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

[Steven P. Donegan]

Not sure if my response cam across properly - but I really DO want input
on harsh environment design...

On Sat, 2007-02-24 at 18:46 -0500, ScottyGrover at aol.com wrote:
>  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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