[Diy_efi] PCB layout/circuit design
Steven P. Donegan
steve
Mon Feb 26 14:00:20 UTC 2007
The 1F caps (there are 2 on the CPU board) are there for UPS purposes -
ie both are 'super caps' - one on the real time clock chip and one on
the power supply side. The CPU has and ADC line connected to the
incoming power (pre regulator, post MOV's and bridge) to detect power
going away for graceful shut down purposes. The other chips have
decoupling capacitors as indicated by their respective manufacturers...
On Mon, 2007-02-26 at 19:17 +1300, Bevan Weiss wrote:
> What's the ESR and ESL of the 1 farad capacitor? I generally only use
> around 1 or 2 1000uF caps for bulk decoupling.
> Then 1uF or 2u2 ceramic 0805 capacitors for more local decoupling.
> Finally 100nF 0805 ceramics for chip level decoupling.
>
> Does the board have a TVS device on the power supply input? It also
> assume it is fused suitably, preferably with a quick blow fuse.
>
>
> Bevan
>
> > We have a 1 farad - ie 10x or more your need in front of this
> > already :-)
> >
> > On Sun, 2007-02-25 at 17:50 -0800, Heitor Lima wrote:
> >
> >> Bill
> >>
> >> We need to use two capacitors , one big as 100uF ,
> >> electrolytic type and another one , 100nF , ceramic
> >> type.
> >> Regards
> >>
> >> Heitor
> >>
> >>
> >> --- Bill Washington <bill.washington at nec.com.au>
> >> 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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> >>>
> >>
> >>
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