OBD II interface chip data sheet.pdf

BR16F84-1.07

OBD II Interface Chip Data Sheet

For PWM, VPW, and ISO 9141-2 Vehicles

VPW (general motors), PWM (Ford products), and ISO 9141-2 (Asian/European)

Final Release Date: Jul 8 2001

Firmware Version: 1.07 Chip ID# 107F

If you are using a completed unit, you don't need to read this data sheet. This data sheet is intended for

those who are building their own hardware using the BR16F84-1.07 Chip, or writing custom software to

employ this device in a particular application.

GENERAL DESCRIPTION:

This device is a CMOS microcontroller which is designed to interface a personal computer or laptop with a

vehicle's On Board Diagnostic (OBD II) interface. It is intended to function with all three of the protocols used

by vehicle manufacturers to implement the OBD II system as defined by SAE and ISO specifications. The

OBD II system became mandatory for 1996 and up vehicles, but some vehicles were already fully or partially

compatible with OBD II requirements prior to 1996. The chip is not suitable for the earlier vehicles such as

OBD I.

The device is intended to function as a simple scan tool and is capable of sending and receiving any OBD II

message defined in SAE J1979 for any of the three types of OBDII bus implementations (PWM, VPW, ISO

9141-2). It can also be used as an inexpensive interface for custom instrumentation monitoring various

vehicle parameters such as speed, RPM, coolant or intake air temperature, engine load, intake air flow rate,

etc.

FEATURES:

Operating Voltage

5 Ma. Typ

Clock: 20 MHz

Inputs: (0-5V levels)

Serial Input (19200 Baud)

PWM receive

VPW receive

ISO 9141-2

Outputs (0-5V levels)

Serial output (19200 Baud)

PWM transmit bus+ (inverted)

PWM transmit bus- (inverted)

VPW transmit

ISO 9141-2 L line transmit (inverted)

ISO 9141-2 K line transmit (inverted)

Operating Current

5.0 V

ELECTRICAL and PHYSICAL CHARACTERISTICS:

This device is a CMOS microcontroller produced by Microchip Technologies. You can obtain detailed

specifications for the chip from Microchip data books, or on the Microchip website, so this section will only

provide a brief summary. It is available in several package designs and temperature ratings. The baseline

BR16F84-1.07 chip is a microchip 16F84A-20/P part, which is a commercial temperature range plastic 18 pin

DIP rated at 20 MHz. The device can be special ordered if other package designs, temp ratings, etc are

desirable.

TYPICAL APPLICATION CIRCUIT:

The typical application is a simple automotive scan tool using a PC or laptop as the host machine. The

interface to the PC is a serial link running at 19200 baud. Only 3 wires are needed, as no handshaking

signals are used. The serial output from the interface circuit is a 5 volt signal, which works well with most

PC's and laptops. The serial input is clipped and current limited, so it is capable of accepting almost any

signal voltage. The interface to the vehicle OBD bus is accomplished with a few external components and an

LM339 comparator chip. The connection to the vehicle uses 6 wires, assuming all three OBD protocols are to

be implemented. Operating voltage for the chip and associated circuits is small, and is derived from the

vehicle's 12V system on pin 16 of the OBD connector.

Refer to the schematic of a typical application. This design uses readily available components.

APPLICATION HINTS

(1) The cable to vehicle can be simple unshielded wires. This cable should be kept as short as possible, and

definitely no longer than 4 feet, especially for PWM operation. Somewhat longer cables could be used if the

circuit's termination resistors are replaced with lower values.

(2) The cable to Computer serial port can also be unshielded. The application circuit has been tested with

unshielded cables up to 30 Ft., but specific implementations may vary, so you may want to experiment if you

are attempting long runs. For every long runs, you might also wish to use an RS 232 transceiver chip in your

design.

(3) Circuit layout is not critical. The circuit has been tested on both hard wired perfboard prototypes and

plugboard breadboards. However, if your layout is very sloppy, you may want to add additional bypass

capacitors.

(4) Free software to utilize the application circuit is available for download. The program will run under DOS,

so it can be used on older computers as well as newer machines running windows. It is small enough that it

will fit on a DOS boot floppy, so you can still run it if your main operating system is not DOS compatible. The

computer need not even have a hard drive.

PARTS LIST :

Value

Description

Qty

Designators

.01 UF

Capacitor, 25V

C5, C6

.1 UF

.47 UF

Capacitor, 50V

20 pf

C2,C1

10 KOhm Resistor, 1/4 Watt

R11, R12, R23, R9, R1, R3, R4, R25, R8

18 KOhm "

R24

1 KOhm "

R19, R18, R7, R20, R22, R5, R6, R13

5.6 KOhm "

R21

510 Ohm "

R16, R17

62 KOhm "

R10

9.1 KOhm "

R15

270 Ohm Resistor, 1/2 Watt

R14

Not used

1N4739

Zener, 9.1V, 1W

1N4148

Diode

D1, D3

PN2222

Transistor, NPN

Q3, Q4, Q5

PN3904

Transistor, NPN

PN3906

Transistor, PNP

7805

Regulator, 5V

IC2

LM339N

Quad Comparator

IC3

16F84

Microprocessor

IC1

20 MHz

Crystal, 20 Mhz

XTL 1

Miscellaneous:

Board, connectors, cable, wire, sockets, enclosure, etc,

Depending on your implementation.

NOTES:

1) All Resistors can be 5% Carbon Film types (Don't use wirewound types)

2) Capacitor voltage ratings can be higher than specified.

3) Capacitors can be ceramic or monolithic.

4) Microprocessor uses Ver 1.07 firmware

5) Transistor packages can be substituted. Example: 2N3904 for PN3904

CHIP COMMANDS AND RESPONSES:

This section details the information needed to send and receive data to the chip from a host computer. It will

enable you to write your own software to utilize the chip with whatever host computer or other control device

you may wish. If you are using the provided program, you need not read this section. Please note that in the

following discussion, all numerical values are in hexadecimal base unless otherwise mentioned.

GENERAL DISCUSSION OF COMMUNICATIONS

This is an overview of communications between the chip and the host computer or controller. There are

exceptions to these rules which are defined in the more detailed sections of this document. All

communications is via a simple 3 wire serial link. No handshaking is used. The chip listens for a message,

completes the task and then returns the results to the host. Then, it immediately waits for the next message.

The typical exchange to or from the chip is a series of bytes. The first byte is called the control byte. The

concept of the control byte is discussed here because it applies to many of the commands and responses

(but not all; exceptions are noted in the specific commands defined below). Normally, the control byte is

simply a number between 0 and 15 decimal (0-F hex) which indicates how many bytes follow this control

byte. For example, if three bytes are to be sent, the control byte would be 3 decimal (or 03 hex), then the

three bytes would follow; 03, byte1, byte2, byte3. This is the method used to ask the chip to retrieve a

particular item of vehicle OBD data, and it is the method used by the chip to return the Vehicles response to

the request message. Note that this only requires using the lower four bits of the control byte. The upper bits

are reserved for certain special commands and responses. These bits are used for the host computer to

make initial connection with the chip, to set the protocol to be used, and for the chip to return data indicating

success or failure. In particular, note that the chip will set the most significant bit of the control byte if the

action was not successful. If the operation was successful, the upper four bits of this control byte will be zero.

These special uses of the control byte are discussed below, and exceptions where the control byte is not

used are also indicated.

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