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PDF ELM408 Data sheet ( Hoja de datos )
| Número de pieza | ELM408 | |
| Descripción | Rotary Decoder | |
| Fabricantes | ELM | |
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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.
Applications
• Digital audio potentiometer controls
• Variable voltage or temperature circuits
• Positioning controls
• Tuning circuits
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)
VDD 1
A2
B3
Write 4
Enable
8 VSS
7 CS
6 U/D
5 4x/2x
Block Diagram
VDD A
Debounce
2
Circuit
Direction
Output
VDD B
Decoding
Debounce
3
Circuit
Logic
7 CS
6 U/D
Rotary
Encoder
Write 4
Enable
5 4x/2x
ELM408DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
1 of 10
Free Datasheet http://www.datasheet4u.com/
1 page  
ELM408
Rotary Encoders
A rotary encoder (sometimes referred to as a
quadrature encoder) is a device that produces digital
(on/off) outputs in response to rotary, or circular,
motion. It is often constructed such that it looks very
much like a potentiometer, or audio volume control
(see the picture of a typical device, at the right).
As the encoder shaft is turned, internal contacts
open and close, creating two waveforms that are
ideally separated in phase by 90 degrees (ie ‘in
quadrature’). Actually, you need to provide external
‘pullup’ resistors and a power supply to create these
waveforms, as the contacts themselves can not do
this. An ideal waveform from a rotary encoder would
look like this:
A
B
Figure 1. Quadrature Waveforms
Due to the 90 degree phase difference, when one
waveform changes, the other is always stable. By
noting the direction of the change and the level of the
other input at that time, you can determine the
direction of motion of the shaft.
Rotary encoders are not ideal, however. Due to
their construction, and variations in shaft speed, the
A typical rotary encoder
waveforms are not perfectly square with the 50% duty
cycles shown. Figure 2 shows a captured trace from a
real rotary encoder that is more representative of what
you will typically find. Note that the two ‘scope
channels (1 and 2) represent the encoder outputs A
and B, respectively. The ch 1 (A) waveform leads the
ch 2 (B) waveform, which usually means that the shaft
is turning in a clockwise direction.
The first rising edge of the channel 2 waveform
shows another problem that occurs with moving
mechanical contacts - multiple pulses due to bounce.
When two contacts meet, the moving one will tend to
bounce, like a ball does when it is dropped on the
floor. Each bounce results in an electrical connection
being made, then broken, which will look like multiple
inputs to a fast electronic circuit. Various mechanical
means are used to reduce the amount of bounce, but it
can never really be eliminated. The following section
discusses how the ELM408 uses electronic means to
remove the bounce.
ELM408DSA
Figure 2. Actual Rotary Encoder waveform
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
5 of 10
Free Datasheet http://www.datasheet4u.com/
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| Páginas | Total 10 Páginas | |
| PDF Descargar | [ Datasheet ELM408.PDF ] | |
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