Rabbit 2000 Microprocessor User's Manual

14. Rabbit 2000 Clocks

The Rabbit 2000 has two built-in oscillators. The 32.768 kHz clock oscillator is needed for the battery-backable clock, the watchdog timer, and the cold-boot function. The main oscillator provides the run-time clock for the microprocessor. Figure 14-1 shows these oscillator circuits.

Figure 14-1. Rabbit 2000 Oscillator Circuits

The 32.768 kHz oscillator is slow to start oscillating after power-on. For this reason, a wait loop in the BIOS waits until this oscillator is oscillating regularly before continuing the startup procedure. If the clock is battery-backed, there will be no startup delay since the oscillator is already oscillating. The startup delay may be as much as 5 seconds. Crystals with low series resistance (R < 35 kW) will start faster. The required oscillator circuit is shown in Figure 14-1(a).

Improvements were made in revisions A-C to reduce the internal power consumption of the RTC circuit. In addition, external circuitry was introduced to further reduce the oscillator power consumption in board-level products based on the Rabbit 2000. Refer to Section B.2.4 for more information.

14.1 Low-Power Design

The power consumption is proportional to the clock frequency and to the square of the operating voltage. Thus, operating at 3.3 V instead of 5 V will reduce the power consumption by a factor of 10.9/25, or 43% of the power required at 5 V. The clock speed is reduced proportionally to the voltage at the lower operating voltage. Thus the clock speed at 3.3 V will be about 2/3 of the clock speed at 5 V. The operating current is reduced in proportion to the operating voltage.

The Rabbit 2000 does not have a "standby" mode that some microprocessors have. Instead, the Rabbit has the ability to switch its clock to the 32.768 kHz oscillator. This is called the sleepy mode. When this is done, the power consumption is decreased dramatically. The current consumption is often reduced to the region of 100 µA at this clock speed. The Rabbit executes about 6 instructions per millisecond at this low clock speed. Generally, when the speed is reduced to this extent, the Rabbit will be in a tight polling loop looking for an event that will wake it up. The clock speed is increased to wake up the Rabbit.

14.2 Clock Spectrum Spreader Module

The clock spectrum spreader is a feature that was introduced on the Rabbit 3000 and migrated to revisions B and C of the Rabbit 2000. The clock spectrum spreader is very effective for reducing EMI and radiated emissions because it will reduce all sources of EMI above 100 MHz that are related to the clock by about 15 dB. See Section B.2.12 for more information.