From your toothbrush to your car, MCUs have become ubiquitous and our reliance on them is growing with the expansion of the Internet of Things (IoT). Today, speed has been replaced by energy efficiency as the key care-about for embedded applications. An MCU’s usefulness as part of an energy efficiency system depends on its ability to solve increasingly complex problems while consuming as little power as possible.
We’re going to look at four considerations when maximizing energy efficiency in embedded applications.
At a high level, all embedded applications are strikingly similar. They are built from a number of components specifically designed to perform a specific task such as power management, processing, connectivity, etc.) The components that perform these functions consume energy from your power source. So when building an energy-efficient system, logic dictates that you should choose components within your budget that are inherently energy efficient.
Modes of Operation
Besides integration, you should also understand the components’ various modes of operation. Most of them have an on mode and an off mode, but there may also be intermediate modes. An application can consist of a number of components, and you have to make a decision on how to control each component in the most efficient way. Designing for energy efficiency can in some ways be harder than designing a system that does not care about efficiency. But in energy-constrained systems, it is well worth the investment.
When working with a sensor in an application, the straightforward approach is to leave the sensor on all the time. With this approach, the MCU can read the voltage across the variable resistor at any time, and calculate the current temperature based on the voltage.
This option is the easiest way to control the sensor, but it’s also the method that consumes the most energy. Now, 33 µA might not seem like much, but when a solar cell that small only produces 10 μW of current, we quickly see the problem. A better setup is where the MCU is able to control the power of the sensor directly, turning it on only when needed.
There are many types of energy sources for embedded applications including wired power, batteries, energy harvesting, and wireless power. A single application might use multiple power sources, but minimizing current consumption is key. Factors including mobility, lifetime, cost, and form factor put some constraints on your application. So uncderstanding the trade-offs that come with each is important.
Batteries - Let’s say you’re a designer and the specificiaton states that the product or application needs to last for at least three years. You’ve decided to use batteries as the energy source. With that choice, comes the reality that you will need to make a tradeoff between lifetime, form factor, and cost.
Energy harvesting – Energy harvesting uses the surroundings to generate energy. And naturally, these methods are dependant upon environmental factors that may be unpredictable. Solar harvesting panels, for example, must be in a bright location, and they need to have a given surface area. They might be able to generate 10 mW/cm^2 under direct sunlight, but can drop to 10 μW/cm^2 when indoors. That is 1000 times less energy to play with! To support nighttime operation, a rechargeable battery is needed as well, which increases cost and penalizes form factor.
Wireless power - Wireless power delivery, also known as remote power delivery, is similar to energy harvesting in that your application picks up energy from its surroundings. The difference is that a power transmitter generates the energy the application is supposed to pick up. For inductive power delivery, the transmitter is generating an alternating magnetic field, and the receiver uses a coil to capture the energy. In this scenario, the maximum distance between the transmitter and the receiver, and also the amount of power that can be delivered, are based on the size of the coil. This puts constraints on form factor and flexibility because the receiver and transmitter must be in very close proximity.
The MCU Itself
Just as application components must be duty-cycled in order to maximize efficiency, the same is true for the MCU itself. Because they are more sophisticated components, MCUs almost always have more than just an on/off button. MCUs have multiple energy modes, where each mode allows a set of capabilities with an associated current consumption overhead.
By requiring the CPU to be off as much as possible in order to save energy, the CPU tasks must be offloaded to the hardware in the MCU. Instead of being in a paradigm where software running on the CPU does everything, software development should focus on setting up hardware to do the heavy lifting and only intervene when hardware needs assistance. This takes the system to an event-driven architecture, allowing massive energy savings.
At the heart of most embedded products lives a microcontroller with power sources that may be limited to small coin sized batteries. When focusing on using available power more efficiently, designers will be able to create energy-friendly products and applications that are smaller, have longer battery life and cost less.
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