ELM404
Output Waveforms
Once the ELM404 has some bounce-free signals
to work with, it can generate outputs based on them.
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.
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.
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.
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
A ‘wiggle’ or vibration causes B to change,
but A does not, so the pulse is ignored
A Input
B Input
CS
debounce
delay
Clk
U/D
Figure 4. Output signals (pin 4 = 0V)
ELM404DSA
Elm Electronics – Circuits for the Hobbyist
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