Rabbit 3000, User Manual

Page: 206 / 266

198 Rabbit 3000 Microprocessor
When the spectrum spreader is engaged, the frequency is modulated, and individual clock
cycles may be shortened or lengthened by an amount that depends on whether the clock
doubler is engaged and whether the spectrum spreader is set to the normal or strong set-
ting. The frequency modulation amplitude and the change in clock cycle length is greater
at lower voltages or higher temperatures since it is sensitive to process parameters. The
spectrum spreader also introduces a time offset in the system clock edge and an equal off-
set in edges generated relative to the system clock. A feedback system limits the worst
case time error of any signal edge derived from the system clock to plus or minus 20 ns for
the normal setting and plus or minus 40 ns for the strong setting at 3.3 V. The maximum
time offset is inversely proportional to operating voltage. The time error will not usually
interfere with communications channels, except perhaps at the extreme upper data rates.
More details on dealing with the clock variation introduced are available elsewhere (see
Chapter 16, “AC Timing Specifications”).
If the input oscillator frequency is 4 MHz or less the spectrum spreader modulation of fre-
quency will enter the audio range of 20 kHz or less and may generate an audible whistle in
FM stations. For this reason it may be desirable to disable the spreader for low speed oscil-
lators (where it is probably unnecessary anyway). However, in practical cases the whistle
may not be audible due to the very low level of the interference from a system with low
oscillator frequency and the spectrum spreader engaged. Each halving of clock frequency
reduces the amplitude of the harmonics at a given frequency by 6 dB or more.
The effect of pure harmonic noise on an FM station is to either completely block out a sta-
tion near the harmonic frequency or to disturb reception of that station. If the spectrum
spreader is engaged then interference will be spread across the band but will generally be
Table 15-1. Spread Spectrum Enable/Disable Register
Global Clock Modulator 0 Register (GCM0R) (Address = 0x0A)
Bit(s) Value Description
7
0 Enable normal spectrum spreading.
1 Enable strong spectrum spreading.
6:0 These bits are reserved.
Table 15-2. Spread Spectrum Mode Select
Global Clock Modulator 1 Register (GCM1R) (Address = 0x0B)
Bit(s) Value Description
7
0 Disable the spectrum spreader.
1 Enable the spectrum spreader.
6:0 These bits are reserved.