a trend moving from 8- and 16-bit CPUs towards 32-bit CPUs to help achieve quicker execution of the active mode tasks. The quicker a task is executed the less energy is consumed because the energy wasted due to static currents is proportional to the time spent in active mode, whereas the useful energy spent in executing the task is more or less a constant value.
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Figure 1 | An edge node’s functionality is described in terms of its integrated microcontroller and the associated transducers, interfaces, processing, and communication to the network.
domain. For example, home automation encompasses anything that is used to control or monitor home or office systems and devices, such as lighting or environmental control, or appliances (e.g. freezer, washing machine, coffee maker, fire alarm). On the other hand, “wearable” or “portable” is anything that is worn or carried on the person while in use. Examples include smart watches, smart glasses, heart rate monitors, pedometers, GPS tracking devices, blood sugar monitors, music or video players, and wireless headsets or microphones. There are also categories for health, environmental, and of course the traditional machine-to-machine (M2M) applications. There is a considerable degree of overlap between categories, for example a heart-rate monitor falls in both the “health” and “wearable” domains. Many edge nodes, especially in the “wearable” domain, are ultra-low-power (ULP) applications. These applications are characterized as battery-powered, with short, occasional periods of activity interspersed with long periods of inactivity, and possibly infrequent human intervention. Ultra-low power highlights energy efficiency as a key performance criterion for such devices, and dictates battery lives of weeks, months, years, or even decades.
ULP MCUs for IoT applications Now that you’ve digested all the acronyms of this section’s subhead, recall from www.embedded-computing.com/topics/iot
our previous discussion that many “things” living on the edge must utilize ULP MCUs to handle user interfaces, collect and transmit sensor data, provide security functions, and manage other tasks. One issue that “thing” designers face is determining if these MCUs are optimized to meet their application’s performance and efficiency requirements to enable the long battery lives that are expected. Ultra-low power implies different things to different applications. In some cases, the lowest active current is required when the power source is severely limited (e.g., energy harvesting). Alternatively, the lowest sleep current is required when the system spends most of its time in standby or sleep mode, waking up infrequently (periodically or asynchronously) to process some task. Furthermore, ULP can also imply great energy efficiency whereby the most work is performed in a limited time period. Overall, the application will require a combination of or tradeoffs on all of the above. There are many factors that enable an MCU to earn its ULP title. One factor is the type and degree of intelligence available through an MCU’s peripherals. For example, peripherals such as SPI, GPIO, PWM, and ADC that we mentioned earlier, if designed correctly by the vendor, can significantly help offload the CPU and thereby allow the device to spend more time in sleep mode. There’s also
Other factors that help yield an ULP MCU include choices of physical IP, low-leakage process nodes, and lowpower memory technologies. Using smaller geometries reduces active power due to smaller gate capacitance and lower operating voltages, but tends to increase leakage current when the clock is stopped. For this reason, power gating becomes more important at smaller geometries. Also from a chip design standpoint, a vendor can implement various forms of gating. Clock gating automatically switches off clock signals to various blocks of circuitry whenever possible. Even more effective is power gating, which switches off power to blocks inside the chip when possible. Even further energy efficiency can be achieved by the use of state retention power gating (SRPG), whereby power is switched off to most logic blocks inside the chip with the status of the digital circuits held in retention elements. One of the biggest factors affecting energy efficiency is the use of low supply voltages. Since power is proportional to the square of the voltage, moving from 3V to 1.5V gives a four-fold reduction in energy, all other things being equal. High efficiency step-down regulators allow this even if the battery voltage is much higher.
IoT designers beware of datasheet parameters While datasheet parameters are typically accurate and essential for anyone doing a system design, one must take care when using these parameters to analyze and compare different devices (this includes MCUs and just about everything else). Vendors tend to utilize IoT Design Guide
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