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Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

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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.

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.

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 these pins and the

circuit common (Vss).

LFmode (pin 4)

This input is used to select the default linefeed mode

after a power-up or system reset. If it is at a high

level, then by default messages sent by the ELM323

will be terminated with both a carriage return and a

linefeed character. If it is at a low level, lines will be

terminated by a carriage return only. This behavior

can always be modified by issuing AT L0 or AT L1

commands (see the section on AT Commands).

RS232Rx (pin5)

A 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 resistor current safely to the supply

connections, protecting the ELM323.) Internal signal

inversion and Schmitt trigger waveshaping provide

the necessary signal conditioning.

RS232Tx (pin 6)

This is the RS232 transmit or data output pin. The

signal level is compatible with most interface ICs,

and there is sufficient current drive to allow

interfacing using only a PNP transistor, if desired.

LED Drive Outputs (pins 7, 8, 9, and 10)

These four pins are driven to low levels when the

ELM323 is transmitting or receiving RS232 or OBD

data. Otherwise, they are at a high level. Current

capability is suitable for directly driving most LEDs

through current limiting resistors. If unused, these

pins should be left open-circuited.

OBDIn (pin11)

The OBD data is input to this pin, with a high logic

level representing the active state of the OBD K line.

No Schmitt trigger input is provided, so the OBD

signal should be buffered to minimize transition

times for the internal CMOS circuitry.

OBDL (pin 12) and OBDK (pin 13)

These are the active high output signals which are

used to drive the OBD bus, using external NPN

transistors. Data transfer normally occurs only by the

K line, but the standards require that the L line be

implemented as well in order to properly initialize the

bus. See the Example Applications section for more

details.

VSS (pin 14)

Circuit common is connected to this pin. This is the

most negative point in the circuit.

Ordering Information

These integrated circuits are available in either the 300 mil plastic DIP format, or in the 150 mil SOIC surface mount

type of package. To order, add the appropriate suffix to the part number:

300 mil Plastic DIP...................................ELM323P 150 mil SOIC........................................ ELM323SM

ELM323DSD

ELM323

1 2 3 4 5 6 7 ... 18 19

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