[Diy_efi] PCB layout/circuit design

Mon Feb 26 01:38:47 UTC 2007

Yes, and thanks - I understand - usually I do a .1 or .01 on every IC
supply pin - I have been a bit lazy on the data logger design as there
are only 3 IC's on the board - I believe (schematic not at hand) 2 of
them do have bypass caps.

That said - as much as I have been doing electronics/etc for 30 years I
am not a professional designer - I believe James may be - so anything he
says I will listen to and take into consideration.

And a 100 uf cap, even at 5V would be a tad large next to every IC :-)

On Mon, 2007-02-26 at 11:33 +1100, Bill Washington wrote:
> James,
>     You said "all ICs should have at least a100uF cap as close as 
> possible to the supply " do you mean 100nF?
> Steven,
>     In PCB layout it is good practise to have a large reservoir Cap 
> (your 1 Farad) close by plus smaller (ceramic 100nF and sometimes 10nF 
> as well) caps right on the IC supply pins (as close as you can get 
> them). The reason for this is that the large reservoir cap has some 
> inductance and resistance in its construction which slows down its 
> response time - the ceramic Caps (which very low ESR) on the IC supply 
> pins handle the sharp rising and falling edge surges and minimise ground 
> bounce which can cause all sorts of problems ....
> Regards
> Bill
> >
> > Subject:
> > [Diy_efi] RE: More ECU progress (Steven P. Donegan)
> > From:
> > "James Holland" <j_holland at btopenworld.com>
> > Date:
> > Sat, 24 Feb 2007 22:41:30 -0000
> > To:
> > <diy_efi at diy-efi.org>
> >
> > To:
> > <diy_efi at diy-efi.org>
> >
> >
> > 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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