ELM408
Rotary Decoder - 2 wire Interface
Description
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.
Features
• 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
Connection Diagram
PDIP and SOIC
(top view)
V
DD
1
2
3
4
8
7
6
5
V
SS
CS
U/D
4x/2x
Applications
• Digital audio potentiometer controls
• Variable voltage or temperature circuits
• Positioning controls
• Tuning circuits
A
B
Write
Enable
Block Diagram
V
DD
A
2
Debounce
Circuit
7
V
DD
B
3
Direction
Decoding
Debounce
Circuit
Output
Logic
6
CS
U/D
Rotary
Encoder
Write
Enable
4
5
4x/2x
ELM408DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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