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

Sat Feb 24 23:24:10 UTC 2007

Thank you profusely for your feedback!

I accept any/all (especially knowledgeable) inputs.

I have so far done the PIC based parts with only input protection
modeled on a successful project - Megasquirt - so any failings my simple
design likely is the same as that design which is in use all over :-)
That is not a disregard in any way of your input - being an engineer I
prefer massive overkill on my designs and I will take every point you've
raised and see what can be done to either eliminate it or make the
decision that it enough of a boundry condition as to not likely apply
for our intended usage. 

And I don't know about what is currently present on the web - but the
power supply has a 1 farad cap about .5 inches from the chips on the CPU
- if that isn't enough I don't know what is :-)

The 'daughter card' has full clamping on all inputs and all of it's
inputs to the CPU card are via SPI - all inputs on the daughter card
have 5.1V zener, and reverse voltage input protection - although in any
normal auto usage the reverse stuff is overkill.

The latest rendition of the power supply has MOV's and 200V bridge
followed by MOV's to make sure any rational DC input is dealt with - and
lots of totally irrational ones too I expect.

Given this particular design is a Data Logger first, and an ECU
Interceptor/spoofer second I think it may be durable enough - but I will
take every point you've raised and evaluate it and modify the boards
from that data.

Going from a 2 side board to a 4 side board to have the various
signal/ground/etc separated will make the board itself fairly pricey -
that will have to be driven by some level of actual board/system failure
-vs- 2 layer - here you need to convince me :-)

Not sure if I touched on this - but the daughter board which provides
all I/O is .20 in away and sandwiched via your usual header things - so
I don't think any ringing will occur even at max frequency likely to be
present.

And PLEASE keep up the feedback!!!!!!!

Oh - funny - the PS I designed for the actual ECU has MOV in front, 200
PIV bridge, MOV in back and then the rest of my design including a zener
clamp in front of and behind the regulator - and regulator is able to
handle 2X 'normal' input voltage :-) There is no more enjoyable overkill
than massive overkill :-)

Given the whole Interceptor fits in a cast aluminum box - hell it may
even survive some level of EMP - the rest of the vehicle sure won't...

On Sat, 2007-02-24 at 22:41 +0000, James Holland wrote:
> 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
> 
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