Issue
146 September 2002
Killing
the EMI Demon
by
Norman Rogers
Reduction Trick #2
Most microprocessors have
I/O and memory devices connected to the same bus with
distinct control signals for the devices. Generally,
there is a lot more activity at a higher frequency for
the memory devices. The Rabbit 3000 has an option to
use separate pins for memory and I/O devices, both address
and data. The advantage is that the physical scope of
the high-speed memory bus is limited to the memory devices.
A separate address and data bus handles I/O cycles and
has a much lower average operating frequency. In particular,
the address lines toggle only during I/O bus cycles,
greatly limiting the emissions from the I/O bus. This
avoids the situation where the fast-toggling address
and data lines of the memory bus have to be run all
over the printed circuit board of a large system. This
scheme also limits the capacitive loading on the memory
bus, which does not have to extend to numerous I/O devices.
