New-Tech Europe Magazine | July 2017

Some MCUs integrate a wide variety of low-power active modes. These modes provide the option to turn off or reduce the speed of the core processor, while selectively keeping the system clock active for the on-chip peripherals. One frequently heard statement is “the higher the performance of the core, the faster the execution of the tasks, then the sooner it can return to sleep mode.” While this might be true in some cases, there is a flaw to this logic. We have to remember that the core consumes more power than any other module in the MCU. Additionally, all of the tasks that require the core must be executed sequentially (FIFO), regardless of the speed. Therefore, the core can’t be turned off until the last task is completed. When a microcontroller can perform some of the required tasks in parallel, using integrated peripherals that can operate independently of the core, then it makes the speed of the core irrelevant while significantly reducing the overall power usage. Core independent peripherals are fully functional while the MCU’s core is in sleep mode. Designing battery-powered applications has become more complex, due to their increasing functionality. Engineers should analyze and fully understand the current-consumption profile of each component in different power and activity modes, in order to achieve the highest battery usage efficiency. The core independent peripheral set found in the next generation of 8-bit microcontrollers enable engineers to be creative with their designs, without sacrificing performance. Note: PIC is a registered trademark of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks mentioned herein are the property of their respective companies.

Figure 2: A graphic representation of a microcontroller’s current consumption over time

corresponding to 200ns to 1us), in a 32-bit architecture, employing deep- sleep techniques to limit leakage currents, it becomes a matter of tens of microseconds, often voiding any/all benefits resulting from the subsequent typical faster execution speed. While we would like to do everything in sleep mode, certain tasks must be performed in active mode where the MCU core consumes the highest amount of power relative to all other modules. This is where things can get a bit tricky. Figure 2 is a simplified graphic representation of the system current consumption over time. The area under the current-consumption line represents the total discharge over time, measured in Coulombs. If the sum of all the areas under the sleep- mode period is much greater than the active mode, then the sleep-current value is more critical since most of the energy consumption takes place in a low-power mode. Vice versa, if the sum of the area under the active-mode period is significantly higher, then the sleep current value and the time spent in sleep mode become irrelevant. Applications with wireless communication, such as Wi-Fi ®

or Bluetooth ® LE, are particularly challenging systems in which to reduce power consumption. Designers of these systems must consider how much data is transmitted or received, since this will directly impact the overall current consumption. Wireless modules can be used in “Beacon Mode,” to wake up periodically and search for signals; or they can go into standby mode when not in use. In such wireless systems the MCU processing speed is actually irrelevant as the application is most often I/O bound, but the MCU wake up time impacts significantly the application profile as the radio circuitry power consumption (typically 10-20 mA) is extended and ends up dominating the application budget. Analog sensors require the use of the MCU’s on-chip ADC module. Typically, the time needed for ADC sampling is much longer than the conversion time. The more time spent in active mode, the more current is consumed. However, some MCUs have ADC modules that allow conversions in sleep mode, which saves power by minimizing the time spent in active mode.