GM OBD-I (OBD1) ALDL Microcontroller LCD Interface \ Scan Tool Project Overview
Project status: Project Completed
Project first started: February 2007
Project finished: Late April 2007
Project Scope: 2007 Senior Design Engineering Project @ Old Dominion University
GM OBD-I ALDL AVR LCD interface pulling data off a 7730 $8D ECU
Heads-up:
This project was done for my 2007 Senor design project class in college. I have copied and pasted a lot of the description out of the project formal report. Much of this information was written in a formal technical writing manor, I have since gone back and added some for more in-depth information others might find useful who have more background knowledge on GM engine control systems then my professor who read the report at the time. That is also why you will see two distinct writing styles, the more formal style used in the report and my more laid back personal how-to style of the added information sections.
Abstract:
A microcontroller based automotive computer interface is described. The device utilizes a propriety serial interface to connect to an automotive engine control computer and retrieve current engine conditions. This information is then processed and output to a LCD. The hardware, software, and interface details are described.
Project Overview:
In this project an interface and display was designed to retrieve automotive diagnostic data from a late 1980's to early 1990's General Motors automotive engine control unit / computer known as an ECU. This interface utilizes an Atmel AVR 8-bit microcontroller to perform serial communication over a one wire serial interface with the ECU. This diagnostic data is then processed by the AVR microcontroller and outputted to a LCD in an easy to read format for the user to view.
Background on Project:
This project will focus on interfacing with an onboard automotive computer using the GM later 8192 baud OBD-I interface. GM started
using the 8192 baud OBD-I interface in 1986 and continued using it widely until 1995 when a universal interface and protocol was mandated
by SAE (OBD-II). The OBD-I interface on GM cars is a proprietary GM interface.
The ECU chosen for this project is one found is many late 1980's and early 1990's GM vehicles and is know by its part number, 1227730
(or 1227727 which is the under hood version). The ECU has a customized version of the Motorola 6811 processor and runs code written
in Assembly. The processor runs at 8.388 Mhz's, has 2 kilobytes of RAM, and a 32 kilobyte UV EPROM for code and calibration data. This
code has since been disassembled and commented by auto hobbyists for the general public use. GM has platform dependent code which is
vehicle specific and is called code "masks". The specific code mask a vehicle runs is determined by engine type, transmission configuration,
which ECU is used in the vehicle, and other vehicle features. The code mask used in this project is $8d, which is used on the 1990-1992
Pontiac Firebird, 1990-1992 Chevrolet Camaro, and the 1990-1991 Chevrolet Corvette. ECU code along with engine specific data such as
fuel maps are hard coded into a removable EPROM chip inside the ECU, which is called the "MemCal", which stands for Memory Calibration
Unit. This made it easy for GM to use the same ECU across different platforms and for technicians to upgrade the calibration in case
a problem was found after production.
Page content last updated: 09-05-2009