ELM408
Pin Descriptions
VDD (pin 1)
4x/2x (pin 5)
This pin is the positive supply pin, and should always
be the most positive point in the circuit. Internal
circuitry connected to this pin is used to provide
power on reset of the internal logic, so an external
reset signal is not required. Refer to the Electrical
Characteristics section for more information.
This input controls how many sets of output pulses
are generated for each complete cycle of the A and
B inputs. If it is high, the CS and U/D outputs will
sequence 4 times for each (one for each of the four
transitions). If the input is low, only two sets of
pulses are generated.
Note that the ELM408 is not capable of generating
1x decoding (one set of output pulses). If your
application needs this, you may be able to use the
ELM401 and generate them in software.
A (pin 2)
This input should be connected to what is normally
known as the ‘A’ signal from a rotary (quadrature)
encoder. The signal at this pin should lead (ie
change state before) the signal at pin 3 when the
device is being moved in a clockwise or up direction.
U/D (pin 6)
The ‘Up/Down’ output serves multiple purposes
when used with a typical digital potentiometer. The
level that it is at when the CS output transitions low
typically sets the ‘mode’ (count up or count down),
while rising edges of the signal are usually used to
generate ‘count’ (ie clock) pulses. Finally, the level
that U/D is at when the CS output returns high will
often determine whether the potentiometer setting is
written to non-volatile memory or not.
An external pullup resistor is required for the encoder
(a typical value is 10 KW), but no external capacitors
are needed (as the internal debounce circuitry
provides the filtering).
B (pin 3)
This input should be connected to what is normally
known as the ‘B’ signal from a rotary (quadrature)
encoder. The signal at this pin should lag behind (ie
change state after) the signal at pin 2 when the
device is being moved in a clockwise or up direction.
CS (pin 7)
This output provides a ‘Chip Select’, or enable
function. It works with the U/D output to provide
various control functions. See the Output Waveforms
section (on page 7) for more information.
An external pullup resistor is required for the encoder
(a typical value is 10 KW), but no external capacitors
are needed (as the internal debounce circuitry
provides the filtering).
VSS (pin 8)
Write Enable (pin 4)
Circuit common is connected to this pin. This is the
most negative point in the circuit.
This input controls the state of the U/D output when
the rising edge of the CS output occurs. If pin 4 is
high, the U/D output will be set high during the CS
transition, which generates an EEPROM write for
many digital potentiometer chips.
All rights reserved. Copyright 2011 Elm Electronics.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
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ELM408DSA
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