ELM406
Output Waveforms (continued)
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
ELM407. 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.
CS
CS
Clk
Clk
400 µsec
200 µsec
U/D
U/D
this will be low for a
counter-clockwise motion
Figure 5. Up or Clockwise Output
Figure 6. Up Output, with Write Enabled
Design Considerations
There are a few details to consider when using the
ELM406 to interface to a rotary encoder. The first is
the fact that the signals available from the encoder are
usually only dry contacts closing and opening. You will
need to provide pullup resistors for these signals in
order to use them in a circuit.
values of no more than about 330 pF, while with a
2.0V supply, the limit would be about 100 pF.
One other issue to consider is that during the initial
circuit startup, there is a period (of about 20 msec)
when the ELM406 is being held in a reset state, and
the outputs are in a tristate condition. During this time,
the outputs will sit at the level they were at before
power up (0V) due to stray capacitance having
discharged through the protection diodes. Since the
CS and the Clk outputs are active low, this may cause
a problem if your controlled circuit is ready too fast. If
you require that pins 6 and 7 go to a high level as
quickly as possible, you may want to install pullup
resistors (of about 10 KW) on these pins to ensure that
the voltage rises quickly to VDD.
The size of the pullup resistor is chosen based on
the encoder specifications. The main concern is the
maximum current carrying capacity, which sets a lower
limit for the pullup resistance. An upper limit for the
resistance is set by the minimum current required for
contact wetting. If you do not provide enough current
through mechanical contacts when they are closed,
they will tend to go open with time. A maximum current
specification is usually in the range of 1 to 10 mA,
while the minimum wetting current would be in the
range of 1 mA. This means that with a 5V supply, a
pullup resistor of 5 to 10 KWis typically required.
After the initial 20 msec period, the ELM406 sets
all pins to their quiescent levels, but does not change
any outputs for an additional 50 msec. This ensures
that the external circuits have had adequate time to
initialize, before being presented with signals to
process.
The second concern is the use of capacitors on
the ‘A’ and ‘B’ signal lines. Many encoder circuits show
these as a way to provide some pre-filtering of the
signal. That is fine, as long as you realize that the
ELM406 inputs are CMOS and do not have Schmitt
trigger waveshaping. This means that you should keep
the rate of change of the input signal as high as
possible to avoid problems (we usually try to maintain
at least 1V/µsec). Typically, with a 5V supply, a 10 KW
pullup, and TTL thresholds, this means capacitor
ELM406DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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