How to Select A Microcontroller

Embedded
How to Select A Microcontroller
Choosing an MCU for your project is not easy. There are a large number of suppliers,
offering varying architectures, peripherals, price points, power consumption, packages
and what not. This article helps you get a better idea of the current situation and select the
right MCU for your application
Dilin Anand
A
search in an online catalogue for microcontroller units
(MCUs) would lead you to a
massive collection of thousands upon
thousands of various MCUs, leaving
you spoilt for choice. There would be
so many good choices that it would be
difficult to make a decision. The following is what the experienced designers in the industry advise.
Architecture
For most embedded systems, you have
the choice between 8-bit, 16-bit and
32-bit cores. When it comes to applications involving computations, 32-bit
cores have a clear edge over 8-bit and
16-bit, in addition to making the most
of the compiler features.
Basavaraj Garadi, chief expert, Robert Bosch Engineering and Business Solutions, shares, “Personally, I am not a
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December 2012 | Electronics For You
great fan of 16-bit controllers. They are
passé. Of late, 8-bit controllers don’t
have significant cost advantage either.
Moreover, when it comes to the available choices in operating systems, the
32-bit core is a clear winner. The RISC
and CISC architectures have become a
little less meaningful with continued
evolution of both the CISC and RISC
designs and implementations.”
“The architecture of a microcontroller refers to the philosophy of the
internal implementation,” explains Jigar Patel, CTO, Mansi Research. “It includes details like how many registers
are used, whether the code can execute
out of the data memory, whether the
peripherals are treated like memory,
registers or something else, whether
there is a stack and how it works, and
so on. A RISC processor can handle
simple math functions much faster, but
a CISC processor will handle complicated functions faster because it can do
them all at once instead of the several
processing commands that a RISC processor would have to take to complete
the same function.”
MCU process technology, architecture and memory technology also need
to be considered as these affect the real
calculation speed rather than MHz and
memory size requirements. “Since RISC core requires a load or
store instruction to access any data item
in the memory, a memory-to-memory
move requires a load operation to first
read the data from the source and temporarily store it into a general register,
then a store operation to move the data
from the register out to the memory.
This requires four CPU clock cycles
to execute and 4 bytes of code storage
space. An RX MCU, on the other hand,
can move the data item directly from
the memory source to the memory with
no need for load/store operation. This
takes only three CPU clocks and 2-byte
code size, thus giving 165DMIPS calculation speed with only 100MHz core,”
explains Wang Kaifei, assistant principal
engineer, Renesas Electronics Singapore.
Personally, I am not a great fan of 16-bit controllers. They are
passé. Of late, 8-bit controllers don’t have a significant cost
advantage either — Basavaraj Garadi, Chief Expert, Robert Bosch Engineering and
Business Solutions
www.efymag.com
Embedded
When it comes to the architecture,
another issue that you may come
across is DSP versus general-purpose
microcontroller.
“Honestly, the line between the two
is narrowing by the day. Microcontrollers
are designed for simple math and control
(of other devices) functions. Some may
also come with hardware multiplication
units. DSPs are processors optimised for
processing the streaming signal. These
have special instructions that speed-up
common tasks such as multiply-accumulate in a single instruction, as well as
other vector or SIMD (single-instruction
multiple-data) instructions. They are
usually designed to operate in one big
loop, processing a data stream. DSPs can
be designed as integer, fixed-point or
floating-point processors,” adds Garadi.
Development tools
As microcontrollers have been available for a very long time, the selection
decision in the industry is now governed by the support from chip vendor
and development tools, not the price
and features.
Siddharth Patel, lead engineerembedded software, EATON India Engineering Centre, explains: “Now we have
multiple vendors offering chips with the
same CPU core and similar features at
almost similar prices across various segments. So now chip vendor support and
development tools play a very important
role among selection verticals.”
Sai Ram Mannar, founder and director, GreenOcean Research Labs, adds,
“The availability and richness of the
development tools is a key aspect. As
these tools are expensive and mostly
tied to the microcontroller, they need
to be evaluated clearly, ensuring flexibility of the tool in supporting multiple
families with ease. The debugging capabilities and an easy graphic user interface (GUI) for configuring the various
peripherals of the MCU also help a lot.”
How much should you
focus on application
requirements?
The application requirements of an
embedded system under development
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December 2012 | Electronics For You
Chip vendor support and development tools play a very
important role among selection verticals — Siddharth Patel, lead
engineer-embedded software, EATON India Engineering Centre
TOP 7 Factors to be Considered
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, P89V51RD2 microcontroller used
I would recommend AT89S in a system
series and P89V51RD2
PIC16F877A microcontroller
(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, you may have to pay additional shipping charges
to get it, which will be costly as well as frustrating.
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.
—Compiled by Frenoy Osburn at EFY’s Community of Electronics Pros
(www.forum.electronicsforu.com)
www.efymag.com
Embedded
The key parameters to be considered while selecting an MCU
should also include the possibility of reduction in the number
of electronic components. This will allow integration of more
functionality — Prashant Mahendrakar, Technical Manager at Sankalp Semiconductor
The 8051 microcontroller architecture
have a big say on the system processing
needs. Based on these requirements, you
select the MCU. The question is: “Should
I choose strictly based on the requirements from a commercial perspective or
consider the greater good of all?
“This part is usually taken very
seriously during MCU selection in
our firm. We usually do not prefer to
use too many different types of MCUs
in our products only to tightly match
the application requirements. This
would leave us with too many tools of
development and maintenance, which
unnecessarily blocks lots of money.
Instead, we have carefully chosen a
few powerful devices and also a few
low-profile devices that we choose
from,” explains T. Anand, managing
director, Knewron.
Plato Pathrose, senior engineer,
design & development, Tata Elxsi,
explains how things work in the automotive industry: “Automotive compliance of the MCU should be considered at first; without this, nothing
could be taken inside the automotive
industry. Next, we look for product
familiarity in development, which
could reduce the development costs
and increase the flexibility in usage.
Following this, we look into the availability of the tools and samples and
also the vendor’s support during the
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December 2012 | Electronics For You
course of development.”
“The key parameters to be considered
while selecting an MCU
should also include the
possibility of reduction in the number of
electronic components
with the MCU. This
will allow integration
of more functionality.
For instance, you can
integrate more memory
(Flash/RAM) and a multiprocessor,”
adds Prashant Mahendrakar, technical
manager at Sankalp Semiconductor.
Don’t forget about memory
The cutthroat competition in the market has pushed software development
so much that we now live in a world
that seems to be obsessed with the
‘Beta Culture.’ Almost every new service or product is run on Beta software.
The designer should keep this in mind
while choosing the memory size for his
product. The software that he has been
given now may soon bloat up as bugs
get squashed and new features added.
So there better be memory available for
the update.
Jigar Patel says, “Some algorithms require substantial RAM to
be implemented in a straightforward
manner. It may be worthwhile looking for a micro with a lot of RAM. On
the other hand, the size of the Flash
memory depends on the size of the
object file generated by the assembler/compiler. The size of the Flash
program memory is an important
factor when the user has decided to
use the internal program memory.
The user must keep in mind to keep
a few percentage (depending on the
nature of the application) of the free
space in the program memory for
future requirements.”
Anand adds, “When choosing
memory sizes (of all kinds), we
give a thought to scalability (if it is
planned) such that tomorrow we
may update the program; this would
need additional memory, so the installed MCU should have the margin
already built-in. This margin has to
be around 15-20 per cent. MCU sizes
usually come in different slots such
as 4kB, 8kB, 16kB and so on. Therefore we go with the closest possible
option. When in doubt, we go with
one slot higher option.”
Scalability is very important for a
commercial embedded system to stay
viable in the market over the long run.
Shrenik Shikhare, embedded software engineer, Maven Systems, says,
“Embedded designers must have
their eye beyond their designs and
think strategically when selecting
platforms with appropriate migration paths for long duration, feature
set and performance. As scalability
is the ability to handle the growing workload that may come in the
future, one should think in all directions—whether the system will perform well to handle the extra work
after adding some hardware into
the existing system, or the existing
When choosing memory sizes (of all kinds), we give a thought
to scalability (if it is planned) such that tomorrow we may
update the program. This would need additional memory, so
the installed MCU should have the margin already built-in. We
go with the closest possible option. When in doubt, we go with
one slot higher option — T. Anand, Managing Director at Knewron
www.efymag.com
Embedded
Availability is an important consideration for commercial
designs. You would have different requirements for different
stages of development—for instance, smaller quantities
for engineering samples, and larger ones once you enter
production. Moreover, make sure that the MCU is available in
the future by ensuring that it’s not nearing obsolescence
— Vivek Tyagi, Country Sales Manager, Freescale Semiconductor India
system itself is capable of handling
the extra work in the future. In short,
your system must be designed for
upcoming workload while fulfilling
the current requirements too.”
“The memory technology used inside the MCU also affects its calculation speed. Some MCUs use MONOS
flash technology, which can run up to
100 MHz with zero wait states. This
differs from other MCUs that use
normal flash technology and need to
insert wait states, dropping CPU performance a lot,” adds Wang Kaifei.
Think about the future
If you want your product to be available in the market for at least a couple
of years, you have to consider the
number of years that the selected microcontroller has been in the market.
An almost obsolete microcontroller
which you have chosen just because
you are very familiar with it, is like a
ticking time bomb ready to blow back
up on you. If the MCU later becomes
obsolete, it will result in having to
rewrite a lot of code and change the
PCB—I am sure you would not want
to be responsible for that.
Vivek Tyagi, country sales manager, Freescale Semiconductor India,
explains, “Availability is an important consideration for commercial
designs. You would have different
requirements for different stages of
development—for instance, smaller
quantities for engineering samples,
and larger ones once you enter production. Moreover, you should make
sure that the MCU is available in the
future by ensuring that it is not nearing obsolescence.”
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December 2012 | Electronics For You
Anand adds, “Many a times,
MCU families come up with specific
programming methods, programmers, IDEs, tools, etc. This means
changing from one family to another
not only changes the MCU cost and
parameters but also the additional
maintenance and overhead cost—
in some cases, total ownership
costs too.”
There are a lot of other considerations to look at while choosing an
MCU family.
“The roadmap of an MCU family ensures that the time and investment made in choosing the family
are futureproofing the designs. It
simplifies migration and helps in
optimisation of the forthcoming
product releases. Pin and package
compatibility would ensure that the
design could scale up or down easily by just replacing the processor
with minimal or no board change.
Software compatibility ensures that
the software written for one device
would be reusable on another device,” says Sai Ram Mannar.
Siddharth adds, “With growing
demand to roll out products faster in
the market, full-fledged BSPs and software compatibility between various
MCU segments have become a major
selection criteria for new designs.
While selecting new MCUs, it is also
important to understand the product
life-cycle in which it is going to be
used. Selection criteria may be different if the MCU is required for enhancing the existing product or for a totally
new product.”
The author is a tech correspondent at EFY Bengaluru
www.efymag.com