User`s manual

Rabbit 4000 Designer’s Handbook rabbit.com 75
9.1.2 Reducing Clock Speed
It is important to know that the lowest speed crystal will not always give the lowest power consumption.
This is because when the crystal is divided internally, the short chip select option can be used to reduce the
chip select duty cycle of the flash memory or fast RAM, greatly reducing the static current consumption
associated with some memories.
Some applications, such as a control loop, may require a continuous amount of computational power.
Other applications, such as slow data logging or a portable test instrument, may spend long periods with
low computational requirements interspersed with short periods of high computational load. At a given
operating voltage, the clock speed should be reduced as much as possible to obtain the minimum power
consumption that is acceptable.
9.1.3 Preferred Crystal Configuration
The preferred configuration for a Rabbit 4000 based system is to use an external crystal or resonator that
has a frequency ½ the maximum internal clock frequency. The oscillator frequency can be doubled and/or
divided by 2, 4, 6 or 8, giving a variety of operating speeds from the same crystal frequency. In addition,
the 32.768 kHz oscillator that drives the battery-backable clock can be used as the main processor clock.
To save the substantial power consumed by the fast oscillator, it can be turned off and the processor can
run entirely off the 32.768 kHz oscillator at 32.768 kHz or at 32.768 kHz divided by 2, 4, 8 or 16. This
mode of operation (sleepy mode) has a clock speed in the range of 2 kHz to 32 kHz, and a VDD_{core}
current consumption in the range of 14 to 22 uA, depending on frequency and voltage.
Figure 9-1 Rabbit 4000 Clock Distribution
CPU
f/2
f or f/2
Clock
Doubler
f/(16,8,4,2,1)
ext pin
CLK
NOTE: Peripherals can
not be clocked slower
than the CPU.
To watchdog timer and
time/date clock
Peripheral
Devices
External 32 kHz
Crystal Oscillator
disable
Internal Fast
Oscillator
f/(8,6,4,2,1)
Spectrum
Spreader