DANIEL HABTEGEBRIEL YOHANNES
- CS3
Introduction
Embedded systems are a ubiquitous component of our everyday lives. We
interact with hundreds of tiny computers every day that are embedded into our
houses, our cars, our toys, and our work. As our world has become more complex,
so have the capabilities of the microcontrollers embedded into our devices. The
ARM® Cortex™-M family represents a new class of microcontrollers much more
powerful than the devices available ten years ago. The purpose of this class is to
present the design methodology to train young engineers to understand the basic
building blocks that comprise devices like a cell phone, an MP3 player, a
pacemaker, antilock brakes, and an engine controller.
An embedded system is a system that performs a specific task and has a
computer embedded inside. A system is comprised of components and interfaces
connected together for a common purpose. This class is an introduction to
embedded systems. Specific topics include microcontrollers, fixed-point numbers,
the design of software in C, elementary data structures, programming
input/output including interrupts, analog to digital conversion, digital to analog
conversion.
In general, the area of embedded systems is an important and growing discipline
within electrical and computer engineering. In the past, the educational market of
embedded systems has been dominated by simple microcontrollers like the PIC,
the 9S12, and the 8051. This is because of their market share, low cost, and
historical dominance. However, as problems become more complex, so must the
systems that solve them. A number of embedded system paradigms must shift in
order to accommodate this growth in complexity. First, the number of
calculations per second will increase from millions/sec to billions/sec. Similarly,
the number of lines of software code will also increase from thousands to
millions. Thirdly, systems will involve multiple microcontrollers supporting many
simultaneous operations. Lastly, the need for system verification will continue to
grow as these systems are deployed into safety critical applications. These
changes are more than a simple growth in size and bandwidth. These systems
must employ parallel programming, high-speed synchronization, real-time
operating systems, fault tolerant design, priority interrupt handling, and
networking. Consequently, it will be important to provide our students with these
types of design experiences. The ARM platform is both low cost and provides the
high-performance features required in future embedded systems. In addition, the
ARM market share is large and will continue to grow. As of July 2013, ARM reports
that over 35 billion ARM processors have been shipped from over 950 companies.
Furthermore, students trained on the ARM will be equipped to design systems
across the complete spectrum from simple to complex. The purpose of this course
is to bring engineering education into the 21st century.
2.1. Binary number systems
To solve problems using a computer we need to understand numbers and what
they mean. Each digit in a decimal number has a place and a value. The place is a
power of 10 and the value is selected from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}. A
decimal number is simply a combination of its digits multiplied by powers of 10.
For example
1984 = 1•103 + 9•102 + 8•101 + 4•100
Fractional values can be represented by using the negative powers of 10. For
example,
273.15 = 2•102 + 7•101 + 3•100 + 1•10-1 + 5•10-2
In a similar manner, each digit in a binary number has a place and a value. In
binary numbers, the place is a power of 2, and the value is selected from the set
{0, 1}. A binary number is simply a combination of its digits multiplied by powers
of 2. To eliminate confusion between decimal numbers and binary numbers, we
will put a subscript 2 after the number to mean binary. Because of the way the
microcontroller operates, most of the binary numbers in this class will have 8, 16,
or 32 bits. An 8-bit number is called a byte, and a 16-bit number is called a
halfword. For example, the 8-bit binary number for 106 is
011010102 = 0•27 + 1•26 + 1•25 + 0•24 + 1•23 + 0•22 + 1•21 + 0•20 =
64+32+8+2 = 106
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Technically you can not convert a binary represented number into a decimal,
because computers do not have any storage facility to represent decimal
numbers.
Practically this might sound absurd, since we are always dealing with numbers in
decimal representation. But these decimal representations are never actually
stored in decimal. Only thing a computer does is converting a number into
decimal representation when displaying it. And this conversion is related to
program construction and library design.
I'll give a small example on C language. In C you have signed and unsigned integer
variables. When you are writing a program these variables are used to store
numbers in memory. Who knows about their signs? The compiler. Assembly
languages have signed and unsigned operations. Compiler keeps track of the sign
of all variables and generate appropriate code for signed and unsigned case. So
your program works with signed or unsigned integers perfectly when it is
compiled.
Assume you used a printf sentence to print an integer variable and you used %d
format converter to print the value in decimal representation. This conversion will
be handled by printf function defined in standart input output library of C. The
function reads the variable from memory, converts the binary representation to
decimal representation by using a simple base conversion algorithm. But the
target of the algorithm is a char sequence, not an integer. So this algorithm does
two things, it both converts binary to decimal representation; and it converts bits
to char values (or ASCII codes to be more precise). printf should know the sign of
the number to carry on the conversion successfully and this information is again
supplied by the compiler constructs placed at compile time. By using these
constructs printf could check whether the integer is signed or unsigned and use
the appropriate conversion method.
Other programming languages follow similar paths. In essence numbers are
always stored in binary. The signed or unsigned representation is known by
compiler/interpreter and thus is a common knowledge. The decimal conversion is
only carried on for cosmetic reasons and the target of conversion is a char
sequence or a string.