Lpc2000

--- In lpc2000@yahoogroups.com, Rob Jansen <rob@m...> wrote:
>
> I'm not an EMI expert,
> > I do have a 51
> > ohm series resistor on all I/O lines including the RXD and TXD of
> > USART0 to make sure that no EMI problems occur.
> >
> > regards,
> > Charles
> >   
> 
> I did do some ESD research/design once for a web tablet. Although
the IC 
> used (forget which one) was said to be ESD safe, firing at the touch 
> screen with a Human Body Model ESD tester resulted in error. I then 
> added R/C combinations to resolve this problem.
> 
> But I'm definitely not an EMI expert and would even think that adding 
> resistors may make the problem worse ...
> Are there any special considerations to take into account with (apart 
> from PCB design and use of separate ground/power planes) ?
> 
> Regards,
> 
>     Rob
> 
> P.s: let me state that I was _not_ the original designer of that web 
> tablet :o)
>
Hi Rob,

  I am not either even though I have been designing our products to
eliminate EMI for CE approvals for the past 4 years on our products. 

  I have also used what Leon suggested as well (the AVX transguards),
but I am using some resistor packs from CTS that are 0.5mm pitch which
allows me to put them about 3mm away from the processor pins.

  We also designed a motion control stage for a large Canandian
company and they wanted 47 ohm resistors on all I/O's from a large 12
x 14 inch, multilayer board. They still had problems with the EMI, but
then we added some ferrits to the motor stages and that helped them
drop to 5db below class B.

  We do have clients (one in particular) who are and suggested adding
an resistor to each processor line to eliminate any EMI and to help
with ESD. They suggested a 30 to 51 ohm resistor on all processor
lines along with 100pf to 1nF capacitor on ports that extend off the
board. We have followed this advice 3 times and each of our boards
that went through CE, class B with no problems. There are still other
considerations like board layout, capacitor and other components to
help reduce the noise as well.

  We design allot of motion control and there is plenty of noise that
is generated that we have to take into account. One saving grace is
the CE approved power supply. We ran experiments with one at an
approved lab and found that it eliminates all the conducted emmissions
from the system if one is there. That saved us from having to put
chokes and such on our boards since we could be pumping as much as 12
amps through the board at any one time.

regards,
Charles