Selecting the optimum MCU for your IoT application
The ongoing growth of the IoT has been accelerated by two major technology trends - the rapid evolution of wireless connectivity and the increasing processing power of the connected devices at the “edge” which perform the sensing, control and data transfer operations underpinning the IoT. A wide variety of wireless connectivity protocols have emerged in recent years, such as Bluetooth LE, Zigbee, and Thread, each with specific attributes suited for different scenarios. At the same time, the falling costs and shrinking form factors of electronics, have driven the development of the Micro Controller Unit, (MCU), a ubiquitous and powerful component at the heart of most of today’s connected IoT devices.
A plethora of MCUs is available on today’s market, at different price points and offering varying levels of resource such as processing power, security features, supported wireless protocols and power consumption. The developer must therefore choose carefully when selecting the right device for the application, to ensure the optimum balance of cost, performance, and speed to market.
This article looks at some of the key features available on the modern MCU and considers the relative importance of each when weighed against emerging IoT trends such as low-power consumption, security, and multi-protocol wireless connectivity. The STM32WB series of MCUs from STMicroelectronics is used to illustrate the variance of features and cost that can be found within MCUs as well as the importance of supporting tools and ecosystem in accelerating speed to market.
The rapidly evolving IoT market
Despite a slowing in growth between 2019 and 2020 caused by the pandemic and global chip shortage, the overall outlook for the global IoT market remains buoyant. Global market research organization, Fortune Business Insights, projects market growth from a 2022 value of US $478.36 billion to US $2,465.26 billion by 2029, a CAGR of 26.4% over the period , driven both by ongoing efficiency initiatives and also changes in working practices as the world learns to live with Covid-19.
North America Internet of Things Market Size 2018-2029 (USD Billion)
Figure 1: Growth projections for the North American market reflect the global outlook
(Source: Fortune Business Insights)
This growth reflects the deployment of innovative IoT applications across multiple sectors, including healthcare technology, connected automobiles, smart city infrastructures, Industry 4.0 factories and various consumer use cases such as domestic lighting control. A number of key trends are emerging across all of these use cases, including:
- Increasing powerful connected devices enable the concept of edge computing where sophisticated algorithms, previously hosted in the data centre, can be executed on the edge device.
- With an increasing reliance on the IoT for mission-critical applications, security is a growing concern. Each connected IoT device represents a threat surface and there are a number of potentially vulnerable points between the edge device and the data centre which must be secured
- Many IoT applications, rely on batteries and alternative power sources and low-power consumption is becoming increasingly important in the design of IoT devices
MCU attributes for IoT device design
The above trends give an insight into the variety of use cases, opportunities and challenges in today’s IoT market and, in-turn, the choices which must be made when setting out to develop an IoT device.
Almost any modern IoT device, from the simplest sensor to the most complex industrial controller will have an intelligent chip at its heart. The complexity of this chip will be dependent upon the application and the developer must select from a wide spectrum of silicon devices which are available on today’s market. At one end of the spectrum, low-cost CPUs handle simple, dedicated tasks while more complex MCUs are now available with a range of functionality integrated onto the chip, figure 2.
Higher levels of integration bring many benefits, reducing design effort, contributing to reduced BoMs and smaller form factors but, since cost rises with the level of integration, care must be taken when selecting the optimal MCU for the application. Factors to be considered include:
Support for Wireless Networking protocols
With wireless connectivity fundamental to the operation of almost any IoT device, support for wireless protocols is a key consideration when choosing a device. A variety of wireless communication technologies including Wi-Fi®, Bluetooth® low energy, Thread and Zigbee® and the emerging MATTER protocol enable device-to-device, device-to-cloud and device-to-mobile communication within home and building automation systems.
The choice of wireless protocol depends on factors, such as
- Battery or mains powered device
- Memory size
- Application data rates – streaming or intermittent, low data rates
- Interoperability considerations
Some IoT devices may require support for multiple protocols. A smart lighting controller may monitor the lighting over a Zigbee network while, simultaneously transmitting a signal over Bluetooth® low energy, allowing a phone to connect and control the lights.
Processing Power and resources
The application complexity will dictate the required processing power and memory size. Multiple cores can be essential where wireless protocols and security functionality are integrated, enabling the separation of application code from the wireless protocol and security stacks, ensuring uninterrupted operation. Multi-core devices also facilitate low power operation since applications can go into sleep mode without shutting down critical communications or security-related tasks.
Peripheral Support
Interaction with IoT devices may be through switches, keypads, touch-sensitive displays, or contactless means such as Bluetooth LE, Zigbee or Thread interfaces. The peripherals and interfaces to be supported will therefore guide the choice of MCU.
Figure 2: Modern MCUs integrate a wide range of functionality on a single chip
Security Support
Digital IoT security measures start at the device level and the MCU plays an important role in hosting these measures. The device must supply the computing resource to host complex cryptographic security algorithms while, at the same time providing hardware security primitives such as secure boot, OTA firmware update capability, memory isolation and unique identity functionality. Modern MCUs integrate these capabilities either through separate cores or dedicated Hardware Security Modules, (HSMs), both of which support the isolation of security functionality and the hardware accelerators required to support cryptographic algorithms.
Power Consumption
Many IoT devices will spend a significant proportion of their lives in idle mode, doing no useful work. To conserve energy – and battery life, most modern MCUs have multiple low-power modes ranging from light sleep/standby through deep sleep to off. The specific functionality of each mode varies from processor to processor, but the principles remain the same, as functionality is switched off, power consumption reduces. Various wake-up methods are employed to restore functionality in response to an external event, with wake-up times dependent upon the “depth” of the sleep mode.
Ease of Development - cost
The viability of many IoT applications is dependent upon speed to market and low development costs and therefore availability of development support and tools are important factors when evaluating an MCU. Most MCU manufacturers provide a range of tools, including pre-integrated modules, evaluation kits and software, such as wireless protocol and security stacks. By taking advantage of these support tools the developer can focus on the application, avoiding the need for specialized resource and reducing development cycles and hence cost.
The STM32WB series
The STM32WB series of MCUs from STMicroelectronics, figure 3, is a leading example of an MCU family optimized for wireless communications. The innovative architecture of STM32WB MCUs is based on two independent cores, with one optimized for wireless connectivity support and the other enabling flexible resource use and power management. The family is split into two sub-lines; the STM32WBx5 integrates a rich feature set in multiple packages and memory sizes and the STM32WBx0 Value Line focuses on the essentials, offering a feature-optimized, cost-effective solution for developers.
All STM32WB MCUs provide comprehensive multi-protocol support for Bluetooth LE 5.3 as well as proprietary IEEE 802.15.4 protocols or full stacks, including the Zigbee PRO 2017 and low-power OpenThread Mesh networking protocols. The MCUs also support concurrent modes of operation (Bluetooth LE and Zigbee or Bluetooth LE and OpenThread), significantly increasing the ease-of-use and overall user experience of these combined technologies.
All STM32WB microcontrollers incorporate extensive embedded security features, including 256-bit AES hardware encryption, PCROP read/write protection, JTAG fuse, public-key cryptography and Customer Key Storage (CKS).
A rich set of peripherals , is available with the STM32WBx5 line, including a practical crystal-less USB 2.0 FS interface, audio support, an LCD driver, touch sensing, up to 72 GPIOs, an integrated SMPS to optimize power consumption optimization, and multiple low-power modes to maximize battery life.
The family is complemented with the STM32WBxM line of modules with the same feature set as the STM32WBx5 line while integrating a full reference RF design with EMC certification covering all major geographical markets. Developers also have access to a powerful development ecosystem which includes a range of evaluation boards and software resources, including HAL and LL peripheral drivers, a full set of middleware and radio stacks (Bluetooth 5.3, OpenThread and Zigbee) together with various preconfigured software examples for several popular IDEs such as Keil MDK-Arm and IAR Systems.
Figure 3: The STM32WB series of MCUs from STMicroelectronics is optimized for wireless communications (click on the image to enlarge)
Let the Application Determine the Optimum MCU
A wide selection of intelligent, MCU devices is available to the IoT developer, each offering different levels of processing power and integrated features. The choice may seem bewildering but a careful analysis of the characteristics of the application will narrow the field.
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