Practical Tips | Teach you how to choose a microcontroller

Latest update time：2021-09-06 00:29

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Selecting the right microcontroller for your product can be a daunting task. Not only do you have to consider the many technical features, but there are also business aspects like cost and lead time that can cripple your project.

Early in a project, you may be tempted to jump in and start selecting a microcontroller before agreeing on the details of the system, but this is of course a bad idea.

Before any thought is given to microcontrollers, hardware and software engineers should first develop high-level specifications of the system, draw block diagrams and flow charts, and only then will they have enough information to make a rational decision on microcontroller selection. When you reach this stage, you can follow 10 simple steps to ensure that you make the right choice.

Step 1: Make a list of required hardware interfaces

Using the overall hardware block diagram, make a list of all the external interfaces that the microcontroller needs to support. There are generally two types of interfaces that need to be listed. The first type of interface is the communication interface, which includes peripheral interfaces such as USB, I2C, SPI, and UART.

If the application requires USB or some form of Ethernet, make special notes. These interfaces have a significant impact on the amount of program space the microcontroller needs to support. Secondary interfaces are digital inputs and outputs, analog to digital inputs, and PWM interfaces.

These two interface types will dictate the number of pins required on the microcontroller. Figure 1 shows a generic example block diagram that lists the I/O requirements.

Step 2: Check the software architecture

Software architecture and requirements have a significant impact on the choice of microcontroller. The severity of the processing requirements will determine whether to use an 80 MHz DSP or an 8 MHz 8051. As with the hardware, any important requirements should be noted.

Step 3: Choose an architecture

Using the information from steps 1 and 2, the engineer should be able to get a preliminary idea of the architecture required. Can the application be implemented with an 8-bit architecture? How about 16-bit? Or does it require a 32-bit ARM core? Between the application and the required software algorithms, these questions will begin to coalesce into a solution. Don’t forget possible future requirements and functional expansions.

Just because an 8-bit microcontroller meets your requirements today does not mean you should not consider a 16-bit microcontroller for future functionality or ease of use. Remember that microcontroller selection can be an iterative process. You may choose a 16-bit part during this step and find in a later step that a 32-bit ARM part is a better fit. This step simply allows the engineer to determine the right direction to move forward.

Step 4: Determine storage requirements

Flash and RAM are two very important components of any microcontroller. Making sure you don't run out of program space, or variable space, is definitely a top priority. When choosing a part, it's easy to go for one that has too much functionality, rather than not enough.

It is not uncommon to find at the end of a design that 110% of the space is needed or that some functionality needs to be cut. After all, you always start out wanting more and then move to a slightly more limited part in the same chip family. Using the software architecture and communication peripherals included in the application, the engineer can estimate the amount of flash and RAM required for the application. Remember to leave room for feature expansion and subsequent versions! This can save a lot of trouble in the future.

Step 5: Start looking for a microcontroller

Now that you have a better idea of the features you need in a microcontroller, you can start your search! A good place to start is with microcontroller suppliers such as Arrow, Avnet, or Future Electronics.

Talk to the FAE about your application and requirements, many times they can recommend new parts that are both cutting edge and will meet the requirements. Just remember that they may be under pressure to promote a certain line of microcontrollers at the moment!

The next best place to start is with chip suppliers that you are already familiar with. For example, if you have used certain microchip parts in the past and have a good relationship with the supplier, start your search on their website.

Most chip vendors have search engines where you can enter your peripheral set, I/O and power requirements, and it will narrow down the list of parts that match your criteria. From that list, the engineer can then proceed to select a microcontroller.

Step 6: Check cost and power constraints

At this point, the selection process has yielded a few potential candidates. Now is a good time to check the power requirements and component costs. If the device will be battery powered and mobile, ensuring the component has low power consumption is a top priority.

If parts do not meet the power requirements, they should be eliminated from the list until one is selected that does. Also, don't forget to check the unit price of the processor. Although many parts have stabilized at around $1 in wholesale prices, if the part is highly specialized or a high-end processor, the unit price can be very important. Don't forget this critical factor.

Step 7: Check Parts Availability

Once you have your list of candidate parts, it’s time to start looking at part availability. Here are a few things to keep in mind: What is the lead time for the part? Do multiple distributors have it in stock? Or is it a 6–12 week lead time? What are your supply requirements? You don’t want to get stuck with a large order and have to wait three months to fill it.

The next question is how old the part is and whether it will remain available for the life of your product. If your product is to be available in 10 years, then you want to look for parts that the manufacturer guarantees will be produced within 10 years.

Step 8: Select a Development Kit

One of the best stages in selecting a new microcontroller is finding a development kit that you can play with and learn about the inner workings of the controller. Once engineers have identified a part they like, they should investigate what development kits are available.

If there is no development kit available, then it is very likely that the part you have chosen is not the best choice and it is time to take a few steps back and find a better part. Most development kits are now priced under $100. If it is more than that (unless it is designed to be used with multiple processor modules), it is clearly too expensive. Other parts may be more suitable.

Step 9: Investigate compilers and tools

Selecting a development kit essentially finalizes your microcontroller selection. The last thing to consider is to check the available compilers and tools. Most microcontrollers offer multiple compiler, sample code, and debugging tool options. It is very important to make sure you have all the necessary tools for the part. Without the right tools, the development process can become tedious and costly.

Step 10: Start experimenting

Even once you’ve chosen a microcontroller, it’s not set in stone. Often, it takes a long time after you get the development kit before you get your first prototype hardware. At this point you can build test circuits and interface with the microcontroller. Select high-risk parts and get them working on the development kit.

You may find that a part you thought was a good fit has some unforeseen problems, and be forced to choose a different microcontroller. In any case, early experimentation will ensure that you make the right choice, and that if changes are necessary, the impact will be minimal!

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