ELM402P データシートの表示（PDF） - Elm Electronics

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ELM402P

Rotary Decoder - 2 wire 2x Interface

Elm Electronics 

ELM402

Output Waveforms

Once the ELM402 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 ELM402 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 ELM402 is simply a series of

pulses, as shown in Figures 4 and 5. When the shaft is

moving in a clockwise direction, there will be pulses at

the Up output, while a counter-clockwise rotation

results in pulses at the Down output. This assumes

that the encoder is a standard one, where the ‘A’

signal leads the ‘B’ for a clockwise rotation. Note that

the Up output pulses are always a result of a change

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

pulses result 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 ELM402 must see a change on one of

the rotary encoder inputs followed by a change on the

other input.

The Up or Down output pulse width is set by the

level at pin 4. If pin 4 is at a high level, the pulse width

will be approximately 2.0 msec in duration, while a low

level on pin 4 will result in 200 µsec wide pulses. Pin 5

can be used to invert the output, as shown in Figure 5.

Note that the ELM402 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

ELM403. 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

Up

Down

debounce

delay

A ‘wiggle’ or vibration causes B to change,

but A does not, so the pulse is ignored

Figure 4. Output signals (pin 5 = 0V)

ELM402DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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