Interresting counting problem for an experimental ECU

[davidbue]

Hello everybody!

Now... I've got a problem that I don't really know how to solve. Any suggestions are appreciated!

I'm currently developing an engine control unit (ECU) for an eco-car. It currently runs on a fuel called dimethyl-ether. If you compare the released energy from burning dimethyl-ether with gasoline, it goes what corresponds to 306 km/l (720 mpg) (http://www.ecocar.dk/)

I'm using a pic18F2220 at 36Mhz to time an injector that delivers the fuel to the engine.


When running the engine at full speed I recieve a 72 khz (TTL like) signal from a rotary encoder.

Once pr. engine revolution I recieve another TTL like signal.


Here's my problem:

- A main routine runs within a while(1) loop, performing all sorts of stuff.

Parallel with this, I want to:

- Count up in a variable, the 72 khz signal starting from when I recieve my "once-pr.revolution pulse"

-After a certain number of pulses from the 72 khz signal, I need to engage the injector.


I've thought about using interrupts, but I fear that 72 khz is to fast.

Maybe one could put an interrupt on the "once pr. revolution" signal, resetting a timer with an external clock that is fed by the 72 khz signal?

Maybe you've got a much better idea?


Thanks in advance!
David

[Ken Johnson]

Yes, that's pretty fast, < 14 usec. You might have a fast interrupt which increments a counter and then opens/closes the injector, with some tight (likely asm) coding.

Aternative, measure the time between the "once per rev" pulses, and set a timer to do the injector. This will lag (only slightly) the engine accel/decel, but might be ok?

Ken

[Guest]

Good suggestions, Ken.

Unfortuenatly sampling only the "once pr. rev" sgnal is not good enough. It is a requirement that I use the 72 khz signal :-(

Maybe a setup with two microcontrollers, where the one only does the counting... hmmm.. But that's not pretty...

[Ttelmah]

Feed the 72KHz signal into Timer1, or Timer3.
Have an interrupt from the 'other pulse', in which you program a CCP channel, set to count the required number of pulses from the timer you are using. Program this to set or clear it's output pin on the required count (direction will depend on what the injector wants). Connect the injector to the output pin, or if the injector needs a precise pulse width, feed this into a non-retrigerable monostable multivibrator, programmed to give the required width. Poll the pin at intervals in the main, and clear it once it is set.

Best Wishes

[rnielsen]

I did something similar to this several years ago but I was using 80C51's. I used a 360 pulse encoder with a 'home' pulse output to signal TDC. Each signal was feed into an interrupt input. The 'home' pulse was fed into int0 and the 360 line was fed into int1. On the 80C51 the int0 could be set to a higher priority. I used a 360 pulse encoder to track each degree of the distributor shaft.

I simply counted pulses and then triggered the injector at the appropriate count. Worked flawless for me. Since I was controlling both the injectors and spark I used a separate controller for each and fed the same encoder signals into each.

Ronald

[asmallri]