Page 105 - Embedded Microprocessor Systems Real World Design

fields and to electrostatic discharge (ESD) . Entire books have been written on the
subject of designing for EMC, so here we concentrate only on those aspects of EMC
design that bear directly on embedded systems.
The first consideration for EMC design is limiting FW emissions. Since micro-
processors use crystals and those crystals operate at RF frequencies, an embedded
system radiates at the processor frequency. Embedded systems are digital, so there
usually are emissions at the odd harmonics of the processor crystal frequency. In
addition, regular signals such as ALE or address lines can radiate at some frequency
other than the processor clock frequency. RF energy can be radiated from PC board
traces and wires that interconnect the system. Multiprocessor systems that have
more than one processor operating at the same frequency are a particular problem
because usually at some point in the test the power from the oscillators will sum,
causing considerable energy to be radiated.

Controlling EMC Emissions
The following are a few guidelines for controlling EMC emissions from your system.
Put a small (50- to 75-ohm) resistor in series with oscillator outputs and signal
lines with more-or-less regular signals, such as ALE. This both matches the output
to the PC board, reducing ringing, and dampens the rise time of the waveform,
reducing the effect of the odd-order harmonics.
Put board-mounted EMC filters on each I/O line. Of course, if you have a very
fast interface (such as video or 100MHz Ethernet) you cannot do this, as it will
affect the signals you want to have. Shield the processor board and all intercon-
nected electronics. Sandwich clock lines between the power and ground planes.
In multiprocessor systems, if you have multiple processors on a single board, all
operating at the same frequency, do not give each processor a separate oscilla-
tor. Have a single oscillator and distribute it to the various processors. If your
multiple processors are on different boards (or you cannot distribute a single
clock for some reason), see if you can stagger the oscillator frequencies slightly.
For instance, instead of running all the processors at 20MHz, run one at 19.966
MHz, one at 20MHz, and one at 20.0333 MHz. This will push the third harmonics
apart by 1OOkHz.
Pick a processor with lower EMC emissions. To run a Microchip PIC processor
at 5MHz, you must put in a 20MHz clock since the PIC divides the clock inter-
nally by four. The third harmonic of a square wave usually contains considerable
energy, and the third harmonic of 20MHz is 60MHz, right in the worst part of
the radiated emissions test spectrum. On the other hand, if you use a processor
in the Atmel AT9OS family, you can run at 5MHz with a 5MHz input-there is
no internal clock division. This clock rate will have lower radiated emissions in
the spectrum that is tested for radiated emissions. Of course, you do not want to

Hardware Design 1 87