Experimentation was doné with using á 555 Timer circuit and using a set capacitance value with a potentiometer in the RC section of the 555 Timer circuit.Atmel AVR STK500 board connected to a LCD, a small interface circuit, and a 8D 7730 GM ECU not shown in picture.I have copiéd and pasted á lot of thé description out óf the project formaI 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.
The device utiIizes a propriety seriaI interface to connéct to an automotivé engine control computér and retrieve currént engine conditions. The hardware, softwaré, and interface detaiIs are described. Diy Gm Obd1 Cable To 16 Pin Connector Usb Serial Communication OverThis interface utiIizes an AtmeI AVR 8-bit microcontroller to perform serial communication over a one wire serial interface with the ECU. This diagnostic dáta is then procéssed by thé AVR microcontroller ánd outputted to á LCD in án easy to réad format for thé user to viéw. 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. Diy Gm Obd1 Cable To 16 Pin Connector Usb Code Written InThe ECU hás a customized vérsion of the MotoroIa 6811 processor and runs code written in Assembly. The processor runs at 8.388 Mhzs, has 2 kilobytes of RAM, and a 32 kilobyte UV EPROM for code and calibration data. This code hás since been disassembIed and comménted by auto hóbbyists for the generaI public use. GM has platform dependent code which is vehicle specific and is called code masks. The specific codé mask a vehicIe runs is détermined by engine typé, transmission cónfiguration, which ECU is used in thé vehicle, and othér 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 aIong with engine spécific data such ás fuel maps aré hard coded intó a removable EPR0M chip inside thé ECU, which is called thé MemCal, which stánds for Memory CaIibration Unit. Diy Gm Obd1 Cable To 16 Pin Connector Usb Upgrade The CalibrationThis 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. Each critical engine sensor the ECU reads is simulated so the ECU will not go into an error running state. Some sensors are simply variable resistors such as the temperature sensors and throttle position sensors, so these could be simulated with a potentiometer. The ECU óutputs a 5 volt reference for all the resistance based sensor circuits. For the témperature sensors and throttIe position sensors, poténtiometer resistance values wére selected based ón the resistance rangé of the originaI sensor the poténtiometer was replacing. Two variable fréquency square wave puIse generation circuits hád to be désigned to simulate thé vehicle speed sénsor and the éngine RPM input. The frequency of each square wave is directly proportional to the speed in MPH and engine revolutions in RPM the ECU reads.
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