Detailed Description

The DS3904/DS3905 contain three, 128-position, NV,

low temperature coefficient, variable digital resistors. All

three resistors also feature a Hi-Z function. The variable

resistor registers (F8h, F9h, and FAh) are factory pro-

grammed with a default value of 7Fh. They are con-

trolled through a 2-wire serial interface, and can serve

as a low-cost replacement for designs using conven-

tional trimming resistors. Furthermore, the DS3904

address pin (A0) allows two DS3904s to be placed on

the same 2-wire bus. The three address pins on the

DS3905 allow up to eight DS3905s to be placed on the

same 2-wire bus.

With their low cost and small size, the DS3904/DS3905

are well tailored to replace larger mechanical trimming

variable resistors. This allows the automation of calibra-

tion in many instances because the 2-wire interface can

easily be adjusted by test/production equipment.

Variable Resistor Memory Organization

The variable resistors of the DS3904/DS3905 are

addressed by communicating with the registers in

Table 1.

Using the Resistor as a Switch

By taking advantage of the high-impedance mode, a

switch can be created to produce a digital output.

Setting a resistor register to 00h creates the low state.

Writing 80h into the same resistor register enables the

high-impedance state. When used with an external

pullup resistor, such as a 4.7k

Ω pullup, a high state

is generated.

Device Operation

Clock and Data Transitions

The SDA pin is normally pulled high with an external

resistor or device. Data on the SDA pin can only change

during SCL low time periods. Data changes during SCL

high periods indicate a start or stop condition depend-

ing on the conditions discussed below. See the timing

diagrams for further details (Figures 2 and 3).

Start Condition

A high-to-low transition of SDA with SCL high is a start

condition, which must precede any other command. See

the timing diagrams for further details (Figures 2 and 3).

Stop Condition

A low-to-high transition of SDA with SCL high is a stop

condition. After a read or write sequence, the stop com-

mand places the DS3904/DS3905 into a low-power

mode. See the timing diagrams for further details

(Figures 2 and 3).

Acknowledge

All address and data bytes are transmitted through a

serial protocol. The DS3904/DS3905 pull the SDA line

low during the ninth clock pulse to acknowledge that

they have received each byte.

Standby Mode

The DS3904/DS3905 feature a low-power mode that is

automatically enabled after power-on, after a stop com-

mand, and after the completion of all internal operations.

Pin Description

PIN

NAME

DS3904

DS3905

DESCRIPTION

SDA

1

2

2-Wire Serial Data. Open-drain

input/output for 2-wire data.

SCL

2

3

2-Wire Serial Clock. Input for

2-wire clock.

3

4

Supply Voltage Terminal

GND

4

5

Ground Terminal

H2

5

6

Resistor 2 High Terminals

H1

6

7

Resistor 1 High Terminals

H0

7

8

Resistor 0 High Terminals

A0

8

9

Address-Select Pin

A1

—

1

Address-Select Pin (DS3905 Only)

A2

—

10

Address-Select Pin (DS3905 Only)

Triple 128-Position Nonvolatile Digital

Variable Resistor/Switch

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7

Table 1. Variable Resistor Registers

ADDRESS

VARIABLE

RESISTOR

POSITION 7Fh

RESISTANCE

NUMBER OF

POSITIONS*

F8h

Resistor 0

20k

(nominal)

128 (00h to

7Fh) + Hi-Z

F9h

Resistor 1

20k

or 10k

(nominal)

128 (00h to

7Fh) + Hi-Z

FAh

Resistor 2

20k

(nominal)

128 (00h to

7Fh) + Hi-Z

*

Writing a value greater than 7Fh to any of the resistor registers

sets the high-impedance mode control bit (RHIZ, the MSB of

the resistor register) resulting in the resistor going into high-

impedance mode. Position 0 is the minimum position. Position

7Fh is the maximum position.