Old 486 Board for ECU??

[Axel Rietschin]

----- Original Message -----
From: "Mike from West Australia" <erazmus at wantree.com.au>
To: <diy_efi at diy-efi.org>
Sent: Tuesday, May 02, 2000 01:57
Subject: Re: Old 486 Board for ECU??


>
> At 05:08 PM 1/5/2000 +0200, "Axel Rietschin"
> <Axel_Rietschin at compuserve.com> wrote:
>
> >How many concurrent PWMs (that's the WHOLE point of this entire
discussion)?
>
> How many do you want ?
>

More than a PC can handle: one for the EGR valve, another for the air bypass
valve, two for the variable distribution (in case the engine has that), one
for the wastegate control, maybe one for the lambda heater (depending on the
type of sensor)... Also, I can think of water injection, intercooler spray
bar, thermostat, banked injection (2 injectors per cylinders with different
pulse width and different crank-related timing)...

> Of course its obviously sensible to go for the latest hardware to make
> the software development far easier and much more so for higher rpm
> and more compensation/operational factors etc etc Part of this is the
> push from the semi co to get devices into sale - sexy apps, marketing
> etc etc.

It not only make it much easier, it actually leaves room for more
interesting things, in the code space and the developer's head.

> Engine at 9000 rpm = 150 per second = dist 75 per second
> For a CAS with 360 deg pulses/rev thats 75*360 = 27KHz

This gives you a base resolution of 37 microseconds. *Far* too coarse to run
any engine at low speed. Forget driveability and low emissions :)

Of course, you can always interrupt at 54KHz, this will give you 18.5us
resolution (still too coarse) and leave you with ~400 instructions. Since
you say you need only about 300, you'll consume "only" 75% of your CPU
cycles in the crank pulse ISR. Not much room left for 'background'
processing , let alone extra 'intelligence' :)

Also, if you have 360 teeth, how do you detect TDC and cylinder number?
Sequential injection requires 720 deg. sync. That's already two interrupts
(unless you sense missing or extra teeth, something better left off to a
TPU), more jitter and less available cycles.

> In the background you no such time critical requirement, you can handle
> 10mS ADC acquisition of AFM or MAP to put a number in a register

Certainly, within the constaints mentionned above, but you keep ignoring
frequency and PWM *inputs* :)

More and more sensors (AFM, MAP, temp) seems to use this type of signals
nowadays, probably because of the improved immunity to interferences. Some
even use CAN.

> Don't get me wrong, I would still rather use peripheral features where
> appropriate, I'm just pointing out you don't need to and that the core
> hardware will do adequately with the addition of ADC and signal
> conditioning.

OK, I'd say that if you don't want to do anything beyond the capabilities of
the simplest ECU you can think off, can live with only one or two analog
inputs and only injectors output, are not concerned about peak performance
nor strict emissions, you can probably work out some experimental thing
based on a PC motherboard. That said, we have yet to hear from someone that
successfully ran an engine past idle speed using such an ECU ;)

--Axel




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