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ELM404

Rotary Decoder - 3 wire 2x Interface

Elm Electronics 

ELM404

Output Waveforms

Once the ELM404 has some bounce-free signals

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

The logic to decode the motion of an encoder

shaft 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 ELM404 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 ELM404 is a series of pulses, as

shown in Figure 4 (representative pulses are enlarged

and shown in Figures 5 and 6). When the encoder is

moving in a clockwise direction, the pulses will be

generated with the U/D output at a high level, while a

counter-clockwise rotation of the shaft results in pulses

generated with the U/D output at a low level. The

direction assumes that the encoder is a standard one,

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

rotation. Note that an Up output is always a result of a

change in A followed by a change in B, while the Down

output is from a change in B followed by a change in

A. Figure 4 also shows what happens when the rotary

encoder shaft is ‘wiggled’, generating pulses on only

the A or only the B input - the logic detects this and

ignores them. In order for an output to be generated,

the ELM404 must see a change on one of the rotary

encoder inputs followed by a change on the other

input.

Note that the ELM404 uses what is known as 2x

decoding of the signals. This means that for each

motion of the shaft between the detents, when the ‘A’

and ‘B’ outputs go through one complete cycle, there

will be two output signals. The other type of decoder

that is very common is the 4x decoder, as used by the

ELM405. We do not currently offer a product for 1x

decoding - if you require that, you will need to use the

ELM401 and decode the signal with your own logic.

A Input

B Input

CS

Clk

U/D

debounce

delay

A ‘wiggle’ or vibration causes B to change,

but A does not, so the pulse is ignored

ELM404DSA

Figure 4. Output signals (pin 4 = 0V)

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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