72 Rabbit 2000 Microprocessor7.2 Rabbit Oscillators and ClocksThere are two crystal oscillators built into the Rabbit. The main oscillator accepts crystalsup to a frequency of 29.4912 MHz (first overtone crystals only). The clock oscillatorrequires a 32.768 kHz crystal, which is powered by VBAT, and can be battery-backed.An external oscillator or clock can be substituted for either crystal by connecting theexternal clock to XTALA1 or XTALB1 and leaving the other crystal pin (XTALA2 orXTALB2) unconnected. If an external oscillator is used for the main clock the output pinCLK (pin 1) should be used if the clock is needed externally. This signal is synchronizedwith the internal clock. In comparison, the internal clock is delayed by approximately 10nanoseconds compared to the external oscillator input XTALB1.The main oscillator is normally used to derive the clock for the processor and peripherals.The 32.768 kHz oscillator is normally used to clock the watchdog timer, the battery back-able time/date clock, and the periodic interrupt. The main oscillator can be shut down in aspecial low-power mode of operation, and the 32.768 kHz oscillator is then used to clockall the things normally clocked by the main oscillator. This results in slower execution atlow power (~200 μA).The on-chip routing of the clocks is shown in Figure 7-1. The main oscillator can be dou-bled in frequency and/or divided by 8. If both doubling and dividing are enabled, thenthere will be a net division by 4. The CPU clock can optionally by divided by 2 and thenoptionally drive the external pin CLK. In many cases the clock is not needed externally,and in that case CLK can be used as a general-purpose output pin. The divide-by-2 optionis available to minimize electromagnetic radiation if the is clock is driven off chip.Figure 7-1. Clock DistributionMain Osc32 kHzOscCPUPeripheralDevicesClockDoubler f/8To Watchdog Timer andTime/Date ClockNote: Peripherals cannot be clocked slower than CPU.DISABLE f/2 ext pinCLKf or f/2
