Correction: The $2.50 MC9S08 with the analog to digital converter is 16
pins and not 14.  We may get it smaller later on if I can get a cheap
programmer.

This chip does not have a pulse width modulator so we are going to do it
using a timer.

How that works is as follows you load the timer with a number that will
give the desired pulse width. You then set the output pin high. When the
timer times out you set the pin low. You then wait until it is time to
send out another pulse and do it all over again.

A simple way to wait is to feed  an oscillator into the reset pin of the
chip running at a desired fixed frequency for triggering the injectors.
It then takes in an analog voltage from the MAF and goes through the
motions of setting the timer and so fourth to output the pulse width.

Things happen so fast, on the order of 20 million instructions per
second, the inherent delays in doing this are minimal. With an
oscillator with a 10 ms period (100 CPS) 200,000 computer instruction
can take place if my arithmetic is correct. Max pulse width would be 8 ms.

What this also does is make the microcomputer software  crash proof as
it is always starting from a reset condition for each pulse.

The chip requires only two capacitors. Less parts than a 555.

---------------------------------------------------------

I discovered a built in fixed frequency oscillator in the chip. Saves
building an external oscillator.

The choices are 31 cycles per second (32 ms) or 125  cycles per second (8
ms). What do you electronic types think? Is 32 ms often enough to squirt
fuel? The advantage is it gives a wider dynamic range and better resolution
for the injectors at a given fuel pressure.

I suppose I could count cycles giving a 16 ms option using every other one.



Paul Lamar


Paul,Just a thought on the Ford mass airflow sensor.I did a little follow
up investigating about using a shaper circuit/integrator chip to make the
nonlinear MAF sensor nearly linear.It definitely would work, however there
are a lot of handling precautions reguarding static electricity involving a
thin layer of silicon in the comparitor. There is a version that has a zenir
diode to protect this problem built into the chip but it still is not up to
the tough as a brick electrical chararistics of the 555. I am personally a
little cautious about static electricity on the composite airframe and near
by lightning damaging the low voltage microchips. I have already had two
inflight engine stoppages on rented certified aircraft. One due to
carburetor malfunction and the other overheated and locked up in cruise/
full rich mixture over about one minute of the onset of the problem. I found
some interesting places to park them and both were eventually repaired and
flown back to the airport.

       Reguarding the mass airflow sensor. The shaper circuit is intended
to make the output linear over a segment of the output curve and then assign
the stages with a modified output voltage  for each stage that results in a
nearly linear output. I was wondering if I might operate only over a portion
of the MAF sensor range of approximately 1/3 of the range. Two ways that
this could be done concern using the wrong size of MAF sensor.A  larger
sensor  would not see the flow required for the steeper part of the curve
and stay in the lower portion of the curve. To use a much smaller MAF sensor
I thought of having a sensor in one of two or three intake openings into the
plenum which would only see a percentage of the airflow.Possibly a sensor of
a different range might have an advantage in its output chararistics. In any
event selection of the resistor and capacitor values used for pulse width
modulation  on the 555 would still provide the fuel air calibration.

george grimes