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

[ScottyGrover at aol.com]

In a message dated 2/24/2007 2:41:57 P.M. Pacific Standard Time,  
j_holland at btopenworld.com writes:

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




James,
Can you, if this is possible, give us a few tips re: the basics of  designing 
circuit boards for the high-temp. environment such as military and/or  
automotive?
Scotty
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