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Microcontrollers & embedded processors FAQ

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Microcontrollers & embedded processors FAQ

Post by Pruthvi Bharadwaj on Thu Jul 17, 2008 8:38 pm


1) What is a Microcontroller?

A controller is used to control (makes sense!) some process or aspect
of the environment. A typical microcontroller application is the
monitoring of my house. As the temperature rises, the controller
causes the windows to open. If the temperature goes above a certain
threshold, the air conditioner is activated. If the system detects
my mother-in-law approaching, the doors are locked and the windows
barred. In addition, upon detecting that my computer is turned on,
the stereo turns on at a deafening volume (for more on this, see the
section on development tools).

At one time, controllers were built exclusively from logic
components, and were usually large, heavy boxes (before this, they
were even bigger, more complex analog monstrosities). Later on,
microprocessors were used and the entire controller could fit on a
small circuit board. This is still common - you can find many [good]
controllers powered by one of the many common microprocessors
(including Zilog Z80, Intel 8088, Motorola 6809, and others).

As the process of miniaturization continued, all of the components
needed for a controller were built right onto one chip. A one chip
computer, or microcontroller was born. A microcontroller is a highly
integrated chip which includes, on one chip, all or most of the parts
needed for a controller. The microcontroller could be called a
"one-chip solution". It typically includes:
CPU (central processing unit)
RAM (Random Access Memory)
EPROM/PROM/ROM (Erasable Programmable Read Only Memory)
I/O (input/output) - serial and parallel
interrupt controller

By only including the features specific to the task (control), cost
is relatively low. A typical microcontroller has bit manipulation
instructions, easy and direct access to I/O (input/output), and quick
and efficient interrupt processing. Microcontrollers are a "one-chip
solution" which drastically reduces parts count and design costs.

2) What is an embedded controller?

Simply (and naively stated) an embedded controller is a controller
that is embedded in a greater system. A rigid definition is
difficult if not impossible to formulate, since the usual response is
"most embedded controllers are...". The problem here is "most". We
can't seem to shake that word from the definition. No matter how
clever you feel your definition is, some wiseguy will come along and
find an exception, or two, or 50.

You COULD say that an embedded controller is a controller (or
computer) that is embedded into some device for some purpose other
than to provide general purpose computing. Of course, someone will
eventually prove you wrong, but who cares?

A common example of a general purpose computer, would be a typical PC
clone. The x86 processor in this machine can't really be considered
an embedded controller, since the machine is typically used for
general purpose computing. However, what is general purpose
computing? Take this same PC clone, turn it into a multi-media
machine, and voila! You have an appliance - much on the order of a
microwave oven or television. Is the x86 processor now considered an
embedded controller Or, is the PC clone itself now considered an
embedded controller, controlling the multi-media peripherals? Hey -
I don't know about you, but I'm getting too old for this nonsense.

Is a microcontroller an embedded processor? Is an embedded processor
a microcontroller? What's the difference between an embedded
processor and a microcontroller? Well, today - not much. With the
continuing process of high scale integration continuing at a dizzying
pace, many standard architecture processors are turning up as
microcontrollers. A few such examples are the Motorola 68EC300,
Intel 386 EX, and the IBM PowerPC 403GB. These chips could be called

So, what's the difference between an embedded processor and a
microcontroller? I wouldn't touch that question with a ten foot
logic probe.

We might be safe by stating that an embedded processor controls
something (for example controlling a device such as a microwave oven,
car braking system, or a cruise missile). Is this always true?
Maybe. Maybe not. You know, it just doesn't end.

The main thing is not to get to hung up on precise definitions.
Black and white? Hell no, we've got grey scale, dithering,
diffusion, you name it! Same thing goes here with embedded
controllers, just go with the flow. It all depends on your point of

Alright, if you really must insist, we'll take a stab at defining
what an embedded controller is - realize however that there will be
many exceptions. Embedded controllers adhere to a philosophy similar
to that of microcontrollers, high integration. By including [many]
features necessary for the task at hand, an embedded controller
(processor) can be a powerful yet cost effective solution. However,
where a microcontroller [almost by definition] is a computer on a
chip, an embedded controller might need external components before it
is considered a "computer." This is especially true regarding RAM.
Since including large amounts of RAM (megabytes) on a processor is
not really practical (due to cost and available silicon real estate)
and because many embedded controllers are real powerhouses requiring
large amounts of RAM, the RAM is often external to the processor.

3) Applications

In addition to the above home monitoring system, embedded processors
and microcontrollers are frequently found in: appliances (microwave
oven, refrigerators, television and VCRs, stereos), computers and
computer equipment (laser printers, modems, disk drives), automobiles
(engine control, diagnostics, climate control), environmental control
(greenhouse, factory, home), instrumentation, aerospace, and
thousands of other uses. In many items, more than one processor can
be found.

Microcontrollers are typically used where processing power isn't so
important. Although some of you out there might find a microwave
oven controlled by a Unix system an attractive idea, controlling a
microwave oven is easily accomplished with the smallest of
microcontrollers. On the other hand, if you're putting together a
cruise missile to solve the problem of your neighbor's dog barking at
3 in the morning, you'll probably need to use processors with a bit
more computing power.

Embedded processors and microcontrollers are used extensively in
robotics. In this application, many specific tasks might be
distributed among a large number of controllers in one system.
Communications between each controller and a central, possibly more
powerful controller (or micro/mini/mainframe) would enable
information to be processed by the central computer, or to be passed
around to other controllers in the system.

A special application that microcontrollers are well suited for is
data logging. Stick one of these chips out in the middle of a corn
field or up in a ballon, and monitor and record environmental
parameters (temperature, humidity, rain, etc). Small size, low power
consumption, and flexibility make these devices ideal for unattended
data monitoring and recording.

4) Flavors

Embedded processors come in many flavors and varieties. Depending on
the power and features that are needed, you might choose a 4, 8, 16,
or 32 bit microcontroller. Standard microprocessors (such as the
Motorola 68000 or National 32032) are frequently used as powerful
embedded controllers. In addition, specialized processors are
available which include features specific for communications,
keyboard handling, signal processing, video processing, and other
Pruthvi Bharadwaj

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