ELM402
Output Waveforms
Once the ELM402 has some bounce-free signals
to work with, it can generate outputs based on them.
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 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 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.
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).
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.
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
A ‘wiggle’ or vibration causes B to change,
but A does not, so the pulse is ignored
A Input
B Input
Up
debounce
delay
Down
Figure 4. Output signals (pin 5 = 0V)
ELM402DSA
Elm Electronics – Circuits for the Hobbyist
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