Low-power microcontrollers prepare for connectivity

Now the Internet of Things (IoT) is a reality, household appliances are an asset that provide consumers with the means to control their costs and time, and provide manufacturers with new revenue streams and service models. Appliance designers can fit products with sensors to monitor usage patterns and energy consumption, sleek user interface displays show usage data whilst also providing timely service and exclusive buying opportunities.

With the expected deployment of the IoT being an order of magnitude greater than that of smartphones, there will be tens of billions of “things” that must be powered. Appliance designers will want to reduce, or at least maintain, their present power-consumption levels in order to meet governmental regulations, while adding IoT capabilities.
To fulfill this need, extremely low power microcontrollers (MCUs) and Bluetooth Smart radios are required. They represent a flexible and cost-effective way to connect objects at the edge of the IoT. It would be timely to explore these aspects of a low-power IoT system.
 
Defining the IoT
There are many interpretations of the IoT, often depending on a market sector. One definition published in Wikipedia summarises the concept of the IoT thus: “The Internet of Things (IoT) is the interconnection of uniquely identifiable embedded computing devices within the existing Internet infrastructure. Typically, IoT is expected to offer advanced connectivity of devices, systems and services that goes beyond machine-to machine (M2M) communications, and covers a variety of protocols, domains and applications”.

Relating this definition to appliances, an IoT appliance would be uniquely identifiable, offer advanced connectivity, such as Bluetooth Smart or Wi-Fi, and connect to the existing Internet infrastructure.
Connecting appliances to the Internet ushers in a new paradigm for both consumers and producers. For consumers, it offers them value in the ability to control cost and manage time. For producers, its value is in the ability to monitor the performance of the appliance, proactively solve maintenance issues, and offer revenue-generation structures (see Table 1).

Table 1: Top five examples of new appliance services

Smartphones as an IoT gateway
A smartphone, low-power MCU and Bluetooth Smart radio offer appliance manufacturers the simplest way to add IoT capabilities to their products. Smartphones now ship with integrated Bluetooth Smart, providing an instant gateway to the Internet, with the added benefit of simplified pairing to the appliance. A smartphone app can control the user experience and manage the data transfer to and from the appliance. Wi-Fi is another method to IoT-enabled appliances that provides a constant channel to communicate sensor data, although pairing can be slightly more challenging.

Bluetooth Smart offers beacon capability, vastly simplifying the pairing process. Beacons can advertise their presence to the smartphone when the two nodes are in close proximity. Wi-Fi pairing, on the other hand, requires pushing a Wi-Fi Direct button on the router, which can often reside in another room.

The value of the appliance’s IoT connection is generated by low-power MCUs mated to a Bluetooth Smart radio. The MCU collects sensor data, such as power consumption or run hours, generated within the appliance, and stores it in a usable format. When a smartphone connects with the appliance, the data is uploaded and either transmitted or displayed. The low-power MCU and radio also maintain compliance with governmental regulations by adding connectivity without measureable increases in power consumption.

Bluetooth primer
While the system is simple, it is worth exploring the components in some detail, beginning with Bluetooth. It is very likely that you have used Bluetooth in some form for years now. Many cars have Bluetooth for audio streaming and a Bluetooth headset for a phone is commonplace. Bluetooth that is of interest for IoT appliances is Bluetooth Smart. This is a new standard that is only recently available from the Bluetooth Special Interest Group (SIG). The standard enables low-power operation, which is ideal for IoT applications. Table 2 show that Bluetooth Classic offers a longer range and throughput of 2.1Mbit/s. For low-data-rate applications like IoT appliances, however, this performance rate is not required. Bluetooth Smart’s advantage is that it connects quickly, has throughput matching the need for IoT operation, and offers lower power consumption.
Bluetooth Smart was designed specifically for devices at the edge of the IoT. As the Wikipedia definition says, an IoT device has to be uniquely identifiable. Bluetooth Smart has that capability, for example, Figure 2 illustrates the organisation of a Bluetooth Smart medical application.

Table 2: Bluetooth comparisons

The hierarchy in Figure 2 shows the Bluetooth blood pressure profile, which has services attributed to it, such as the device service and blood pressure service. The profile includes unique user identification (UUID), uniquely identifiable information (in this example, the manufacturer), which is a requirement for the IoT. This is just one example that is part of the Bluetooth Smart GATT, or generic attribute, profiles. The profiles are typically supported in the Bluetooth device directly, as shown in Figure 3. There are profiles for many other applications, including a custom profile, which is particularly suitable for appliances.

Figure 2: Bluetooth Profile Hierarchy

Energy saving
In addition to uniquely identifiable attributes, what makes Bluetooth Smart radios particularly suitable for IoT appliances, is their power needs. The radio has the ability to stay paired with a smartphone without requiring a constant connection. As connections require power, this saves energy. The Bluetooth radio features a ‘connect interval’ and ‘slave latency’, which make this possible. Figure 4 illustrates the ‘connect interval’ is the time period between which the slave, or peripheral, transmits to the smartphone (or ‘central’), before entering a low-power state. This time varies from a few milliseconds to several seconds, with the regularity of the connection determined by ‘slave latency’. These parameters, when combined, allow for data to be transmitted as frequently as every 7.5ms, or as infrequently as every 33minutes for maximum energy savings.

Figure 3: Block Diagram of Bluetooth Smart module

Low-power MCU features
Of course, the other half of the power equation is the microcontroller, or MCU. Power consumption is largely determined by the power mode state and clock speed.

Figure 4: Bluetooth smart communication period

Many new low-power MCUs include power modes. This is the ability to change the configuration of the MCU under software control. Typical examples are ‘run’, ‘doze’, ‘idle’, ‘low-voltage sleep’, and ‘deep sleep’. Each of these modes has key attributes that affect power. For example, the PIC MCU has doze and low-voltage sleep modes. In doze, the MCU can run code at a lower frequency than its on-chip peripherals. This reduces current consumption, but still allows key peripherals like a UART (universal asynchronous receiver/transmitter) to communicate at the proper baud rate. Low-voltage sleep switches out the high-performance, on-chip regulator for a low-current regulator, allowing full MCU state retention using a current of only hundreds of nano-Amps. A transition from run to low-voltage sleep reduces current consumption by 99.9%.

Clock switching
Low-power MCUs also offer on-the-fly clock switching. This is the ability to change clock frequency based upon the task. For example, math-intensive filter algorithms running on sensor data, should run at full clock speed. If in a simple loop and awaiting an interrupt, it is possible to dial back clock speed to save current. Using these methods, current consumption is reduced from five milliAmps to 26µA, a savings of 99%. The bottom line is that low-power MCUs make it easy to save energy.

Step out to the edge
Joining the edge of the IoT is achievable for a broad range of appliance designs. By using the built-in features of low-power MCUs and Bluetooth radios, it is now possible to create a connection to the IoT from an appliance that maintains regulatory energy compliance. This connection allows the collection, processing and transmission of data to smart phones in a way not possible just a few years ago. With smartphones remaining a staple in modern day life, they provide an instant gateway to value-creating applications. Consumers value connectivity as it allows management of their busy lives from the one device they always have near, their smartphone. Manufacturers also benefit from this connectivity through gaining an insight into product performance and usage, allowing the utilisation of modern marketing and service techniques to reduce lifetime costs and enhance revenue as well as gaining insight for the development of next-gen appliances.
The IoT is here, and it represents new opportunity, so step out there.

Microchip Technology at PCIM Europe 2017: Stand 7-138