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sensor functions and use either an 8-bit MCU or a 32-bit device to run a small radio frequency (RF) protocol stack. These devices are typically battery powered and connect to gateways where heavier processing and data transmission occurs. Sensor Nodes typically transfer small amounts of data and often have to operate on batteries for several years. The devices must also be portable, reliably connected, and able to operate under varied environmental conditions regardless of RF interference or physical barriers. Because these devices are part of networks, the setup of networks, aggregation of sensor data, and display of information must also be considered. The combined selection of the appropriate MCU and wireless or RF connectivity for these devices as well as development tools and software stacks for application development are critical to their successful design. Selecting a Microcontroller (MCU) Sensor node selection is largely a factor of the function and purpose designers are trying to fulfill. For a simple end sensor node that senses and transmits data a few times a day, an 8-bit MCU might be the correct answer. However, for advanced end nodes or gateway devices that build in intelligence or need to run an RF protocol stack or other sophisticated algorithms, a 32-bit MCU is a more appropriate choice. Some 32-bit MCUs such as those based on the ARM® Cortex®M4 core also include a floating-point unit (FPU) that proves useful for implementing complex algorithms. The higher processing capacity of 32-bit MCUs enables them to complete processing sooner in order to enter sleep mode and preserve power. Additionally, the larger flash and RAM sizes available with 32-bit MCUs allow designers to implement the entire networking stack and application code on the MCU without needing an additional processor in the system. ARM Cortex-M4 based 32-bit MCUs include an FPU with a DSP instruction set. Matrix multiplication, used in monitoring applications that use multiple sensors to get a more accurate reading than individual sensors, is an operation where fast single-cycle MACs (multiply-accumulate) significantly reduce the computation time. A common algorithm to correlate the sensor readings is the Kalman Filter, which relies heavily on matrix multiplications. Developing applications becomes simpler with the integrated FPU. 32-bit floating-point operations are executed in hardware and the algorithms can be written directly with floating point values, taking advantage of the great dynamic range and precision of floating point numbers. Code can be more easily developed by removing the need for overflow checks needed with fixed-point processors. A good example of where ARM Cortex-M4 based MCUs are useful is when a device combines GPS, accelerometer, and gyro measurements to improve location accuracy. Selecting the Appropriate Wireless and RF Connectivity Solution Considerations for selecting the RF protocol/technology are link budget, energy consumption, and cost. Wireless developers must determine whether a Sub GHz or 2.4GHz transceiver will best serve their application needs. Given the high profile of 2.4GHz wireless standards such as Bluetooth and Wi-Fi™, many manufacturers assume that 2.4GHz is the de facto transceiver frequency of choice. Wi-Fi provides high data rate connectivity and is the most widely used protocol for bandwidth-intensive applications such as wireless cameras, while Bluetooth provides easy point-to- point connectivity with smart phones, which most people use to control their connected home applications. Transceivers based on 2.4GHz offer high data rates (greater than 1Mbps) and a small antenna (less than one-third the size of a 900MHz antenna) and are suited for short-range consumer electronics devices. However, a 2.4GHz radio has limited range, poor wall penetration, and higher power consumption. The high data rates require a wider receiver channel bandwidth, which further limits the sensitivity and range. In addition, the 2.4GHz spectrum is crowded and subject to significant interference from Wi-Fi devices, Bluetooth nodes and microwave ovens. 15

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