PRELIMINARY TECHNICAL DATA

Nonvolatile Memory Digital Potentiometers

AD5235

REV PrD 6 NOV, 2000

5

Information contained in this Product Concept data sheet describes a product in the early definition stage. There is no guarantee that the

information contained here will become a final product in its present form. For latest information contact Walt Heinzer/Analog Devices, Santa Clara,

CA. TEL(408)562-7254; FAX (408)727-1550; walt.heinzer@analog.com

OPERATIONAL OVERVIEW

The AD5235 digital potentiometer is designed to operate as a true

variable resistor replacement device for analog signals that remain

potentiometer allows both scratch pad register (RDAC register)

changes to be made, as well as 100,000 times of nonvolatile

electrically erasable memory (EEMEM) register operations. The

EEMEM update process takes approximately 20.2ms, during this

time the shift register is locked preventing any changes from taking

place. The RDY pin flags the completion of this EEMEM save.

The EEMEM retention is designed to last 10 years without refresh.

The scratch pad register can be changed incrementally by using the

software controlled Increment/Decrement instruction or the Shift

Left/Right instruction command. Once an Increment, Decrement or

Shift command has been loaded into the shift register, subsequent

CS strobes will repeat this command. This is useful for push button

control applications. Alternately the scratch pad register can be

programmed with any position value using the standard SPI serial

interface mode by loading the representative data word. The

scratch pad register can be loaded with the current contents of the

nonvolatile EEMEM register under the program control. At system

power ON, the default value of the scratch pad memory is the value

previously saved in the EEMEM register. The factory EEMEM

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

readout of the internal register contents. The serial input data

register uses a 24-bit instruction/address/data WORD. The write-

protect (

WP) pin provides a hardware EEMEM protection feature

disabling any changes of the present EEMEM contents.

SERIAL DATA INTERFACE

The AD5235 contains a four-wire SPI compatible digital interface

(SDI, SDO,

CS, and CLK). Key features of this interface include:

•

Independently Programmable Read & Write to all registers

•

Direct parallel refresh of all RDAC wiper registers from

corresponding EEMEM registers

•

Permanent storage of the present scratch pad RDAC register

values into the corresponding EEMEM register

•

30 bytes of user addressable electrical-erasable memory

The serial interface of AD5235 digital potentiometer uses a 24-bit

serial word loaded with MSB first. The format of the SPI

compatible word is shown in Table 1. The Command Bits (Cx)

control the operation of the digital potentiometer according to the

command instruction shown in Table 2. The Address Bits (Ax)

determine which register is activated. The Data Bits (Dx) are the

values that are loaded into the decoded register. The last

instruction executed prior to a period of no programming activity

should be the NOP instruction. This will place the internal logic

circuitry in a minimum power dissipation state.

SE R IA L

RE G IS T E R

CO UNT E R

C O MMA ND

PR O C ES SO R

& AD D R ES S

DE CO DE

VA L ID

C O MMA ND

CS

CL K

SD I

R

PU LL U P

+5 V

SD O

GN D

PR

Figure 2. Equivalent Digital Input-Output Logic

The equivalent serial data input and output logic is shown in figure

2. The open drain output SDO is disabled whenever chip select

CS

is logic high. The SPI interface can be used in two slave modes

CPHA=1, CPOL=1 and CPHA=0, CPOL=0. CPHA and CPOL

refer to the control bits, which dictate SPI timing in the following

microprocessors/MicroConverters: ADuC812/824, M68HC11, and

MC68HC16R1/916R1.

Table 1. AD5235 24-bit Serial Data Word

M

S

B

L

S

B

AD5235

C3

C2

C1

C0

A3

A2

A1

A0

X

X

X

X

X

X

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

Command bits are identified as Cx, address bits are Ax, and data bits are Dx. Command instruction codes are defined in table 2.