AD5232 datasheet(14/24 Pages) AD

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AD5232

Data Sheet

Rev. C | Page 14 of 24

THEORY OF OPERATION

The AD5232 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 a |VDD − VSS| < 5.5 V. The

digital potentiometer wiper position is determined by the RDACx

allowing as many value changes as necessary to place the poten-

tiometer wiper in the correct position. The scratch pad register

can be programmed with any position value using the standard

SPI serial interface mode by loading the complete representative

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

saved into a corresponding EEMEMx register. Thereafter, the wiper

position is always set at that position for any future on-off-on

power supply sequence. The EEMEM save process takes approx-

imately 25 ms. During this time, the shift register is locked,

preventing any changes from taking place. The RDY pin indicates

the completion of this EEMEM save.

SCRATCH PAD AND EEMEM PROGRAMMING

The scratch pad register (RDACx register) directly controls the

position of the digital potentiometer wiper. When the scratch

pad register is loaded with all 0s, the wiper is connected to

Terminal B of the variable resistor. When the scratch pad register

is loaded with midscale code (1/2 of full-scale position), the wiper

is connected to the middle of the variable resistor. When the

scratch pad is loaded with full-scale code, which is all 1s, the

wiper connects to Terminal A. Because the scratch pad register

is a standard logic register, there is no restriction on the number

of changes allowed. The EEMEMx registers have a program

erase/write cycle limitation that is described in the Flash/EEMEM

Reliability section.

BASIC OPERATION

The basic mode of setting the variable resistor wiper position

(by programming the scratch pad register) is accomplished by

loading the serial data input register with Command Instruc-

tion 11, which includes the desired wiper position data. When

the desired wiper position is found, the user loads the serial

data input register with Command Instruction 2, which copies

the desired wiper position data into the corresponding non-

volatile EEMEMx register. After 25 ms, the wiper position is

permanently stored in the corresponding nonvolatile EEMEM

location. Table 6 provides an application programming example

listing the sequence of serial data input (SDI) words and the

corresponding serial data output appearing at the serial data

output (SDO) pin in hexadecimal format.

At system power-on, the scratch pad register is refreshed with

the last value saved in the EEMEMx register. The factory preset

EEMEM value is midscale. The scratch pad (wiper) register can

be refreshed with the current contents of the nonvolatile EEMEMx

The application programming example shown in Table 6 lists

two digital potentiometers set to independent data values. The

wiper positions are then saved in the corresponding nonvolatile

EEMEMx registers.

Table 6. Application Programming Example

SDI

SDO

Action

0xB040

0xXXXX1

Loads 0x40 data into the RDAC1 register;

Wiper W1 moves to 1/4 full-scale position.

0x20XX1

0xB040

Saves a copy of the RDAC1 register contents

into the corresponding EEMEM1 register.

0xB180

0x20XX1

Loads 0x80 data into the RDAC2 register;

Wiper W2 moves to 1/2 full-scale position.

0x21XX1

0xB180

Saves a copy of the RDAC2 register contents

into the corresponding EEMEM2 register.

1 X = don’t care.

Note that the PR pulse first sets the wiper at midscale when it is

brought to Logic 0. Then, on the positive transition to logic high,

it reloads the DAC wiper register with the contents of EEMEMx.

Many additional advanced programming commands are avail-

able to simplify the variable resistor adjustment process.

For example, the wiper position can be changed, one step at

a time, by using the software controlled increment/decrement

command instructions. The wiper position can be also be changed,

6 dB at a time, by using the shift left/right command instructions.

After an increment, decrement, or shift command instruction is

loaded into the shift register, subsequent CS strobes repeat this

command instruction. This is useful for push-button control appli-

cations (see the Advanced Control Modes section). The SDO pin

is available for daisy chaining and for readout of the internal

instruction/address/data-word.

EEMEM PROTECTION

The write protect (WP) pin disables any changes of the scratch

pad register contents, regardless of the software commands,

except that the EEMEM setting can be refreshed using Instruction

Command 8 and PR. Therefore, the WP pin provides a hardware

EEMEM protection feature. Execute an NOP command (Com-

mand Instruction 0) before returning WP to logic high.

DIGITAL INPUT/OUTPUT CONFIGURATION

All digital inputs are ESD protected, high input impedance that

can be driven directly from most digital sources. The PR and WP

pins, which are active at logic low, must be biased to VDD if they

are not being used. No internal pull-up resistors are present on

any digital input pins.

The SDO and RDY pins are open-drain, digital outputs when pull-

up resistors are needed, but only if these functions are in use.

A resistor value in the range of 1 kΩ to 10 kΩ optimizes the power

and switching speed trade-off.

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