MSP430 FRAM Microcontrollers Enable Energy Harvesting

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MSP430 FRAM Microcontrollers Enable Energy Harvesting [Copy link]

For many people, the first exposure to energy harvesting may have been in the early days of solar-powered portable calculators, and while these types of calculators are no longer mainstream, the technology and concepts used are still used in our daily lives. We can now see energy harvesting in many applications, such as sensor nodes, wind turbines, and indoor power supply applications. However, even though the discussion of this technology has evolved greatly from the past, when it comes to energy harvesting, developers still face the same challenges as decades ago.

In order to generate the required energy without negative effects, it is usually necessary to have a solar panel of large physical size and a huge thermal energy collection device, or to obtain energy by emitting vibrations in different frequency ranges from the equipment, all depending on the system used. Therefore, in many cases, the cost of this system even exceeds the advantages of replacing traditional power sources. Of course, there are exceptions if these limitations must be ignored for certain reasons. For example, in remote areas where power lines cannot reach, wind or solar energy collection can provide a viable alternative energy source for battery-powered systems, although the initial cost of this approach will be higher.

Let’s look at some of the key challenges facing current energy harvesting solutions.

Figure 1 - Simplified general block diagram

First, from the simplified general block diagram above, you can see that this system consists of inputs and outputs, including sensors, buttons, LEDs, displays, sounders, and increasingly wireless connectivity. The edge nodes of this typical Internet of Things (IoT) architecture can communicate via Wi-Fi, Bluetooth, NFC/RFID, or other proprietary interfaces. The power required for these wireless connections is as low as a few uA, and the maximum is only tens or hundreds of mA, which can power the related RF ICs and subsystems within tens of milliseconds.

Figure 2 - Typical RF power usage chart

In many applications, designers want to store sensor or other data in non-volatile memory because the acquired data can be recovered even when power is lost. Therefore, existing general-purpose memory technologies such as EEPROM or FLASH are not always the best choice in these energy-constrained situations.

Fortunately, technology is moving in a direction that makes energy harvesting systems feasible. One such technology integration is TI’s family of ferroelectric random access memory, or FRAM, microcontrollers (MCUs). FRAM technology combines many of the benefits of SRAM memory with the nonvolatility of FLASH memory. A key advantage is the ultra-low-power, nonvolatile FRAM writes, which, unlike FLASH, do not require a pre-erase cycle, saving time and power. Another advantage is the inherent low-voltage writes to FRAM cells. Traditional flash or EEPROM technologies require an integrated charge pump to complete a pre-erase cycle, which typically requires 5-10 mA of current and hundreds of milliseconds of operation. In applications that require frequent nonvolatile writes, this additional power consumption can consume significant battery power or harvested energy.

The cost of buying disposable batteries may not be very high, but the impact they have is far-reaching. Billions of new batteries are sold worldwide each year, and only a small fraction of them are recycled, which creates a lot of landfill waste . Another disadvantage of disposable batteries is that both the battery itself and the entire system need to be replaced at some point in some situation, which creates potential challenges. Imagine if the battery is installed in a system deployed at the bottom of the ocean or on the top of a mountain, how should we replace it? In fact, the cost of battery replacement can be very large. Although rechargeable batteries can reduce the number of batteries to be replaced, they will not necessarily solve all the challenges of battery replacement. Rechargeable batteries do provide benefits when they are charged using energy harvesting.

Currently, solar energy, thermal energy, motion energy (vibration or other dynamic effects), and RF are widely accepted. Other energy sources are also in the process of development, such as the possibility of harvesting energy from electrochemical reactions in human blood, or from such reactions inside plants and trees.

Ideally, these sources would be continuous, but in reality they are not. In the case of solar harvesters, drifting clouds may block out the sun, and lights in indoor facilities cannot be turned on forever. Vibration-based harvesters usually operate near a resonant frequency, which limits their operating range, and thermal harvesters lose efficiency or stop operating completely if they cannot maintain the appropriate temperature difference. Ultimately, we cannot rely on this source to maintain continuous 7x24 hour operation, so redundancy is needed. In some cases, this may be a second harvester or a rechargeable battery. Even solar-powered calculators include a CR2025 battery to serve as a backup to the solar energy when the office is dark.

Dealing with power loss is a major consideration for designers of energy harvesting nodes. Modern microcontrollers run through a startup sequence when powered on, which often takes several milliseconds and consumes precious power. If power is lost, most microcontrollers need to restart and run this startup code each time power is restored.

FRAM memory itself is the enabling device for a highly innovative software utility called Compute Through Power Loss (CTPL). We can even think of CTPL as a non-volatile interrupt handling routine where, when a power loss is detected (usually using a comparator or ADC input), key parameters and microcontroller state are saved to non-volatile memory (NVM). In the event of a power outage, FRAM offers the advantage that designers can continue working directly from where they left off rather than starting from scratch.

The low-cost MSP430FR6989 MCU Launchpad development kit with 128KB FRAM MSP430 microcontroller enables simple demonstrations.

By combining FRAM technology, Compute Through Power Loss code and the Energy Harvesting BoosterPack plug-in module, we have laid a good foundation for many energy harvesting sensor nodes. The power-good signal provided by the bq25570 can be used as a trigger for Compute Through Power Loss activation, saving time and precious energy after a power outage.