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Taking advantage of MCU sleep modes to boost energy savings

Posted: 21 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Microcontrollers? energy consumption? sleep mode?

If the selected MCU can conduct memory transfers and other transactions between internal and external peripherals autonomously, it is important to make full use of such facilities. For example, an I2C bus transceiver or serial port that can autonomously receive and buffer packets to memory allows the MCU to remain in sleep mode until the arrival of an incoming message that requires action. Some MCU peripherals require external hardware or CPU intervention to manage a serial port effectively, which constrains the designer to use relatively high-power sleep modes to avoid losing incoming data.

It often takes detailed analysis to evaluate the interactions between response time and the savings achievable from different sleep modes. However, tools are available to help work out how these interactions will play out (figure 3) by providing an effective and inexpensive way of determining how different sleep strategies will affect the power consumption of an application and its performance. The key is first to know how the sleep modes function.

Figure 3: Power management tools can be effective in picking the right MCU sleep mode strategy.

Run faster to save energy
Selecting a processor clock speed that is appropriate for your application is another technique that can be used to fine-tune both the design's operating power and the time it spends in Sleep Mode. In many cases, the processor speed that results in the lowest overall energy use may not be the slowest clock speed (figure 4).

Figure 4b: Overall energy use versus clock speed.

The idea of running a device faster to conserve energy may seem counter-intuitive, but a semiconductor's energy consumption is the sum of its static consumption, which is present regardless of clock frequency, and its active consumption, which increases with operating speed. If we assume that doubling the clock frequency means cutting the time it takes the CPU to complete a task by half or more, the increase in dynamic power can be offset by the reduced processing time. This is because the static power consumption applies for a shorter period.

Additional power savings can often be achieved by maximizing the transmission speeds of wired and wireless links. This is because physical layer communication interfaces often consume as much or more power than the MCU itself. Unless there are other considerations C such as longer link negotiation times at higher transmission rates C the transceiver's power consumption can be minimized by insuring the processor's clock speed is sufficient to allow it to handle data at the transceiver's maximum available transmit/receive rate. As a result, running faster to sleep longer is often an effective energy-saving strategy.

About the author
Anders Guldahlis an Application Engineer at Energy Micro, supporting customers, developing energy friendly code examples, and writing application notes. Anders also worked in Energy Micro's Simplicity team, designing development kits for the EFM32 Gecko microcontrollers, LESENSE peripherals and capacitive touch. He holds a Masters degree in control systems engineering from The Norwegian University of Science and Technology (NTNU) in Trondheim, Norway.

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