![](http://datasheet.mmic.net.cn/Rabbit-Semiconductor/668-0003-C_datasheet_95667/668-0003-C_78.png)
72
Rabbit 2000 Microprocessor User’s Manual
7.2 Rabbit Oscillators and Clocks
There are two crystal oscillators built into the Rabbit. The main oscillator accepts crystals
up to a frequency of 29.4912 MHz (first overtone crystals only). The clock oscillator
requires 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 the
external clock to XTALA1 or XTALB1 and leaving the other crystal pin (XTALA2 or
XTALB2) unconnected. If an external oscillator is used for the main clock the output pin
CLK (pin 1) should be used if the clock is needed externally. This signal is synchronized
with the internal clock. In comparison, the internal clock is delayed by approximately 10
nanoseconds 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 a
special low-power mode of operation, and the 32.768 kHz oscillator is then used to clock
all the things normally clocked by the main oscillator. This results in slower execution at
low power (~200 A).
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, then
there will be a net division by 4. The CPU clock can optionally by divided by 2 and then
optionally 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 option
is available to minimize electromagnetic radiation if the is clock is driven off chip.
Figure 7-1. Clock Distribution
Main Osc
32 kHz
Osc
CPU
Peripheral
Devices
Clock
Doubler
f/8
To Watchdog Timer and
Time/Date Clock
Note: Peripherals cannot be clocked slower than CPU.
DISABLE
f/2
ext pin
CLK
f or f/2