ELM ELM320SM, ELM320P Datasheet

ELM320

OBD (PWM) to RS232 Interpreter

Description

1238765

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2

3

5

6

4

7

Features

Since the 1996 model year, North American
automobiles have been required to provide an OBD,
or On Board Diagnostics, port for the connection of
test equipment. Data is transferred serially between
the vehicle and the external equipment using these
connections, in a manner specified by the Society of
Automotive Engineers (SAE) standards. In addition
to operating at different voltage levels, these ports
also use a data format that is not compatible with the
standard used for personal computers.

The ELM320 is an 8 pin integrated circuit that is
able to change the data rate and reformat the OBD
signals into easily recognized ASCII characters. This
allows virtually any personal computer to
communicate with an OBD equipped vehicle using
only a standard serial port and a terminal program.
By also enhancing it with an interface program,
hobbyists can create their own custom ‘scan tool’.

This integrated circuit was designed to provide a
cost-effective way for experimenters to work with an
OBD system, so many features such as RS232
handshaking, variable baud rates, etc., have not
been implemented. In addition, this device is only
able to communicate using the 41.6KHz J1850 PWM
protocol that is commonly used in Ford Motor
Company vehicles.

• Low power CMOS design

• High current drive outputs - up to 25 mA

• Crystal controlled for accuracy

• Configurable with AT commands

• Standard ASCII character output

• High speed RS232 communications

• 41.6KHz J1850 PWM Protocol

Connection Diagram

PDIP and SOIC

(top view)

VDD VSS
XT1
XT2

OBDIn Rx

OBDOut
Tx

Applications

• Diagnostic Trouble Code Readers

• Automotive Scan Tools

Block Diagram

Tx

Rx

ELM320DSB

3.58MHz

XT1 XT2

Timing and

Control

RS232

Interface

Interpreter

OBD

Interface

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OBDIn

OBDOut

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ELM320

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.

XT1 (pin 2) and XT2 (pin 3)

A 3.579545MHz 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 Applications section for a
typical circuit.

Rx (pin5)

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 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.

VSS (pin 8)

Circuit common is connected to this pin. This is
the most negative point in the circuit.

Ordering Information

All rights reserved. Copyright 2001 - 2002 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.

ELM320DSB

These integrated circuits are available in either the 300 mil plastic DIP format, or in the 200 mil SOIC surface

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

300 mil Plastic DIP............................... ELM320P 200 mil SOIC.....................................ELM320SM

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ELM320

Absolute Maximum Ratings

Storage Temperature.......................-65°C to +150°C

Ambient Temperature with

Power Applied....................................-40°C to +85°C

Note:
Stresses beyond those listed here will likely damage

the device. These values are given as a design
guideline only. The ability to operate to these levels

Voltage on VDD with respect to VSS............0 to +7.5V

is neither inferred nor recommended.

Voltage on any other pin with

respect to VSS........................... -0.6V to (VDD + 0.6V)

Electrical Characteristics

All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.

Characteristic Minimum Typical Maximum ConditionsUnits

Supply voltage, VDD 4.5 5.0 5.5 V

VDD rate of rise 0.05 V/ms

Average supply current, IDD 1.0 2.4 mA

Input low voltage VSS 0.15 VDD V

Input high voltage VDD V0.85 VDD

see note 2

see note 3

Output low voltage 0.6 V

Output high voltage VVDD - 0.7

Rx pin input current

RS232 baud rate

-0.5

+0.5

9600

Notes:

1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX as the core embedded
microcontroller. For further device specifications, and possibly clarification of those given, please refer to the
appropriate Microchip documentation (available at http://www.microchip.com/).

2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved
using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as
may be obtained through direct connection to solar cells, or some charge pump circuits.

3. Device only. Does not include any load currents.

4. This specification represents the current flowing through the protection diodes when applying large voltages
to the Rx input (pin 5) through a current limiting resistance. Currents quoted are the maximum that should be
allowed to flow continuously.

5. Nominal data transfer rate when a 3.58 MHz crystal is used as the frequency reference. Data is transferred
to and from the ELM320 with 8 data bits, no parity, and 1 stop bit (8 N 1).

mA

baud

Current (sink) = 8.7mA

Current (source) = 5.4mA
see note 4

see note 5

ELM320DSB

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