Internet of Things
How to specify a Wi-Fi-enabled MCU
By Alex Li, Microchip Technology
T
he trend in today’s industrial internet of things (IoT) is doing more with less, squeezing more functions onto a single system on a chip (SoC) instead of using
numerous separate devices. Benefits include less risk, a smaller size and a lower parts count. These all-in-one devices are exemplified by the Wi-Fi microcontroller unit (MCU) that integrates Wi-Fi connectivity with a processor and the general purpose input/output (GPIO). Choosing the best Wi-Fi MCU is a challenge because for the project to work, it must offer both robust Wi-Fi connectivity and a high-performance MCU. Because it forms the heart of the system, getting the right MCU is critical if we don’t want to undertake major changes to the software later in the project.
The critical role of the ADC
Although analog-to-digital conversion is one of the first stages in the signal chain, it is also one of the most overlooked when specifying a Wi-Fi MCU. Because it is critical to the performance of the entire system, it’s vital to understand key metrics about the analog-to- digital converter (ADC). Designers often focus on the
ADC’s number of bits, but far more important is the effective number of bits (ENOB) the ADC can use to perform the conversion. This is important, as the fewer the bits available to perform the conversion, the less precisely the SoC will represent the input signal.
ADCs can also introduce errors in areas such as
quantization and timing and can also be affected by wide temperature swings (See Figure 1). It’s vital to get information from the manufacturer about the Wi-Fi MCU’s ENOB,
32 October 2021
performance over temperature, linearity, and accuracy.
Supporting peripherals All Wi-Fi MCUs will support at least a few interface standards, but engineers shouldn’t think they can use the same Wi-Fi MCU in another design. To accommodate the changing needs of industrial production processes, the Wi-Fi MCU should have enough GPIO to accommodate more relays, switches, and other components easily. This means that the device should support Ethernet MAC, USB, CAN, CAN-FD, SPI, I2C, SQI, UART, and JTAG.
essential for IoT applications. The first line of defence is the MCU’s own integrated crypto engine. Its ciphers should include AES
by the Wi-Fi Alliance. The latest WPA3 version includes features to simplify Wi-Fi security, enable more robust authentication, give
Figure 2. An integrated development such as this one reduces risk by providing the designer with debugging and other tools from the prototype stage through the finished product.
The best security comes from within With cyber-attacks on the increase, security is
encryption with key sizes up to 256 bits, DES and TDES, and authentication should include SHA-1 and SHA-256, and MD-5. Provisioning their product for a cloud service is a major challenge for designers, as each cloud service provider has its own certification and keys. Fortunately, some manufacturers, including Microchip Technology, make this process easier, potentially helping cut weeks from the design process while meeting all security and
provisioning requirements. Instead of storing
credentials in a vulnerable flash memory, consider a hard-coded security element which cannot be read by external software. Microchip’s Wi-Fi MCUs, such as the WFI32, employ this approach in the company’s Trust&GO platform. The Trust&GO platform needs only an inexpensive Microchip development kit which the designer uses to create the required manifest file. Once the C code for the secure element is working, the design is ready for production.
Figure 1. Low-grade ADCs have poor accuracy and linearity and are sensitive to environment and temperature.
Components in Electronics Devices also need the latest Wi-Fi security certified
stronger cryptographic features, and maintain network resilience. A Wi-Fi MCU also needs to be able to communicate with the most popular access points so should have passed interoperability testing – the manufacturer should be able to prove this.
You’ll need help As a designer, you don’t want to be left out on a limb by the manufacturer of the Wi-Fi MCU – some may only provide basic information but to get your solution from development to production, you’ll need a comprehensive integrated development environment (IDE) platform. As in Figure 2, this should include every analog and digital function performed by the Wi-Fi MCU, as well as all the external components needed for particular applications. Ideally, the IDE will let you see how design changes will affect RF performance as well as regulatory compliance.
Summary
IoT is moving toward more processing power at the edge of the network, with Wi-Fi MCUs helping achieve this by integrating multiple functions in a single device.
Integrating such devices into an IoT subsystem can be simple, as long as the Wi-Fi MCU manufacturer can offer the vital resources - a high level of security, a straightforward means of provisioning to accommodate the needs of cloud service providers, and a comprehensive IDE that aids progress from prototype to production.
www.cieonline.co.uk
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