Selecting the right microcontroller for a product can
be a daunting task. Not only are there a number of technical features to
consider, there are also business case issues such as cost and lead-times that
can cripple a project. At the start of a project there is a great temptation to
jump in and start selecting a microcontroller before the details of the system
has been hashed out. This is of course a bad idea. Before any thought is given
to the microcontroller, the hardware and software engineers should work out the
high levels of the system, block diagram and flowchart them and only then is
there enough information to start making a rational decision on microcontroller
selection. When that point is reached, there are 10 easy steps that can be
followed to ensure that the right choice is made.
Step 1: Make a list of required hardware
interfaces
Using the general hardware block diagram, make a
list of all the external interfaces that the microcontroller will need to
support. There are two general types of interfaces that need to be listed. The
first are communication interfaces. These are peripherals such as USB, I2C,
SPI, UART, and so on. Make a special note if the application requires USB or
some form of Ethernet. These interfaces greatly affect how much program space
the microcontroller will need to support. The second type of interface is
digital inputs and outputs, analog to digital inputs, PWM’s, etc. These two
interface types will dictate the number of pins that will be required by the
microcontroller. Figure 1 shows a generic example of a block diagram with the
i/o requirements listed.
Step 2: Examine the software architecture
The software architecture and requirements can
greatly affect the selection of a microcontroller. How heavy or how light the
processing requirements will determine whether you go with an 80 MHz DSP or an
8 MHz 8051. Just like with the hardware, make notes of any requirements that
will be important. For example, do any of the algorithms require floating point
mathematics? Are there any high frequency control loops or sensors? Estimate
how long and how often each task will need to run. Get an order of magnitude
feel for how much processing power will be needed. The amount of computing
power required will be one of the biggest requirements for the architecture and
frequency of the microcontroller.
Step 3: Select the architecture
Using the information from steps 1 and 2 an
engineer should be able to start getting an idea of the architecture that will
be needed. Can the application get by with eight bit architectures? How about
16 bits? Does it require a 32 bit ARM core? Between the application and the
required software algorithms these questions will start to converge on a
solution. Don’t forget to keep in mind possible future requirements and feature
creep. Just because you could currently get by with an 8 bit microcontroller
doesn’t mean you shouldn’t consider a 16 bit microcontroller for future
features or even for ease of use. Don’t forget that microcontroller selection
can be an iterative process. You may select a 16-bit part in this step but then
in a later step find that a 32 bit ARM part works better. This step is simply
to start getting an engineer to look in the right direction.
Step 4: Identify Memory Needs
Flash and RAM are two very critical components
of any microcontrollers. Making sure that you don’t run out of program space or
variable space is undoubtedly of highest priority. It is far easier to select a
part with too much of these features than not enough. Getting to the end of a
design and discovering that you need 110% or that features need to be cut just
isn’t going to fly. After all, you can always start with more and then later
move to a more constrained part within the same chip family. Using the software
architecture and the communication peripherals included in the application, an
engineer can estimate how much flash and RAM will be required for the
application. Don’t forget to leave room for feature creep and the next
versions! It will save many headaches in the future.
Step 5: Start
searching for microcontrollers
Now that there is a better idea of what the
required features of the microcontroller will be the search can begin! One
place that can be a good place to start is with a microcontroller supplier such
as Arrow, Avnet, Future Electronics or similar. Talk with an FAE about your
application and requirements and often times they can direct you to a new part
that is cutting edge and meets the requirements. Just keep in mind that they
might have pressure on them at that time to push a certain family of
microcontrollers!
The next best place to start is with a silicon
provider that you are already familiar with. For example, if you have used
Microchip parts in the past and had a good experience with them, then start at
their website. Most silicon providers have a search engine that allows you to
enter your peripheral sets, I/O and power requirements and it will narrow down
the list of parts that match the criteria. From that list the engineer can then
move forward towards selecting a microcontroller.
Step 6: Examine Costs
and Power Constraints
At this point the selection process has revealed
a number of potential candidates. This is a great time to examine the power
requirements and cost of the part. If the device will be powered from a battery
and mobile, then making sure the parts are low-power is absolutely precarious.
If it doesn’t meet power requirements then keep weeding the list down until you
have a select few. Don’t forget to examine the piece price of the processor
either. While prices have steadily been approaching $1 in volume for many
parts, if it is highly specialized or a high-end processing machine then price
might be critical. Don’t forget about this key element.
Step 7: Check part availability
With the list of potential parts in hand, now is
a good time to start checking on how available the part is. Some of the things
to keep in mind are what the lead times for the part? Are they kept in stock at
multiple distributors or is there 6 – 12 week lead time? What are your
requirements for availability? You don’t want to get stuck with a large order
and have to wait three months to be able to fill it. Then there is a question
of how new the part is and whether it will be around for the duration of your
product life cycle. If your product will be around for 10 years then you need
to find a part that the manufacturer guarantees will still be built in 10
years.
Step 8: Select a
development kit
One of the best parts of selecting a new
microcontroller is finding a development kit to play with and learn the inner
working of the controller. Once an engineer has settled their heart on the part
they want to use they should research what development kits are available. If a
development kit isn’t available then the selected part is most likely not a
good choice and they should go back a few steps and find a better part. Most
development kits today cost under $100. Paying any more than that (unless it is
designed to work with multiple processor modules) is just too much. Another
part may be a better choice.
Step 9: Investigate
compilers and tools
The selection of the development kit nearly solidifies
the choice of microcontroller. The last consideration is to examine the
compiler and tools that are available. Most microcontrollers have a number of
choices for compilers, example code and debugging tools. It is important to
make sure that all the necessary tools are available for the part. Without the
right tools the development process could become tedious and expensive.
Step 10: Start
Experimenting
Even with the selection a microcontroller
nothing is set in stone. Usually the development kit arrives long before the
first prototyped hardware. Take advantage by building up test circuits and
interfacing them to the microcontroller. Choose high risk parts and get them
working on the development kit. It may be that you discover the part you
thought would work great has some unforeseen issue that would force a different
microcontroller to be selected. In any event, early experimentation will ensure
that you made the right choice and that if a change is necessary, the impact
will be minimal!
7 Steps to select microcontroller:
1. 8-bit vs
32-bit core. 8-bit cores are currently the most widely used MCU cores. For almost all
tasks, an 8-bit core like 8051, PIC and AVR will suffice. However, as the
project complexity increases, some may want to utilise the 32-bit cores like
ARM, which provide you with more features and faster clock speeds but tend to
have a higher learning curve than 8-bit cores. Some 32-bit cores also have
floating-point units, which enable you to process math/DSP related functions
faster and much more efficiently. However, you should get familiar with 8-bit
cores before moving to 32-bit cores.
2. Programming
language. This is indirectly related to the MCU but directly to the tool chain used.
The most popular programming languages for MCUs are Assembly (ASM) and Embedded
C. Working with ASM is much more time-consuming but gives you great insight
into the architecture and actual working of the MCU. However, when the code
size efficiency does
not matter as much
as the time to develop, Embedded C is the way to go. Again, you can write
efficient code in ‘C’ and some projects require you to use ‘C’ to keep things
simple. For most of the 8051s available in the Indian market, Keil supports the
code in ASM and Embedded C. For the PIC series, MPLAB has a CCS plugin (I use
this as opposed to Hi-Tech Compiler), which enables you to code a PIC in ‘C’.
So before you select the chip, make sure a tool chain with the right options is
available for development. Do consider the cost of the same as many of them do
not have free versions.
3. GPIOs and
features. Manufacturers release many MCUs of similar specifications but different
features and resolutions. Select a chip that has all the features with the
resolution as per your needs. Try to keep external chip interfacing to a
minimum. This will help you in your design. Also make sure your MCU has
sufficient input/output (I/O) pins for your needs.
Try using alternate
modes for peripherals—like 4-bit LCD mode—to save some I/O pins.
4. Cost. If it is your first
time working with an MCU, there is no need to purchase an expensive MCU. We all
have had our share of
burnt MCUs, and likely you will too. So keep the cost low initially. Weigh all
the points mentioned here (and elsewhere
too) and select an appropriately priced MCU that suits your budget. For
absolute beginners, I would recommend
AT89S series and P89V51RD2 (expensive but has PWMs) from 8051 family, and
PIC16F877A (has ADCs too) from PIC family. In general, 8051s tend to be cheaper
than PICs, which are cheaper than Atmega’s, but PICs have much more
functionality.
5.
Availability. Select an MCU that will be available for a long time to come. There is no
general rule of thumb to know this, but selecting a popular one is your best
bet. (The ones mentioned above are most widely used in many universities.) If
for some reason you select an MCU which is not available in India
6. Online
support. This is crucial when you’re just venturing out and when you move on to
32-bit cores. Selecting an MCU which has good online support will help you with
your ideas and solve most of your problems as the experience of other users is
available for your reference. Getting to know existing bugs with tool chains or
MCU documentation will help you avert the problem in advance and speed up
development.
7. Packaging. For most DIY
projects, a DIP package is suitable and easy to work with. SMD packages are a
bit difficult to work with without proper tools and therefore not recommended
for first timers. Once the design is ready, you can always use SMD equivalents
to make it more compact.
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