5231

Data Sheet

Rev. D | Page 14 of 28

THEORY OF OPERATION

The AD5231 digital potentiometer is designed to operate as a

true variable resistor replacement device for analog signals that

remain within the terminal voltage range of VSS < VTERM < VDD.

The basic voltage range is limited to VDD − VSS < 5.5 V. The

digital potentiometer wiper position is determined by the

RDAC register contents.

The RDAC register acts as a scratchpad register, allowing as

many value changes as necessary to place the potentiometer

wiper in the correct position. The scratchpad register can be

programmed with any position value using the standard SPI

serial interface mode by loading the complete representative

data-word. Once a desirable position is found, this value can be

stored in an EEMEM register. Thereafter, the wiper position is

always restored to that position for subsequent power-up.

The storing of EEMEM data takes approximately 25 ms; during

this time, the shift register is locked, preventing any changes

from taking place. The RDY pin pulses low to indicate the

completion of this EEMEM storage.

The following instructions facilitate the user’s programming

needs (see Table 7 for details):

0. Do nothing.

1. Restore EEMEM content to RDAC.

2. Store RDAC setting to EEMEM.

3. Store RDAC setting or user data to EEMEM.

4. Decrement 6 dB.

5. Decrement 6 dB.

6. Decrement one step.

7. Decrement one step.

8. Reset EEMEM content to RDAC.

9. Read EEMEM content from SDO.

10. Read RDAC wiper setting from SDO.

11. Write data to RDAC.

12. Increment 6 dB.

13. Increment 6 dB.

14. Increment one step.

15. Increment one step.

SCRATCHPAD AND EEMEM PROGRAMMING

The scratchpad RDAC register directly controls the position of

the digital potentiometer wiper. For example, when the scratchpad

register is loaded with all zeros, the wiper is connected to

Terminal B of the variable resistor. The scratchpad register is a

standard logic register with no restriction on the number of

changes allowed, but the EEMEM registers have a program

erase/write cycle limitation (see the Flash/EEMEM Reliability

section).

BASIC OPERATION

The basic mode of setting the variable resistor wiper position

(programming the scratchpad register) is accomplished by

loading the serial data input register with Instruction 11 (0xB),

Address 0, and the desired wiper position data. When the

proper wiper position is determined, the user can load the serial

data input register with Instruction 2 (0x2), which stores the

wiper position data in the EEMEM register. After 25 ms, the

wiper position is permanently stored in the nonvolatile

memory. Table 5 provides a programming example listing the

sequence of serial data input (SDI) words with the serial data

output appearing at the SDO pin in hexadecimal format.

Table 5. Set and Store RDAC Data to EEMEM Register

SDI

SDO

Action

0xB00100

0xXXXXXX

Writes data 0x100 to the RDAC

register, Wiper W moves to 1/4

full-scale position.

0x20XXXX

0xB00100

Stores RDAC register content into

the EEMEM register.

At system power-on, the scratchpad register is automatically

refreshed with the value previously stored in the EEMEM

register. The factory-preset EEMEM value is midscale, but

it can be changed by the user thereafter.

During operation, the scratchpad (RDAC) register can be

refreshed with the EEMEM register data with Instruction 1

(0x1) or Instruction 8 (0x8). The RDAC register can also be

refreshed with the EEMEM register data under hardware

control by pulsing the PR pin. The PR pulse first sets the wiper

at midscale when brought to logic zero, and then, on the

positive transition to logic high, it reloads the RDAC wiper

register with the contents of EEMEM.

Many additional advanced programming commands are

available to simplify the variable resistor adjustment process

(see Table 7). For example, the wiper position can be changed

one step at a time using the increment/decrement instruction or

by 6 dB with the shift left/right instruction. Once an increment,

decrement, or shift instruction has been loaded into the shift

register, subsequent CS strobes can repeat this command.

A serial data output SDO pin is available for daisy-chaining and

for readout of the internal register contents.

EEMEM PROTECTION

The write protect (WP) pin disables any changes to the

scratchpad register contents, except for the EEMEM setting,

which can still be restored using Instruction 1, Instruction 8,

and the PR pulse. Therefore, WP can be used to provide a

hardware EEMEM protection feature. To disable WP, it is

recommended to execute a NOP instruction before returning

WP to logic high.

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