[Diy_efi] Fixed frequency PWM batch mode FI

[Bernd Felsche]

Les Dittert tapped away at the keyboard with:
[snip]
> While it is possible to control the rate of the PWM, it would then
> perhaps require a crank sensor which is added complexity.

You already have some sort of positional sensor on the car; else how
do you time the spark?

VW's Digifant uses only the ("static") distributor with its Hall
sensor to obtain 4 timing points per crank revolution. That is
sufficient unless you want a really smooth idle or are into refined
torque or emissions control.

To keep it simple, use a free-running timer that's synchronised to
the engine's timing. Most uCs have timer compare registers on at
least some of their timers... use that to determine when you want to
open the injector and use the same high-resolution timer with a
later comparison value to determine when to close the injector.

The limit of precision is determined by the slowest speed you want
to operate the engine without causing a timer overflow (minimum
crank speed) between _timing edges_. Injector timing resolution in
the order of 2 us is not out of the question.

See http://bernd.felsche.org/tech/EFI/vapour/Resources.html#Timers
for a early DRAFT plan of how to use timers in an ECU of that sort.

-- 
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