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ELM403DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM403

Output Waveforms

Once the ELM403 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 ELM403 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 ELM403 is simply a series of

pulses, as shown in Figure 4. 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.

Outputs are usually generated each time that one

of the inputs change, so for each full cycle of both of

the inputs, there will be 4 output pulses. This is what is

known as 4x decoding. The other type of decoder that

is very common is the 2x decoder, as used by the

ELM402. We do not currently offer a product with 1x

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

ELM401 and decode the signal with your own logic.

The duration of the output pulse is set by the level

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

approximately 2.0 msec long, while a low level will

result in 200 µsec wide pulses. Pin 5 can be used to

invert the outputs, so that they are normally at a high

level, and switch to a low level for each pulse.

Output pulses do not always occur when one of

the inputs change. After startup, the first output can

not be generated until the ELM403 has seen both of

the inputs change, and no output is generated until

then. Also, if an input is seen to be ‘chattering’ the

ELM403 will block the output. Chattering can occur

when the shaft is wiggled, or when it is subjected to

severe vibration. Figure 5 on the next page shows how

the ELM403 handles the multiple inputs due to chatter.

If this logic were not in place, multiple outputs could

occur due to vibration (as found in an automobile, or

an industrial setting), and the setting could ‘creep’ with

time – even though the shaft was not turned.

Generally, output pulses will occur in groups of

four (between each detent), so resistance values or

settings will repeat consistently as the shaft is rotated

in one direction and then in the other. Depending on

when the shaft changes direction, however, the pulses

may not be generated in groups of four. If you are

designing a circuit and require that the controlled

variable must repeat exactly as the shaft is turned in

either direction, you may prefer to use a 2x decoder

such as the ELM402.

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

A Input

B Input

Up

Down

debounce

delay

direction changes