Microcontroller shielding questions

[Darrell Norquay]

At 09:14 AM 9/17/96 PDT, Wade Barnes wrote:

>I am designing a small (2"x2") controller board for use in the automotive
>and robotics industries.  The board is based on the MC68HC16Z1
>microcontroller.  Since the engine compartment of a car is filled with EMI
>we are attempting to make the board as resistant as possible to the effects
>of the EMI.

>Can anyone give me some information on how to determined the type of
>shielding to use (box type and thickness, feed through type and value)?
>What is necessary?  What is commonly used and what is affordable?

Sorry, I can't quantify any of the EMI/RFI effects that you mentioned, I'd
be surprised if anyone could.  Mebbe stick the car in a Faraday cage and see
what you get.  As far as processor shielding, you would at least need a good
metal enclosure, some of the commercially available die cast aluminum boxes
would be ideal.  All wiring entering or leaving the box should run through a
feedthrough RFI filter.  The RFI filtered feedthrough DB style connectors
from Conec are very good.  They have a ferrite core through which all pins
run, some also have feedthrough capacitors.  The important thing here is to
have the bypass caps grounded to the box right where the wire comes through
the box.  Bypass caps on the circuit board are not enough.  Even an inch of
unbypassed wire in the box is enough to radiate RFI sufficient to scramble
the processor.  Panasonic and Phillips make nifty little RFI filters which
are essentially a feedthrough capacitor with a ferrite bead on each side of
the through line, forming a pi filter.  They are very effective.  

Filtering on the supply lines is a must, both power and ground, as well as
protection against transients and load dump.  Signal lines will also require
transient protection as well.  Use differential inputs on analog leads
wherever possible, this will minimize the effects of common mode noise and
ground differentials.  Shielded leads on all inputs are also a good idea,
but remember not to use the shield as a current carrying conductor and only
ground it at one end.  Use hard pullups on digital signals, and generally
keep impedances low to minimize noise pickup.  Take a tip from the OEM's and
use the body as shielding, ie mount the computer on the passenger
compartment side of the firewall.

You'll generally find that the protection and filtering circuitry that you
need will take up more board space than the controller itself, you'll have a
hard time fitting all this on a 2x2 board...  Consider making the system
building block style, with the controller as a piggyback module on the
signal conditioning board.  In this way, you can keep the controller
generic, and just make different signal conditioning boards for different
applications.  Some of the stuff I have suggested may seem like overkill,
but it's much better to overdesign the first time for any contingency.  A
few dollars worth of extra components will save many hours of dinking around
later.  Good luck, and keep us posted.

regards
dn
dnorquay at awinc.com