ELM320

Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

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............................... ELM320P

208 mil SOIC..................................... ELM320SM

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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.

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.

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

also normally 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 high

logic level representing the active state (and a

low, 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 –

see the Example Application section for a typical

circuit.

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 47K

Ω) is installed in series.

(Internal protection diodes will pass the input

currents

safely

to

the

supply

connections,

protecting the ELM320.) 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 state. Since the

J1850 PWM standard requires a differential bus

signal, the user must create the complement of

this signal to drive the other bus line. See the

Example Application section for more details.

Circuit common is connected to this pin. This is

the most negative point in the circuit.

ELM320DSC