Rabbit 2000™ Microprocessor - UTN

9. Low-Power Design and SupportTo get the most computation for a given power level, the operating voltage should be approximately3.3 V. At a given operating voltage, the clock speed should be reduced as much as possibleto obtain the minimum power consumption that is acceptable.Some applications, such as a control loop, may require a continuous amount of computationalpower. Other applications, such as slow data logging or a portable test instrument, may spendlong periods with low computational requirements interspersed with short periods of high computationalload.The current (and thus power) consumption of a microprocessor-based system generally consists ofa part that is independent of frequency and a part that depends on frequency. The part that is independentof frequency consists of leakage or current or current drawn by special circuits such aspullup resistors or circuits that continuously draw power. Ordinary CMOS logic uses power whenit is switching from one state to another, and this is the power that is dependent on frequency. Thepower drawn while switching is used to charge capacitance or is used when both N and P FETs aresimultaneously on for a brief period during a transition.Floating inputs or inputs that are not solidly either high or low can also draw current because bothN and P FETs are turned on at the same time. To avoid excessive power consumption, floatinginputs should not be included in a design (except that some inputs may float briefly during poweronsequencing). Most unused inputs on the Rabbit can be made into outputs by proper softwareinitialization to remove the floating property. Pullup resistors will be needed on a few inputs thatcannot be programmed as outputs. An alternative to a pullup resistor is to tie an unused output tothe unused inputs. If pullup (or pulldown) resistors are required, they should be made as large aspossible if the circuit in question has a substantial part of its duty cycle with current flowingthrough the resistor.Rabbit 2000forf/2disable f/2ext pinCLKMainOscillatorClockDoublerf/8CPU32 kHzOscillatorTo watchdog timer andtime/date clockPeripheralDevicesNote: Peripherals cannot be clocked slower than the CPU.Figure 13. Rabbit Clock DistributionDesigner’s Handbook 47