Rabbit 6000 User Manual page 23

The spectrum spreader either stretches or shrinks the low plateau of the clock by a maximum of 3 ns for the
normal spreading and up to 4.5 ns for the strong spreading. If the clock doubler is used, this will cause an
additional asymmetry between alternate clock cycles.
Both normal and strong modes reduce clock harmonics by approximately 15 dB for frequencies above 100
MHz; for lower frequencies the strong setting has a greater effect in reducing the peak spectral strength as
shown in Figure 2.2.
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10
5
Figure 2.2 Peak Spectral Amplitude Reduction by Spectrum Spreader
Two registers control the clock spectrum spreader. These registers must be loaded in a specific manner
with proper time delays. GCM0R is only read by the spectrum spreader at the moment when the spectrum
spreader is enabled by setting bit 7 of GCM1R. If bit 7 of 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 writing a
1 to bit 7 of GCM1R.
The spectrum spreader is applied to the main clock before the clock doubler, so if both are enabled there
will be additional asymmetry between alternate clock cycles.If the clock doubler is used, the spectrum
spreader affects every other cycle and reduces the clock high time. If the doubler is not used, then the
spreader affects every clock cycle, and the clock low time is reduced.
Rabbit 6000 User's Manual
Strong Spreading
Normal Spreading
50 100 150
Frequency (MHz)
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200 250 300
350
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