ELM408

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Connection Diagram

PDIP and SOIC

(top view)

1

2

3

4

8

7

6

5

Rotary Decoder - 2 wire Interface

The ELM408 is an 8 pin integrated circuit that is

used to convert the output from a rotary encoder into

two signals (chip select, and up/down) that can be

used to control various devices. The low power

CMOS technology used ensures that only a very

small current is required over the entire 2.0 to 5.5

volt operating range.

There is no need for external filtering or

debounce circuits with the ELM408, as this is all

performed

within

the

integrated

circuit.

After

debouncing

the

encoder

signals,

the

ELM408

determines the direction of shaft rotation, and then

generates the appropriate outputs. A write sequence

can also be generated if the Write Enable input is at

a high level when the outputs are generated.

The ELM408 provides both 2x and 4x decoding

of

a

rotary

encoder

signal.

See

the

Output

Waveforms section for more information.

A

B

Description

• Digital audio potentiometer controls

• Variable voltage or temperature circuits

• Positioning controls

• Tuning circuits

Applications

Block Diagram

1 of 10

• Low power CMOS design

• Wide supply range – 2.0 to 5.5 volts

• Complete debouncing of the encoder inputs

• No external filtering needed

• 2x and 4x decoding

• Can generate a write sequence

• Startup delay timer

• High current drive outputs

Features

ELM408DSA

Write

Enable

CS

U/D

B

7

A

Debounce

Circuit

2

CS

5

Direction

Decoding

4

Write

Enable

Output

Logic

3

U/D

Rotary

Encoder

Debounce

Circuit

6

4x/2x

4x/2x