You might consider using one or more of the following techniques to reduce
the susceptibility of your circuit: (1) use type 28 ferrites (Steward EMI
ferrite chip beads) on both VSS and VDD pins on your PIC. (2) Use a POR
reset chip on your MCLR line rather than a simple pull up. (3) Put a TVS on
the input to your 5V regulator and a power filter on the output of the
regulator (like the BNX002 series from muRata). Use a dedicated input chip
like the MAXIM MAX6818 for your inputs which will give you good ESD
protection. Use a four layer PCB and partition the PCB into a clean area
with a dedicated ground plane where the PIC and other sensitive circuits
live. Put the inputs with ferrites in each input line towards the edge of
the PCB and tie the ground plane in this area directly to the ground pin at
the power input to the board. Tie all un-used inputs on the PIC to ground.
Finally, shield all the input lines to the PCB and tie the shield directly
to the chassis. Tie the ground plane of the PCB to the chassis at one point
and use capacitors to tie the other mounting points of the PCB to the PCB
ground. This way you will have a good single point ground at low frequencies
and a multi point ground at higher frequencies.
Automotive designs are very unforgiving of even small mistakes in PCB
layouts and inadequate fault tolerant design. If you really have to test the
prototype on a wire wrap board, keep the board away from the ignition system
for now and test it in proximity only with a good four layer board mounted
on a good chromate metal chassis; you might get better results that way.
Good luck
Madhu

>-----Original Message-----
>From: pic microcontroller discussion list
>[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Chris Miller
>Sent: Monday, April 19, 2004 9:49 PM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Input trigger circuit in a very bad EMI environment.
>
>
>I have Pic event timer circuit that needs to operate very close to
>a high performance automotive ignition system.  I have wire
>wrapped my prototype and tested near the ignition system.  As soon
>as the ignition fires my circuit triggers.  Obviously this is a
>big problem.  Last year I designed a similar event timer but it
>was not Pic based,  It withstood the ignition system very well.
>However the old design was never tested in the wire wrapped stage,
>we only tested the final PCB version.   I know my wire wrapped
>version is more suceptable to EMI but what I'm really interested
>in is improving my input trigger filter.
>
>I already have a substantial filter on my triggers but they go
>direct to the PIC interrupt pins which are schmidt trigger inputs.
> I would like to try an opto-coupled design but I do not have 2
>seperate power supplies.  Even with just one supply wouldn't the
>opto design be superior to the schmidt trigger input since you
>must pull current through the optos LED in order to trigger the
>output?  One other advantage is that the trigger leads that go out
>into the world are not the same leads that ultimately trigger the PIC.
>
>I have attached a PDF of my input circuit.  C1 is 1uf C2 is 33pf
>C3 is 1000pf R1 is 500ohm and R3 is 4.7K.  This circuit seems to
>work well on my old design but I do not have a PCB version of my
>PIC project to test it on yet.  I do have a wire wrapped version
>which fails miserably.
>
>I have wired a simple opto H11L on my prototype in place of the
>input circuit in the schematic.  I feel it needs some additional
>filtering.  Can anyone suggest a good filter method?  This
>ignition system is capable of generating a 6" arc @ 1amp.  This
>stuff is serious.
>
>Any help would be greatly appreciated,
>Chris Miller
>Chris@Electrimotion.com
>

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