Designing Embedded Hardware, 2nd Edition

Low Power Design

There are several ways you can reduce the power consumption in your embedded system. The use of low-power devices is the most obvious place to start. The power consumption of different chips varies considerably, and there are many low-power variants of common devices available. RISC processors often have lower power consumption than comparable CISC processors, so they are often used in preference to CISC in low-power applications. The PIC and AVR microcontrollers can have current draws of less than 5 mA (and as low as 10 nA when in sleep mode). This is considerably less than the 35 mA used by a 68HC11 microcontroller.

Many memory chips and peripherals will automatically enter a low-power mode when they are not in use. Others may be placed in low-power mode by toggling a digital input or by an appropriate software command. The power consumption of some devices can be reduced even further by turning them off when not in use. If the processor is executing code from RAM and outputting data to a serial port, then the power to the ROMs and any other I/O devices may be turned off since they are not in use. Implementing this requires separate power sources for the chips that are to be disabled, switched via software control. Some voltage regulators (discussed in the next section) have shutdown inputs, allowing the subsystem they are powering to be turned off.

Further, some low-power devices (such as sensors) may need very little current, so little that they can be directly ...