Rabbit 3000, User Manual

Page: 223 / 266

User’s Manual 215
16.4 Current Consumption Mechanisms
The following mechanisms are important for the current consumption of the Rabbit 3000
while it is operating.
1. A current proportional to voltage and clock frequency that results from the charging of
internal and external capacitances. At 3.3 V (see 2 below) approximately 57% of the
current is due to charging and 43% to crossover current.
2. A current proportional to clock frequency and to Vc= V(0.5×V - 0.7). This is the cross-
over current that results from a brief short circuit when both the P and N transistors of a
CMOS buffer are turned on at the same time. This component drops as the voltage
drops relative to the other component, and becomes negligible at 1.4 V.
3. The current consumed by the built-in main oscillator when turned on. This current is
proportional to Vc above, and is equal to 1 mA at 3.3 V.
4. The current drawn by the logic that is driven at the oscillator (crystal frequency). This
is considered distinctly because it varies with the crystal frequency, but is not reduced
when the clock frequency is divided. This current becomes zero if the main oscillator is
turned off. This current is 2.5 mA at 3.3 V when the crystal frequency is 14.7 MHz.
This current is divided between capacitive and crossover components in the same man-
ner as the currents in (1) and (2) above.
All of the above components can be combined in the following formula:
Itotal = 0.32×V×f + 0.23×Vc×f + 0.30×Vc + 0.029×V×fc + 0.025×Vc×fc
where Vc = V×(0.5×V - 0.7), fc = frequency of crystal oscillator, and f = clock frequency
in MHz