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What you need to know when choosing a Wi-Fi MCU for the Industrial Internet of Things

As the Industrial Internet of Things (IIoT) grows, the trend is to perform more functions in a single system-on-chip (SoC) rather than in multiple discrete devices, as the result is a smaller bill of materials, less design risk and footprint The area is smaller. A good example is a Wi-Fi microcontroller (MCU), which integrates Wi-Fi with a processor and GPIO to meet the needs of various applications. Wi-Fi MCU selection is a matter of careful consideration. There are several factors to consider, and it is important to understand them.

When evaluating Wi-Fi MCUs for the IIoT, designers need to consider multiple features such as ADCs, interfaces, security, and interoperability.

As the Industrial Internet of Things (IIoT) grows, the trend is to perform more functions in a single system-on-chip (SoC) rather than in multiple discrete devices, as the result is a smaller bill of materials, less design risk and footprint The area is smaller. A good example is a Wi-Fi microcontroller (MCU), which integrates Wi-Fi with a processor and GPIO to meet the needs of various applications. Wi-Fi MCU selection is a matter of careful consideration. There are several factors to consider, and it is important to understand them.

Low-cost Wi-Fi options exist on the market today, but often sacrifices in peripheral count and overall performance. This means that choosing the best Wi-Fi MCU is challenging and risky, as a Wi-Fi-enabled MCU must not only have robust Wi-Fi connectivity, but also high-performance MCU capabilities. Missing one of these can lead to delays or even failures in design projects.

As the core of the system, the MCU is the most critical part of a Wi-Fi-enabled device, so it is necessary to check its performance at the beginning of the project to avoid redesigning all software and circuits caused by frequent device replacement.

Don’t forget ADCs

Analog-to-digital conversion is one of the most overlooked functions when specifying a Wi-Fi MCU, even though it is the first processing component in the signal chain after the signal is input. This means that its performance affects the entire system, so it’s important to understand the key metrics of analog-to-digital converters (ADCs) and how Wi-Fi MCU manufacturers should address them.

One of the first specifications that designers look at is the number of bits in the ADC. This can be confusing because in practice the actual number of bits will be lower than the datasheet specification, sometimes significantly lower. More importantly, one should pay attention to the effective number of bits (ENOB) that the ADC can use to perform the conversion. This will always be lower than the spec in the data sheet, but the closer the match between ENOB and the spec in the data sheet the more important as this will vary widely between ADCs. The fewer bits available to perform the conversion, the less accurate the SoC’s input signal.

In addition, like all Electronic devices, ADCs have several negative effects on the signal, including errors such as quantization and timing errors, as well as errors in offset, gain, and linearity. ADCs are also plagued by their sensitivity to wide temperature fluctuations common in many IIoT operating environments (see Figures 1a and 1b), so it is important to contact the manufacturer to determine the ENOB, temperature performance, linearity and accuracy of the device .

Figures 1a and 1b: Low-level ADCs have poor accuracy and linearity and are sensitive to ambient and temperature. (Image: Microchip Technology)

Peripheral support

All Wi-Fi MCUs support at least some interface standards, so it’s easy to assume they’re sufficient. Engineers often regret this assumption when they try to use the same Wi-Fi MCU in another design. This is becoming more and more common when building or modifying IIoT systems, as most devices are made by different manufacturers at different times.

As the system grows, it may add more interfaces and sometimes need to support features like touch sensing and LCD. If the SoC has spare GPIOs, more relays, switches and other components can be controlled with little or no pin sharing. Therefore, the interfaces supported by the device should include Ethernet MAC, USB, CAN, CAN-FD, SPI, I2C, SQI, UART and JTAG (and possibly touch and Display, etc.) to ensure that almost every situation can be accommodated now and future.

Safety starts from within

Security is essential for every IoT application, but industrial scenarios are mission critical. Once a danger enters an IIoT network, it can pervade an entire organization or even an entire company. The first level of security required is in the MCU’s integrated cryptographic engine, where encryption and authentication can be performed sequentially or in parallel. Ciphers should support 256-bit AES encryption, DES and Triple DES, and authentication should include SHA-1, SHA-256 and MD-5.

One of the most challenging tasks in design is that people are provisioning their products for cloud services. Each cloud service provider has its own certificates and keys, so configuring the device becomes complicated and requires a lot of knowledge about encryption.

Fortunately, some manufacturers, including Microchip, make this process easy, saving a lot of time and money. It’s important to note that most Wi-Fi MCUs store password credentials in flash memory, where the data is accessible and vulnerable to software and physical attacks. The highest security is achieved by storing this information in a hard-coded secure section, as the internal data cannot be read by any external software. For example, Microchip’s Wi-Fi MCUs such as the WFI32 (see Figure 2) take this approach in the company’s Trust&GO platform to securely configure their MCUs to connect to AWS IoT, Google Cloud, Microsoft Azure, and third-party TLS networks .

The time reduction and confusion caused by this approach cannot be underestimated. Save weeks or more during the design process while ensuring a reliable and verifiable approach to meeting all security and backup requirements.


Figure 2: The WFI32 Wi-Fi module isolates credentials by storing them in hardware, making them virtually immune to hacking. (Image: Microchip)

Pre-configured or custom secure elements store credentials generated inside the device’s Hardware Security Module (HSM) at the time of device manufacture, so that they are not exposed during and after production. The Trust & Go platform only requires an inexpensive Microchip development kit, and designers can use tutorials and code examples to create the required manifest files in the included design kit. Once the C code for the secure element is running in the application, the design can be sent to production.

Another form of required security is state-of-the-art Wi-Fi security certified by the Wi-Fi Alliance. The latest version is WPA3, which builds on its predecessor, WPA2, but adds features to simplify Wi-Fi security. It also enables stronger authentication, enhanced encryption strength and maintains network resiliency.

All new devices must be WPA3 certified to use the Wi-Fi Alliance logo, so every Wi-Fi chip and Wi-Fi MCU should be certified to provide the highest security. Therefore, please check in advance to ensure that the Wi-Fi MCU is WPA3 certified.

ensure interoperability

There is always a chance that a Wi-Fi MCU will not be able to communicate with some access points on the market due to mismatches in RF, software and other factors. The inability to connect to popular access points can damage a company’s reputation. While there is no guarantee that a Wi-Fi MCU will work with every access point (AP) on the planet, problems can be minimized by ensuring that the Wi-Fi MCU passes interoperability testing with the most popular APs on the market . This information can be found on the manufacturer’s website.

you need more help

Last but not least, design support is required. Without a comprehensive integrated development environment (IDE) platform, designers will have to scour the web for a variety of available or unavailable, simple or reliable resources. For example, some Wi-Fi MCU manufacturers provide basic details and prototyping instructions about the product, but only stop there, rather than including the information needed to get the design into production.

Manufacturers should provide a comprehensive IDE (see Figure 3) that includes all analog and digital functions performed by the Wi-Fi MCU, as well as all external components required for implementation in a specific application. IDE platforms also need to provide a way to visualize how design changes are reflected in overall performance and to evaluate designs for RF performance, regulatory compliance, and more. Some of these basic tools are free, while others are expensive, including evaluation boards.


Figure 3: Integrated development reduces risk by providing designers with debugging and other tools from the prototype stage to the final product. (Image: Microchip)

Summarize

The IoT trend is to move more processing power to the edge of the network, rather than just in cloud-based data centers. For this, as many functions as possible must be integrated in the least space and components. Wi-Fi MCUs do this by integrating multiple functions into a single device rather than function-specific discrete components.

Integrating these devices into an embedded IoT subsystem can be relatively straightforward, assuming sufficient resources are available. Includes a high level of security, providing a straightforward approach to meeting the needs of cloud service providers, and a comprehensive IDE that supports the designer’s full cycle from prototype to production.

About the author:

Alex Li is a Product Line Manager at Microchip Technology, responsible for technical marketing and global growth of Wi-Fi products. During his career in the semiconductor industry, Mr. Li has held technical engineering and marketing positions in the US and Singapore. Before joining Microchip, he worked in Semtech Corporation’s Consumer and Sensing Division Product Line Marketing. Previously, Mr. Lee served as a senior applications engineer at Arrow Electronics, serving a customer base in the Asia Pacific region. Before joining Arrow, he was a Senior Principal Product Application Engineer at NXP Semiconductors in Singapore and was a member of the global sales and marketing team providing business development strategies for NXP’s Asia Pacific region. Mr. Li holds a Bachelor of Science degree in Electrical and Computer Engineering from the National University of Singapore and a Master of Business Administration (MBA) from Washington University’s Olin School of Business in St. Louis, Missouri.

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