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ELM408

Rotary Decoder - 2 wire Interface

Elm Electronics 

ELM408

Output Waveforms

Once the ELM408 has some bounce-free signals

to work with, it can generate outputs based on them.

The output sequences that the ELM408 generates

depends on the direction of shaft rotation, and on the

level at the Write Enable input (pin 4). The direction

logic always assumes that the encoder is a standard

one, where the ‘A’ signal leads the ‘B’ for a clockwise

or ‘up’ rotation. Note that the level at pin 5 only

determines when the sequences are output, and does

not affect the waveshape or timing of them.

Figures 4 to 7 below show the sequences that the

ELM408 can generate. In all cases, the basic time

interval is 200 µsec (if it does not seem apparent).

Figures 4 and 5 show the two types of sequences that

would occur if the Write Enable is low (ie disabled).

The level at the U/D output when the CS signal goes

low determines whether the the controlled device will

count up (level is high), or down (level is low) when the

U/D pin next goes from low to high.

For many digital potentiometers, the level at the

U/D pin when the CS output returns high determines

whether the current setting of the digital potentiometer

is to be stored in non-volatile memory or not. If the

ELM408’s Write Enable input is high, then the U/D

output will be held high during the transition, causing a

write for many digital potentiometers. The resulting

waveforms are shown in Figures 6 and 7.

The logic to decode the motion of an encoder

shaft, and so decide when to provide output

sequences is not as simple as it would first appear.

Some authorities recommend simply monitoring an

input and when it changes, provide an output based on

the level of the other input. This does not always work,

as the encoder can output multiple signals from only

the ‘A’ or or only the ‘B’ contact if the shaft is moved

ever so slightly when at the detent or at the mid-point

position (between detents). Simply seeing one input

change is not sufficient to say that there is any

significant shaft rotation.

The ELM408 monitors both ‘A’ and ‘B’ transitions,

and determines the outputs based on the sequence in

which the transitions have occurred. This is a better

way to guarantee that the output signals are generated

properly. The internal logic also performs some self-

checking, and monitors for problems such as an output

pulse being initiated before the previous one had

completed, which might occur for some very fast

inputs (the second one will be ignored in this case).

The output of the ELM408 is a series of pulses, as

shown in Figures 8 and 9. The first figure shows 2x

CS

200

U/D

µsec

Figure 4. Up (Clockwise) with

Write Enable = Low

CS

200

U/D

µsec

Figure 5. Down (Counterclockwise) with

Write Enable = Low

CS

U/D

200

µsec

Figure 6. Up (Clockwise) with

Write Enable = High

CS

U/D

400

µsec

Figure 7. Down (Counterclockwise) with

Write Enable = High

ELM408DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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