ELM334
Pin Descriptions
VDD (pin 1)
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 microprocessor, so an external
reset signal is not required. Refer to the Electrical
Characteristics section for further information.
of time that the pin 4 input remains low (as long as it
is greater than the 26 msec debounce time). The
state of the RLED and the GLED lines is not updated
if the pushbutton is being pressed.
ClosedSw (pin 6), and OpenSw (pin 7)
These two inputs are for monitoring the position of
the door (or other contacts). Since these switches
might be attached to some very simple switch
mechanisms, a long 0.5 sec debounce timer (pickup
and dropout) is used on each input. This allows the
LED outputs to possibly be used with logic circuits,
as well as driving LEDs.
RLED (pin 2), and GLED (pin 3)
These two outputs are for driving LEDs (through a
suitable current limiting resistance). Logic ensures
that only one output is active at a time, allowing the
two outputs to drive a single dual type LED (that is
red if energized in one polarity, and green if the
polarity is reversed). During powerup, the red LED
will be lit for 0.5 sec, followed by the green for
0.5 sec, as a LED test.
The OpenSw input is not required for circuit
operation. If this input is not used, the LEDs will still
flash alternately when the door is opened, but after
30 seconds, the flashing will stop and the red LED
will light solidly. Please refer to the Circuit Operation
section for more details.
PB (pin 4)
A momentary low level on this pin will be debounced,
and then used to initiate a 0.5 second output pulse
on pin 5. If unused, it is preferrable to connect this
pin to VDD (but it may be left open-circuited, as there
is an internal pullup resistor).
If either pin 6 or pin 7 are not used, it is preferrable to
connect them to VDD (but they may be may be left
open-circuited, as they both have internal pullup
resistors).
VSS (pin 8)
Control (pin 5)
Circuit common is connected to this pin. This is the
most negative point in the circuit.
An active high level pulse will appear at this pin, in
response to a low level on pin 4. Only a single
0.5 sec pulse will be output, regardless of the length
Ordering Information
These integrated circuits are available in either the 300 mil plastic DIP format, or in the 208 mil SOIC surface
mount type of package. To order, add the appropriate suffix to the part number:
300 mil Plastic DIP............................... ELM334P
208 mil SOIC..................................... ELM334SM
All rights reserved. Copyright ©1999, 2008 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.
2 of 7
ELM334DSC
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
ELM [ ELM ELECTRONICS ]
