Choosing the Best Low-Power Microcontroller (MCU)
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Choosing the Best Low-Power Microcontroller (MCU)

Posted Date: 2024-01-17

Designing a low-power microcontroller (MCU) is a complex task, and selecting the right MCU for a specific embedded design is equally challenging. The multitude of application-specific considerations involved makes comparing microcontroller specifications a daunting task.

From a system architecture perspective, in order to determine which MCUs truly meet the "low power" criteria, designers need to carefully sift through the various claims made by semiconductor suppliers. The task is not simple as different vendors employ different and often confusing metrics.

The power consumption of an MCU can usually be divided into the following parts:

Total power consumption = working mode power consumption + standby (sleep) mode power consumption

However, another important criterion to consider is the transition time of the MCU from standby to active state.The MCU starts performing useful processing after all digital and analog components are fully stable and operational, so this (wasted) energy should also be included when calculating total power consumption:

Total power consumption = working mode power consumption + standby (sleep) mode power consumption + wake-up power consumption

Because the needs of each application are different, system designers may focus more on some of these elements. For example, applications like water meters spend most of their time in standby, so their long duty cycles obviously require very low standby power consumption. And applications like data loggers frequently enter and exit working states, so limiting the time spent in wake-up transitions becomes critical.It is worth noting that manufacturers who develop solutions specifically for MCUs do not try to guess which metric is most important, but design from the ground up, aiming to minimize every part of the above formula. To do this, a strong foundation of mixed-signal technology is needed to address the architecture-level and circuit-level challenges of minimizing power consumption in both the analog and digital domains.Exploring these variables can help highlight the types of issues system designers need to understand when choosing the best MCU solution.

Working mode power consumption

For CMOS logic gates, dynamic power consumption can be expressed by the following formula:

Working mode power consumption = C x V^2 xf

Where C represents the load capacitance, V represents the supply voltage, and f represents the switching frequency.

The capacitance term depends on the design and processing technology, while the frequency term depends on the processing needs of the application. However, as the formula shows, supply voltage has a significant impact on the overall MCU power consumption. therefore,Adding voltage regulation function to MCU design can save a lot of working mode power consumption by providing a lower stable power supply to the MCU circuit.Switching converters may be a solution, but they are best suited for regulated environments with large voltage conversion ratios. For battery-powered applications (where the average voltage slew ratio is small, close to 1:1 at the end of battery life), a better solution may be to integrate an on-chip Low Drop Out (LDO) linear voltage regulator, since it Provides acceptable efficiency at lower complexity and cost than switching solutions.

To highlight the advantages of using an LDO regulator, we can revisit the CMOS dynamic power equation:

Working mode power consumption = C x V^2 xf

= V x (C x V xf)

= V x I, where dynamic current I = C x V xf

Dynamic current is usually standardized for a frequency of 1MHz and a specific supply voltage. For example, a new ultra-low-power MCU has a dynamic current consumption of 160µA/MHz at 1.8V. Without power regulation, this increases to (160) x (3.2/1.8) = 284 µA/MHz at a supply voltage of 3.2V. When using an LDO, the battery current will remain at 160µA/MHz over the entire supply range.

As can be seen, thisThe advanced power architecture maintains constant operating current over the entire operating voltage range, helping system designers achieve significant power savings.Therefore, it is very important for system designers to determine the current consumption of the MCU over the entire operating voltage range-- and not just the 1.8V minimum operating condition often cited by vendors. Quoting an optimistic current value, assuming the voltage supply is no less than typical, does not accurately reflect real-world application conditions. For example, in 2 x AA/AAA and coin cell applications, where the battery operates at close to the initial 3V for most of its life, it may be misleading to only quote a 1.8V specification MCU power consumption is about 50% higher than the often quoted figures.

Furthermore, since power dissipation is proportional to switching frequency,It is important for system designers to normalize the quoted current values ​​to current per MHz.Combining these two factors allows for a side-by-side comparison of MCUs based on the following metrics:

Current consumption/MHz at 3V

Some vendors may try to confuse concepts by equating "MHz" with system clock speed, when what actually makes sense is the instruction clock speed. This is misleading because the clock speed can run at double (or more) the actual instruction speed, thus doubling (or more) the power efficiency. therefore,Need to ensure that all values ​​are normalized to the instruction clock speed. Doing this, together with using typical supply voltages, allows the actual operating mode current consumption budget to be correctly derived.

Standby (sleep) current

In order to achieve maximum energy efficiency (and battery life), you need to ensure that each task of the MCU is completed in the shortest possible time, consuming the least amount of power at the lowest possible current and voltage, so that the device is extremely low most of the time. power consumption in sleep mode.In some applications, the sleep mode current is the main parameter affecting the total energy consumption. However, what is often overlooked is that the lowest sleep current an MCU can achieve is primarily limited by its leakage current.For example, a device with 20 inputs that has a leakage current specification of 100nA per input may consume up to 2µA in sleep mode.

Leakage current is affected by many factors, but the most important is the underlying process technology used. Sometimes, vendors choose to use 0.25 or 0.35 micron process technology to reduce sleep current due to leakage, but this choice results in higher operating current. In other cases, MCU vendors may choose to use 0.18 micron or smaller process technology to reduce active mode current, but this will result in higher leakage current.The unique approach to solving this problem is to leverage mixed-signal expertise to implement a specially designed advanced power management unit (PMU) to limit leakage and enable ultra-low sleep current, regardless of the underlying process technology.

Achieving minimum sleep mode current when using 0.25 micron or smaller process technologies requires cutting power to the digital core.Modules that operate in sleep mode, such as power management circuitry, I/O pad components, and the real-time clock, must operate from an unregulated supply voltage to avoid drawing additional current in the LDO.Cutting power to the digital core also prevents sleep mode currents caused by its off-state leakage; however, the MCU needs to retain all RAM contents and all register states in sleep mode so that code execution can resume to the point of interruption.This retention can be achieved by a very low current sleep mode latch biasing scheme, or by using special retention latches that do not leak significantly in sleep mode.The MCU also requires a form of continuous supply voltage detection (i.e. "blackout detection") to reset the device if the supply voltage drops below the minimum holding voltage, as this may result in state corruption.

Therefore, from a system designer's perspective, it is important to examine the underlying leakage current specifications to determine which MCU vendors leverage their mixed-signal expertise to solve this complex problem. Designers should also be aware that most vendors offer several different standby current options. Most vendors emphasize their minimum sleep mode current, which is typically when the real-time clock and power-off detector are disabled. Some vendors even quote shutdown mode current, which retains no memory and requires a reset to wake up, which is often not practical. therefore,Since most applications require full RAM and register retention, it is important for system designers to make comparisons based on the following metrics:

- Current in sleep/standby mode, real-time clock and power-off detector disabled (RAM retained)

- Current in standby/sleep mode, real-time clock disabled, power-off detector enabled

- Standby/sleep mode current, real time clock and power outage detector are enabled

This allows system designers to use the correct values ​​when calculating the entire standby mode power budget based on their application's duty cycle.

awaken energy

As mentioned earlier, in systems using sleep mode, a large amount of power can be wasted waking the MCU and preparing it to acquire or process data. In fact, in some applications, the MCU consumes as much energy when resuming from standby as it does when fully processing the data. Therefore, when designing an MCU, it is necessary to be able to wake up and stabilize in a very short time to minimize the time in a state of wasting power.

The MCU should be able to wake up from sleep mode from external events or internal timers. The most flexible periodic wake-up source is the real-time clock, which can run either from an external crystal oscillator (for applications requiring precise timing) or from a low-frequency internal oscillator (for applications where accuracy is not critical).Avoid using a slow-starting crystal oscillator as a high-speed system clock; an accurate, fast-starting on-chip oscillator is a better choice.

also,Since many products wake up periodically to use the on-chip ADC sampling input, sufficient time is required for the digital circuitry to wake up and for the analog circuitry to stabilize before valid measurements can begin.The startup behavior of analog blocks can have a significant impact on the active mode time; voltage regulators or references using external decoupling capacitors can take several milliseconds to stabilize. Sometimes, MCU vendors only quote the wake-up time for digital circuits and ignore the time it takes for analog circuits to stabilize. therefore,It is important for system designers to analyze the overall wake-up and settling time of analog and digital circuits to calculate the true cost of this wasted power.

Other considerations

There are other ways to further reduce power consumption in the system. For example, MCUs can often operate at 1.8V or even lower (in which case there may be no ADC functionality; the instruction clock frequency is reduced), so a 2 x AA/AAA battery configuration is often used. An innovative way to reduce power consumption (and environmental impact) is to convert the design to a single-cell configuration, where the battery can be used until the end of its life (0.9V).To achieve this, the MCU must integrate a highly optimized DC-DC converter, the lowest working voltage of the battery that can be used is 0.9V for alkaline batteries. This approach also saves suppliers and/or consumers money on batteries.

Another way to reduce power consumption is to use a highly integrated MCU that includes ADCs, DACs, and other peripherals, because applications can control enabling and disabling these peripherals. For example, some MCUs offer specialized low-power ADCs with a burst mode that can take analog measurements while the CPU is off to further reduce power consumption.

For most applications, it's probably best to simply go back to the original power consumption equation to get to the point:

Total power consumption = working mode power consumption + standby mode power consumption + wake-up power consumption

Each application will be affected by a combination of various elements such as standby power, active mode power, and wake-up power. Therefore, the system designer may find that the most helpful way to start any analysis is to put the power consumption numerical value Systematically broken down into the elements above.Once these values ​​are obtained, system designers can consider the application's duty cycle—the proportion of time the application is expected to spend in standby, active, and wake modes—to calculate a value for average power consumption. The resulting values ​​should give the system designer an approximation that can be used to objectively evaluate and compare MCU options to achieve the lowest possible system-level power consumption.


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