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New Zealand Engineering 1998 March
Software
Development for Embedded Systems</h3>
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Software
Development for Embedded Systems
<HR><strong>Paul
Wilson </strong>formerly senior lecturer and
<br /><strong>John Collins</strong> principal
lecturer in software engineering, Electrotechnology Department, Auckland
Institute of Technology
</p>
<p><IMG SRC="../gifs/comp.GIF" NOSAVE BORDER=0 HEIGHT=166 WIDTH=208 ALIGN=RIGHT><em>There
may be some confusion among many people between the use of the terms "embedded
system" and "micro-controller". In general terms an embedded system has
a programmable processor used for real time control. The embedded controller
may be a general purpose processor or a specialised device designed for
a specific category of applications. An example of the specialised type
is the digital signal processor (DSP) commonly found in video and audio
applications.</em></p>
<p>In contrast, a micro-controller is a general purpose, commonly single
chip, microcomputer system that can be used for a variety of applications.
Some examples of these single chip micro-controllers are the Intel 8051
and Motorola 68HC11 families. Different micro-controllers provide varying
sub-systems, apart from the semi-standard RAM and ROM, which may include
parallel and serial I/O, programmable timers and analog to digital conversion.
These sub-systems obviously determine the suitability of the micro-controller
for particular applications. The micro-controller can be, and most often
is, used as an embedded system processor.</p>
<p>Another option for developers of embedded systems is the use of a "standard"
PC as the controller. This is perhaps best exemplified by the PC/104 system.
The PC/104 cards are much smaller (3.6x3.8 inches) than standard ISA bus
PC systems and stack together, thus eliminating the need for a motherboard
or back-plane.</p>
<p>The single common factor for all these various options for providing
a controller is that they all require programming. Unlike many applications
programs it is uncommon for an embedded system program to be altered once
it has been developed. An example of this is the embedded system in most
modern washing machines - how often is this program updated once it has
been installed? Obviously there are a few larger systems, such as those
using a PC/104 system perhaps for plant control, that will have periodic
upgrades or enhancements where the plant itself is altered.</p>
<p>When designing an embedded system controller program, especially for
single chip micro-controllers, there is a requirement to make the code
as efficient as possible. The requirement for efficient programs is two-fold.
Firstly, memory size of both ROM and RAM is limited in micro-controllers
and efficient programs require less memory for storage and execution. Secondly,
efficient programs run faster which can provide the real-time performance
required.</p>
<p>Programs can be written in two types of language _ assembly language
or a high level language such as the C programming language. Assembly language
has a close correspondence to the native machine language of the microprocessor.
High level languages are easier for programmers to use, but the price paid
for ease of use is programs which require more memory and which run more
slowly than assembly language programs. This is particularly true for small
programs where the overheads incurred by using a high level language are
often a large proportion of the size of the final program code.</p>
<p>Assembly costs</p>
<p>Using assembly language has its own costs however. Assembly language
programs are more difficult to write and test. If the program is going
to be installed in thousands or even millions of units then the additional
cost of developing the program in assembly language may be small compared
with the cost savings to be made from obtaining a smaller program. For
example, a small assembly language program may require 2000 bytes of program
memory whereas the same program written in a high level language may require
3000 bytes. The assembly language program can then be stored in a cheaper
microcontroller than the high level language program. In a large production
run, the cost savings of the cheaper hardware may be much larger than the
additional cost of developing in assembly language.</p>
<p>Another cost of developing in assembly language is the cost of maintenance.
Maintenance is the changing of the program to incorporate new requirements
in the system specification. It does not include fixing errors in the original
program or design. Most products evolve over time as users develop new
requirements, or as new features are added by the manufacturer. The cost
of maintaining an assembly language program is higher than for a high level
language program. This is particularly so if the original programmer is
not available to do the job, and is even more so if the original programmer
is not even available for consultation.</p>
<p>Maintenance is easier if the program is well documented and if programming
standards are used. Well structured high level languages, such as C, impose
their inherent structure on the programmer. Assembly language allows the
programmer much more freedom in the way code is written, so the programmer
must be more disciplined to produce a well structured program. The programmer
must design and write the program to avoid the "spaghetti code" which results
from the undisciplined use of jump and branch instructions. One way to
do this is to design the program using the program constructs available
in high level languages, and then code these constructs in assembly language.
This effectively limits the use of jump and branch instructions to well
structured selection statements (if statements) and loops (for and while
statements). Combined with well written program comments and accurate design
documentation, this approach results in an assembly language program which
is easier to test, debug and maintain.</p>
<p>Where program execution speed is an important issue, it is possible
to write programs as a combination of assembly language and a high level
language. Parts of the program which require high speed may be written
in assembly language and linked with the rest of the program written in
a high level language. This results in lower software development costs
than writing the whole program in assembly language, although the program
will still be larger than if it is written completely in assembly language.</p>
<p>Portability</p>
<p>Another issue that may need to be considered is code portability. This
is the ability of the program to be ported (transferred) to a different
microprocessor. Assembly languages are very specific to a microprocessor
family and assembly language programs need to be rewritten if the program
is to be ported to another microprocessor. High level languages for different
microprocessors are very similar so only a small number of program changes
are required when porting a program to another microprocessor. For both
languages, porting will also require program changes that reflect the architectural
differences between the original microprocessor and the new microprocessor.</p>
<p>High level languages for embedded systems now have direct access to
all hardware facilities of the microcontroller including interrupt handling
and bit manipulation when this is supported by the microcontroller. These
are no longer reasons for choosing to program in assembly language.</p>
<p>The choice of programming language for an embedded system now is to
use assembly language when program memory space is limited, to use assembly
language (possibly in conjunction with a high level language) when high
program execution speed is required, and to use a high level language in
other situations to obtain the advantages of lower development cost, easier
testing and debugging, and easier maintenance.
</p>
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