Selection method of low-power MCUs

Driven by the Internet of Things, the industry has created a huge demand for various battery-powered devices. This, in turn, has led to increased energy efficiency requirements for microcontrollers and other system-level devices. As a result, ultra-low-power MCUs are setting new records for many power-related metrics. In choosing the right ultra-low-power MCU microcontroller to master the necessary skills, the application also needs some design direction and ideas for better application. This article focuses on how to choose an ultra-low power MCU.

(1) In low-power designs, the average current consumption often determines the battery life. If an application uses Eveready high power 9V1222 type battery with a rated current of 400mAh, the average current consumption must be less than 400mAh/8760h, i.e., 45.7uA, to provide -year battery life.

(2) The power-down mode is the most important of all the features that enable the MCU to meet its current budget. Low-power MCUs have power-down modes that provide different levels of functionality. Low Power Mode 0 (LPMO) shuts down the CPU but keeps other functions running normally. lpm1 and lpm2 modes add various clock functions to the list of disabled functions. lpm3 is the most common low-power mode and keeps only the low-frequency clock oscillator and peripherals that use that clock running. lpm3 is often called the real-time clock mode because it is the mode that allows the timer to use the low power 32768Hz clock source to run with a current consumption of less than 1uA while also periodically activating the system. Finally, LPM4 completely shuts down all functions on the device, including ram storage, with a current consumption of only 100nA.

(3) The clock system is the key to MCU power consumption. Applications can enter and exit various low-power modes hundreds of times per second. The ability to enter or exit low-power modes and process data quickly is extremely important because the CPU currently wastes while waiting for the clock to stabilize. Most low-power MCUs have "instant start" clocks, which can be ready for the CPU in less than 10-20us. It is important to understand which clocks are instant-start and which are not. Some MCUs have a dual-stage clock activation feature that provides a low-frequency clock (typically 32768Hz) during high-frequency clock stabilization, which can be up to 1ms. The CPU normally runs for about 15us, but it runs less frequently and less efficiently. If the CPU needs to execute only a small number of instructions, e.g., 25, it needs 763usa CPU low frequency to consume less current than high frequency, but it is not enough to compensate for the difference in processing time. Some MCU in 6us time can provide a high-speed clock for CPU, processing the same 25 instructions only need about 9us (6us activation + 25 instructions 0.125us instruction rate)), and can achieve instant start high-speed serial communication.

(4) If the MCU clock system provides multiple clock sources for peripherals, the peripherals can still operate when the CPU is asleep. For example, a single A/D conversion may require a high-speed clock. If the MCU clock system provides a high-speed clock independent of the CPU, the CPU can go to sleep while the A/D converter is running, thus saving CPU power consumption.

(5) Event-driven functionality coexists with the flexibility of the clock system. Interrupts cause the MCU to exit the low-power mode, so the more interrupts the MCU has, the more flexibility it has to prevent wasted current CPU polling and reduce power consumption. Polling means there is a difference between doing and not doing power budgeting because it wastes CPU bandwidth and requires extra current while waiting for an event to occur. A good low-power MCU should have sufficient interrupt capability to provide interrupts for all its peripherals and numerous external interrupts for external events.

(6) Button or keyboard applications can demonstrate the benefits of external interrupts. Without interrupt capability, the MCU must frequently poll the keyboard or button to determine if it has been pressed. Not only does polling itself consume power, but controlling the polling interval also requires a timer, which draws additional current. With interrupts, the CPU can stay asleep for the entire process and only activate when a button is pressed.