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.

 

 

 

 

 

 

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.

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Rabbit 3000 Microprocessor

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Jameco Electronics 2000, 3000 Rabbit Oscillators and Clocks, Global CPU Register, Global Revision Register, Gcpu, Grev