The First Steps to Choosing the Right MCU for an Automotive Design

Both Microchip and STMicroelectronics recently made headlines with new automotive microcontrollers.

Microchip’s new PIC18-Q41 and AVR DB MCU families seek to meet the demands of automotive MCUs such as low cost, small size, high performance, and low power by focusing on increased microcontroller integration.

On the other hand, STMicroelectronics' new Stellar MCU family is designed to execute multiple independent real-time applications, eliminating the tradeoff between determinism and virtualization.

With so many MCU options available, and evidently more continually being developed, it can be confusing to select the component that will meet all the requirements of your design. Because each project includes such specific requirements, the datasheet will be essential to provide more in-depth clarification for choosing the right automotive MCU.

In this article, though, we'll consider the basic pre-requisite considerations when considering a group of MCUs—that is, before you begin narrowing your selections down by specs.

 

First, Determine Whether the MCU is Automotive Qualified

Automotive electrical systems can be extremely complex with dozens of separate control systems, all connected by what can be up to a mile of wire. In addition to technical complexity, there are stringent standards set by the SAE for electronics involved in automotive design.

This means that the first step in selecting an automotive MCU is to ensure that it’s automotive qualified.

 

Automotive network types. Image used courtesy of Renesas

 

Since there are so many interconnected MCUs in a car, multiple network types and protocols have been defined by the SAE and other automotive consortiums. The main networks and protocols to be aware of are CAN, LIN, FlexRay, MOST, and Ethernet AVB. 

A quick look over of a datasheet is all it takes to identify the presence of CAN ports or Ethernet ports. You'll also be able to determine if the MCUs are designed to the protocol specifications listed above.

 

Consider the MCU's Intended Application

As mentioned before, there can be many MCUs coexisting within an automotive system all serving different functions. Hence, the next step in choosing the right MCU for your design is to consider the application it needs to serve. 

To help you break down such an open-ended consideration, you might consider, for instance, what peripherals are necessary. Certain applications might require complex peripherals found on higher-end microcontrollers. An example could be an ADC with enhanced features, such as multiple-channel 12-bit continuous conversions. For less complex applications a lower-end microcontroller with single-channel 8-bit conversion ADC might be suitable.

 

Graphic showing the wide number of applications within a vehicle. Image used courtesy of NXP and SemiMedia

 

You will likely also need to consider how mission-critical your application may be. If your MCU is responsible for deploying an airbag with a quick response time, high bus speeds with a low tolerance for error might be the priority for your MCU. Other applications might be able to operate with a greater tolerance for error, such as adjusting seating and mirrors. In these cases, a lower bus speed will do the job while prioritizing low power. 

 

Narrow Down By Basic Specs

Once you've established that an MCU is automotive qualified and outlined the core tenants of the component's end application, you'll need to dig deeper into the device's datasheet, application note, and any other technical errata. 

NXP and Freescale Semiconductor recommend that designers interrogate an MCU's RAM and NVM capabilities to ensure it aligns with the end device's goals; for instance, seats, steering and, radio will require more memory storage while simple switch applications will not. 

 

Block diagram of Microchip's new AVR128DB28/32/48/64 automotive MCUs. Image used courtesy of Microchip

 

Alternatively, you might size up how the package size of a certain MCU fits into an allotted amount of board space in your design. You should also determine whether low power or performance is the primary goal of an application and assess the datasheet's specifications accordingly. 

Other considerations of note could include the minimum number of I/Os you'll need, the availability of supporting software tools, and the future scalability of the MCU. 

 

Conclusion

While it's impossible to fully discuss all of the considerations for choosing an automotive MCU in a brief article, there are some key takeaways here. First, before you can even consider an MCU for automotive applications, you need to ensure that it’s automotive qualified. This can be determined based on whether or not the MCU under question supports automotive networks and protocols.

Beyond that, every application will have its own set of requirements. Understanding your application and choosing an MCU with complimentary specs will be crucial to finding the right component for your design.

As more and more automotive-facing MCUs like those from STMicroelectronics and Microchip find their way to the market, learning to dependably select the right MCU for your application will become a useful skill.