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
XT1 (pin 2) and XT2 (pin 3)
A 3.579545 MHz NTSC television colourburst
crystal is connected between these two pins.
Crystal loading capacitors (typically 27pF) will
www.DataSheet4aUlso.cnoomrmally be connected between each of the
pins and the circuit common (Vss).
OBDIn (pin 4)
The OBD data is input to this pin, with a low logic
level representing the active state (and a high,
the passive). No Schmitt trigger input is
provided, so the OBD signal should be buffered
to minimize transition times for the internal
CMOS circuitry. The external level shifting
circuitry is usually sufficient to accomplish this -
refer to the Example Applications section for a
Rx (pin 5)
The computer’s RS232 transmit signal can be
directly connected to this pin from the RS232
line as long as a current limiting resistor
(typically about 47KΩ) is installed in series.
(Internal protection diodes will pass the input
currents safely to the supply connections,
protecting the ELM322.) Internal signal inversion
and Schmitt trigger waveshaping provide the
necessary signal conditioning.
Tx (pin 6)
The RS232 data output pin. The signal level is
compatible with most interface ICs, and there is
sufficient current drive to allow interfacing using
only a single PNP transistor, if desired.
OBDOut (pin 7)
This is the active low output signal which is used
to drive the OBD bus to its active (high) state.
Typically this is accomplished by switching a PNP
type transistor on - see the Example Applications
section for more details.
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
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............................... ELM322P
208 mil SOIC..................................... ELM322SM
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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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