Issue
146 September 2002
Killing
the EMI Demon
by
Norman Rogers
REDUCTION TRICK #1
A key feature used in the
Rabbit 3000 to mitigate the problem of noise on the
I/O lines is the provision of two sets of power supply
pins. One set is used for the processor core; the other
is for the output drivers that are located in the I/O
ring on the periphery of the die (see Figure 2).
 |
| Figure 2—The connection of separate
power and ground pins for the core and I/O ring
of a processor is shown here. A PC board filter
blocks core noise from power planes. You can also
see how I/O buffers spread power supply noise. |
If the I/O buffers are
supplied with the same power that is made dirty by the
fast transients in the processor core, every output
pin of the processor will spread EMI. The EMI that tries
to come out of the power pins for the core can be blocked
by a combination of decoupling capacitors and PC board
trace inductance. This keeps the PC board power planes
a relatively clean source of power for the processor
I/O ring. The design team figured this feature decreases
EMI amplitudes by 10 dB, which is a factor of three
in EMI electrical field strength measured by the prescribed
calibrated antenna. This is a lot because it’s common
to flunk the tests by 5 dB.
