Jameco Electronics Rabbit 3000 - Manual (Page 221)

212 Rabbit 3000 Microprocessor
15.1 Power Supply Connections and Board Layout
Refer to Technical Note TN221, PC Board Layout Suggestions for the Rabbit 3000
Microprocessor, for recommendations on laying out a PC board to minmize EMI emsis-
sions.
15.2 Using the Clock Spectrum Spreader
The spectrum spreader is very powerful for reducing EMI because it will reduce all sources
of EMI above 100 MHz that are related to the clock by about 15 dB. This is a very large
reduction since it is common to struggle to reduce EMI by 5 dB in order to pass government
tests.
Figure 15-1. Peak Spectral Amplitude Reduction from Spectrum Spreader
The spectrum spreader modulates the clock so as to spread out the spectrum of the clock
and its harmonics. Since the government tests use a 120 kHz bandwidth to measure EMI,
spreading the energy of a given harmonic over a wider bandwidth will decrease the
amount of EMI measured for a given harmonic. The spectrum spreader not only reduces
the EMI measured in government tests, but it will also often reduce the interference cre-
ated for radio and television reception.
The spectrum spreader has three settings under software control (see Table 15-1 and
Table 15-2): off, standard spreading and strong spreading.
Two registers control the clock spectrum spreader. These registers must be loaded in a spe-
cific manner with proper time delays. GCM0R is only read by the spectrum spreader at the
moment when the spectrum spreader is enabled by storing 0x080 in GCM1R. If GCM1R
is cleared (when disabling the spectrum spreader), there is up to a 500-clock delay before
the spectrum spreader is actually disabled. The proper procedure is to clear GCM1R, wait
for 500 clocks, set GCM0R, and then enable the spreader by storing 0x080 in GCM1R.
15dB
10
5
100 50 200 150 250
350
300
Normal Spreading
Strong Spreading
MHz