[Diy_efi] RE: More ECU progress (Steven P. Donegan)

[James Holland]

I have been following this project with some interest. I had a look on the
website but you seem to have the same schematic for the datalogger and I/O
board. I presume the I/O board schematic hasn't been posted. I've designed
electronics for military automotive issues so I'm aware of the problems of
designing for this kind of environment.
I like the idea of this but I think that you will have some noise issues
with the design as it stands. The PCB design lacks a good ground plane and
the decoupling could do with being improved, all ICs should have at least a
100uF cap as close as possible to the supply. The PIC should have two, one
on each supply pin. I'm a bit surprised that the PIC doesn't have a separate
Analogue supply for the A/D as it stands the analogue and digital returns
are mixed which isn't good. The A/D inputs should really have anti-aliasing
filters close to the inputs, a simple RC would do and only the C really
needs to be up by the input. The R would also provide some input protection
to the PIC. Input voltage clamping is also a good idea. The PIC is pretty
well protected but voltages outside of its supply rails throw the A/D
readings all over the place. 
It looks like you are using the 5V supply as the reference for the A/D, what
supply are the I/O lines referenced to?
The digital I/O could do with some resistance in line as well, a couple of
hundred ohms would provide output short circuit protection and give some
filtering on the inputs. If your digital inputs are coming off another board
then you could see some ringing on them, that could be more of a problem on
PortB which doesn't have Schmitt inputs. 
The 5V regulator has plenty of protection, that's good. Automotive supplies
are horribly noisy. The 28V military stuff I design has to be protected
against +280V/-130V spikes. The EMC test involves 100V for 100mS. You could
reduce the protection needed by using a regulator specifically designed for
automotive use, there are a few of them out there. I have used the LM2940S
in the past and that works well but I think the best spec only comes in an
SMD package.
I hope this doesn't sound too harsh, I don't see any reason why it won't
work but I think you will lose some A/D resolution by having to digitally
filter the inputs. I learnt the hard way with a few board redesigns along
the way. The Microchip website has a very good on line seminar about PCB
layout and design for A/Ds, its well worth checking out.
HTH
James