Jameco Electronics Rabbit 3000 - Manual (Page 89)

80 Rabbit 3000 Microprocessor
7.2 Rabbit Oscillators and Clocks
The Rabbit 3000 usually requires two separate clocks. The main clock normally drives the
processor core and most of the peripheral devices, and the 32.768 kHz clock drives the
battery-backable time-date clock and other circuitry.
Main Clock
An oscillator buffer is built into the Rabbit 3000 that may be used to implement the main
processor oscillator (Figure 7-1). For lowest power an external oscillator may be substi-
tuted for the built-in oscillator circuit. An oscillator implemented using the built in buffer
accepts crystals up to a frequency of 27 MHz (first overtone crystals only). This frequency
may be then doubled by the clock doubler. The component values shown in the figure for
the oscillator circuits are subject to adjustment depending on the crystal used and the oper-
ating frequency.
The Rabbit 3000 has a spectrum spreader unit that modifies the clock by shortening and
lengthening clock cycles. The effect of this is to spread the spectral energy of the clock
harmonics over a fairly wide range of frequencies. This limits the peak energy of the har-
monics and reduces EMI that may interfere with other devices as well as reducing the
readings in government mandated EMI tests. The spectrum spreader has two operating
modes, normal spreading and strong spreading. The spreader can also be turned off.
Table 7-3. Global CPU Register
Global CPU Register (GCPU) (Address = 0x2E)
Bit(s) Value Description
7 0 Program fetch as a function of the SMODE pins.
(read only) 1 Ignore the SMODE pins program fetch function.
6:5 read These bits report the state of the SMODE pins.
4:0 00001 CPU identifier for this version of the chip.
Table 7-4. Global Revision Register
Global Revision Register (GREV) (Address = 0x2F)
Bit(s) Value Description
7 0 Program fetch as a function of the SMODE pins.
(read only) 1 Ignore the SMODE pins program fetch function.
6:5 read These bits report the state of the SMODE pins.
4:0 00000 Revision identifier for this version of the chip.