Making use of low-power modes on MCUs

In general, a product described as "green" has minimal or no harmful effect on the environment. In the case of a product that uses power, green means power use is minimal. However, what is meant by "minimal" or "low power" often is not clearly defined, partly because the requirements for a low-power microcontroller (MCU) vary, depending upon how the MCU will be used.

For example, in a battery-operated thermostat low power is primarily defined by the minimum power mode that allows the device to drive the LCD display; a reduction in power leads to extended battery life. In an electricity meter, low power refers to the active current consumed by the system during operation. An electricity meter also requires the time of day to be maintained regardless of the presence of the system's primary power supply, for example during a power failure. Thus, MCUs with flexible power modes allow a designer to tailor system operation depending on the application.

In the past, MCUs had an active mode to allow for device operation; idle and doze modes to reduce or eliminate the CPU switching power while allowing the peripherals to operate; and sleep modes that allowed limited peripheral operation with minimal power consumption.

As today's MCUs move to more advanced silicon processes that minimize cost and reduce active current, a number of new low-power modes are being added to increase flexibility. To explore some of the operating modes available on today's advanced MCUs, we will examine how these new low-power operating modes are used in a variety of applications.

A software Battery Life Estimator (BLE) tool and 16bit MCU will be used to provide a comparison of the various power modes when implemented in different applications. The BLE from Microchip is a free software tool that allows a designer to estimate the battery life of their system and to determine which of the available operating modes is best suited to their application.

Apps requiring low power MCUs
Thermostats have become more complex, needing to display more information and cover multiple regions.锟紸s a result, significant amounts of on-chip Flash program memory are often required to store their complex menus in multiple languages. In general, advanced processes are required to produce MCUs with large memories at competitive prices. As semiconductor processes advance, there tends to be a reduction in operating, or active, current with an increase in the leakage current of the transistor.

The increase in leakage current is most visible in the current specifications for low-power modes, such as sleep mode. The sleep currents on advanced MCUs are typically in the 3 to 5?A range. The typical thermostat application spends the majority of its time doing little more than driving a segmented LCD display. The segmented LCD display is typically driven in a sleep mode that allows the driver to operate while the CPU and most peripherals are powered down. On a periodic basis, the thermostat must wake and enter an active mode, read the temperature, update the display, and perhaps signal the furnace, fan, or AC units to turn on.锟紿owever, over 99% of the time only the sleep mode is required. This large amount of time in the sleep mode makes the sleep current an area where improvement can greatly benefit the battery life of the system.

New lower power modes
To provide MCUs with a sub?A power mode, suppliers have introduced new low-power "deep sleep" modes that consume 10 to 50?A and can add a clock calendar (RTCC) with an additional 400 nA of current. Extremely low currents are achieved by shutting down the entire device with the exception of a small amount of memory, a real-time clock and perhaps a watch dog timer. However, these deep sleep modes do not allow peripheral operation or maintain the data RAM on the device. The loss of the RAM contents requires the device to execute a restart routine prior to resuming program execution when waking from deep sleep.