Rabbit Rabbit 4000, Руководство пользователя

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Chapter 2 Clocks 9
2. C
LOCKS
2.1 Overview
The Rabbit 4000 supports up to three separate clocks—the main clock, the 32 kHz clock,
and the 20 MHz Ethernet clock. The main clock is used to derive the processor clock and
the peripheral clock inside the processor. The 32 kHz clock is used to drive the asynchro-
nous serial bootstrap, the real-time clock, the periodic interrupt, and the watchdog timers.
The Rabbit 4000 has a spectrum spreader on the main clock that shortens and lengthens
clock cycles. This has the net effect of reducing the peak energy of clock harmonics by
spreading the spectral energy into nearby frequencies, which reduces EMI and facilitates
government-mandated EMI testing. Gated clocks are used whenever possible to avoid
clocking unused portions of the processor, and separate power-supply pins for the core and
I/O ring further reduce EMI from the Rabbit 4000.
The main clock can be doubled or divided by 2, 4, 6, or 8 to reduce EMI and power con-
sumption. The 32 kHz clock (which can be divided by 2, 4, 8, or 16) can be used instead of
the main clock to generate processor and peripheral clocks as low as 2 kHz for significant
power savings. Note that dividing the 32 kHz clock only affects the processor and periph-
eral clocks; the full 32 kHz signal is still provided to the peripherals (RTC and watchdog
timers) that use it directly. The periodic interrupt is automatically disabled since there is
not enough time to process it when running off the 32 kHz clock.
The Ethernet clock can be driven by the processor clock, the processor clock divided by 2,
or by the input on PE6. The Ethernet clock needs to be 20 MHz to conform to the 10Base-T
specification. See Chapter 20 for more details on the Ethernet clock.