sampling a M.A.P. input

Nicholas Parker nrparker at ihug.co.nz
Mon Mar 19 07:57:29 GMT 2001


Hi,
I am plodding along with my long term ECU / EFI project, based on an Atmel
AVRMEGA103 micro, coded in C.  I've designed my prototype hardware (Protel),
but am wondering, before I shell out to get boards made;  What kind of
analogue circuitry do a MAP sensors usually have following them, if any?
Do most systems sample always at the same crank angle in order to 'cancel'
out intake manifold cylinder pulsations?
On my 'system' I have an ADC which puposefully scans each individual channel
(8 total) at about 30Hz quite slow I suppose.  The AVR's A2D takes 70uS or
so to sample & convert on demand, this also stuffing up the current
conversion! Anyway I put a simple 'peak hold' + time const type circuit (few
op-amps and a diode pretty much) on the MAP input to my circuitry, so I
don't have to sample synchronously.  Has this been the approach of most
people?  Or am I better off adding some 'sample & hold' circuitry and
sampling the MAP input always at the same crank angle?  Related to this;
Should an intake manifold be a lot larger than a cylinders volume in order
to minimise pulsations?
Cheers, Nick Parker.

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