Supplier eBooks

16 Many IoT sensor nodes have relatively low data rate requirements. For these applications, sub-GHz radios offer substantially higher range than 2.4GHz radios. Sub GHz radios also have much lower power consumption and can provide connectivity for devices that must operate for years on a single battery. These factors, combined with low system cost, make Sub GHz transceivers ideal for low data rate applications that need maximum range and multi-year battery life, such as smoke detectors, door/window sensors, and outdoor systems such as weather stations and asset tracking. One of the main advantages of using Sub GHz wireless is its long-range capability and ability to effectively pass through walls and other barriers. Narrowband transmissions can transmit data to distant hubs, often several miles away, without hopping from node to node, thus reducing the cost of deployments with fewer base stations/repeaters. RF waves at lower frequencies can travel longer distances for a given output power and receiver sensitivity. Achieving good range depends on the antenna gain, receiver sensitivity, and transmitter power. The antenna gain is usually limited by cost and device form factor. Receiver sensitivity sets the lower limit on power that can still be received and understood and receiver sensitivity determines how well it can distinguish the desired transmission from other signals and noise in the area. Thus, having a receiver with both good sensitivity and selectivity makes it possible to achieve longer range. A primary factor affecting radio sensitivity is the data rate. The lower the data rate, the narrower the receiver bandwidth is and the greater the sensitivity of the radio. On the transmitter side, range is determined by the output power level. A 6dB increase in link budget will double the range in an outdoor, line-of-sight environment. However, regulatory standards limit the allowed output power, and increasing the transmitter power also increases the current consumption, which can have a negative effect on battery life. While many of the existing Sub GHz networks use proprietary protocols and are closed systems, the industry is moving towards standards based, interoperable systems. The IEEE 802.15.4 standard is gaining popularity worldwide and is being adopted by various industry alliances such as Wi-SUN and Zigbee. Designers should also consider a transceiver's support for mandatory features in Sub GHz bands defined in the 802.15.4 standard. Silicon Labs' family of EZRadioPRO ® Sub GHz transceivers have shown 8 to 10 miles of range in both high frequency bands and low frequency bands using standard GFSK modulation. A unique feature of EZRadioPRO is the ability to support ultra-narrow band applications all the way down to 100bps in a 200Hz bandwidth. This allows extremely long range communication with standard GFSK modulation for low data rates. Transceivers with programmable data rates such as EZRadioPro allow developers to trade off higher data rates and sensitivity to fine tune the data rate so the radio transmits in the least amount of time. Simplifying Design with Integrated Components Thus far we have discussed MCU and wireless requirements separately. For an IoT system, however, the combination of MCU and wireless device selection and their interaction must be considered together. Instead of using discrete components, designers now have the ability to make use of integrated devices that combine MCUs and transceivers. By taking advantage of chips that integrate the two devices, designers do not have to worry about interconnections between the radio and MCU, making for simpler board design, more straightforward design processes and less susceptibility to interference due to shorter bond wires. Designers are also able to rely upon a fully tested wireless MCU solution as a starting point for their development. Examples of devices that fulfill these criteria include the Silicon Labs EZR32™ family of wireless MCU devices. By making use of single chips that combine ARM Cortex-M based MCUs and radio transceivers, board component count, board layout, and overall cost can be reduced. Devices such as the EZR32 wireless MCUs from Silicon Labs integrate ARM Cortex-M3 or Cortex-M4 MCUs with EZRadio® and EZRadioPRO transceivers. In addition to ensuring that transmitters provide sufficient power and receivers provide the necessary sensitivity, designers should also make sure the necessary peripherals and communication interfaces are supported. The high power +20dbm transmitters and receiver sensitivity of up to -133dBm of EZR32 wireless MCUs makes them ideal for low data rate and long range applications. Meeting Low Power Requirements One of the main considerations in IoT implementations is the power consumption of the whole system. Typical applications run on batteries that are expected to last for up to 20 years. In most of these applications, the MCU typically stays in low-power sleep mode for a dominant portion of the time, waking up only once in a while to read sensors or to transmit and receive some data. There are two aspects to the power consumption of the MCU subsystem; dynamic power consumption when the MCU is active, which is proportional to the clock frequency, and the static power related to the leakage current (mostly constant) that plays a bigger role in sleep states. Thus, the total power consumption is affected by the active mode current, the sleep mode current, and the amount of time spent in active mode. In applications that spend the majority of the time off, sleep

• Cover