www.mr2turbo.info wrote:JMR_AW11 wrote:
In theory could I just monitor the bus passively and wait for the MCU to request the parameter?
It doesn't really work like that.
Wont stuff I want like temperature values be somewhere within the normal traffic though?
I don't know enough about the rev3 ECU to answer your question.
However, on some ECUs this bus can also be used for interfacing to sensor ADCs etc.If so it would be much easier to simply measure the basic temp sensors directly with your own external ADC rather than try to intercept repetitive digital data that is doing the same job.
The interface for diagnostics is different, rather like a back door into the MCU chip itself.
There is code inside the chip that will respond to (the correct) commands stuffed into the MCU via this bus. Normally this powerful code routine never ever gets run as the MCU jumps over it if there is no correctly formatted external data request.If it does get a request the new code routine will dutifully go off and retrieve the data and spit it out of the data bus.
It can do this hidden data request routine hundreds of times a second.The data request could be to ask for a specific fuel cut flag status or it could ask to see if the ECU has any error codes stored or it could just ask for the current throttle position or how fast the car is travelling or you can datalog the self learning as it runs closed loop or show any knock detection/corrections.
It really is the mutts nuts and this is because it was designed by the manufacturer for use in the initial ECU development.They needed to monitor EVERYTHING in real time and this hidden backdoor into the ECU permits it.
The request commands are different for every ECU revision and I have to reverse engineer them by looking through the ECU ROM code.
It does take a while to do this and I've done it for several ECUs now. However, on the rev3 I have yet to dump out the ROM from both MCU chips and I've had to guess/hack the commands by trial and error and that takes far too long to do. I've also had to write my own developer tools for this MCU chip as it uses a unique instruction set and it took me ages to do the disassembler and assembler modules etc.
This is another reason why progress has been slow! I will look at the rev3 again later in the year because I want to try the reprog board in it to see how much control it allows on this complex ECU.
The board will slot straight in but because the rev3 ECU has two MCU chips the control will be limited.
