Embedded Systems 423
What would happen if we try to drive the LED through logic high (by sourcing current), as shown
in Figure 14.9(b)? In general, the LED would remain apparently off as the sourcing current is too low
to make it glow.
In ATmega8, this problem is eliminated as all port pins are capable of sourcing and sinking same
amount of current and its maximum limit is 20 mA when Vcc is 5V. For Vcc of 3V, this maximum sourc-
ing or sinking limit of port pins is reduced to 10 mA.
14.5.2 Power Consumption and Saving
The reader may note here that ATmega8 is capable of operating at a wider voltage input range. Its ATmega8L
version offers an input voltage range between 2.7V and 5.5V. This is especially suitable for battery pow-
ered systems and we know that most of the embedded systems are battery-operated. We have already
discussed about the power management features of ATmega8. As a matter of fact, the average current con-
sumption of 3.6 mA during its active mode is reduced to 1.0 mA during its idle mode and further reduced
to 0.5 μA in its power-down mode. Therefore, from the organizational point of view, ATmega8 is capable
of saving a substantial amount of power utilizing its power source (generally a battery) in an optimum way.
14.6 DESIGN ISSUES
As far as embedded system design is concerned, the design issues arising out of it may be enormous.
In general, every embedded system design places some unique issue to the system designer. However,
some of the common and major issues may be related to
R Device selection
R Communication
R Power optimizing
R Compactness
R Cost-effectiveness
R Robustness and
R Compatibility.
We shall now individually discuss these issues.
14.6.1 Device Selection
This issue of embedded system design is becoming more and more serious every day due to the intro-
duction of more advanced microcontrollers in the market. It is needless to mention that selection of a
proper microcontroller plays the central role in the embedded system design as most of the other issues
ATmega8 is a very powerful and versatile microcontroller with many unique architectural
features and well-suited for embedded system designers. Interested readers should consult
the manufacturer’s data sheets for more details regarding this.
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424 Computer Architecture and Organization
would depend upon this selection. In this case, the designer must be well versed not only with the well
established and popular microcontrollers but also with the latest arrivals in the microcontroller market
and their architectural features. Just for the sake of example, it may be indicated that as on today, we nd
not only the microcontrollers like Intel 8051, PIC (various versions), AVR (many models), ARM series
and MCS-96, but also a whole range of Renesas microcontrollers.
Figure 14.10 Range of renesas microcontrollers
Founded in 2003, Renesas Technology Corporation of Japan, a joint venture of Hitachi Ltd. and Mit-
subishi Electric, offers a variety of powerful microcontrollers of different architectures and computing
powers to t-in practically all types of embedded systems. Covering a range from 8-bit to 32-bit, these
microcontrollers provide on-chip ash and SRAM of variable sizes, together with I/O ports, Timers,
WDT, UART and several other features. There are more than 500 models of Renesas microcontroller
to select an ideal device for a particular application. This may be readily understood from Figure 14.10,
which presents the whole range of microcontrollers offered by Renesas. The operating frequency range
is more than 300 MHz in some selected cases of high-speed high-performance Super-H family proces-
sors, while it is less than 5 MHz in some other cases like low-frequency low-power microcontrollers.
As the reader may observe from Figure 14.10, the range of application of Renesas microcontroller is
DVD/HDD recorder, TV, USB related devices, SH-Mobile and PLC, to mention a few. All these would
project a rough idea to the reader about the complexity, about the selection or identi cation of a proper
microcontroller for a speci c embedded system design project.
14.6.2 Communication
Communication is another challenging issue for embedded systems designers. In many embedded sys-
tems, some type or other sensors are used, which are generally located at a remote corner. Communicat-
ing the signals from these sensors to the embedded controller needs a lot of planning and veri cation.
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Embedded Systems 425
In general, noise pick-up, thermal oscillations, input impedance and similar issues have to be solved by
the embedded system designer to ensure proper functioning of the designed system.
14.6.3 Power Optimizing
Optimization of power is one of the critical issues in embedded system design as most of the embedded
systems are battery powered. It is already indicated that various power saving modes are available in
most of the microcontrollers so that the power may be properly consumed and optimized to prolong the
battery-life of the designed systems.
14.6.4 Compactness
Many embedded systems, especially those that are related to instrumentation as well as consumer elec-
tronics, demand to be compact as far as its outer packaging is concerned. Their keys and displays need
to be easily accessible or readable for the user. This imposes some extra constraint in system design and
also in its layout. Sometimes a special attention is demanded from the system designer related to these
issues at the very early stage of system design.
14.6.5 Cost-effectiveness
Cost-effectiveness is a very important feature, valid for all types of system design and embedded system
design is no exception in this case. Various alternatives for an economic design might have to be consid-
ered along with availability of components in the market, paying adequate attention to their future avail-
ability for replacement related to servicing, if necessary. It may be interesting to note that some manufac-
turers design and fabricate multiple systems using two or three different microcontrollers and distribute in
the market that one whose microcontroller is available at the lowest price. As the price of these microcon-
trollers keep uctuating, the microcontroller, used in the system being marketed, also changes accordingly.
14.6.6 Robustness
This feature is not applicable to all systems, but to some embedded systems. It is specially expected in
instrumentation and medical equipments and also in some consumer items. By the term robustness, we
mean that the system must be properly functional even in adverse conditions. These adverse conditions
may be power supply uctuations, mechanical vibrations and dynamic impacts, higher humidity or
similar cases. A robust unit would always perform and even if it is unable to function properly, it would
place some feedback to the user about the reason behind its failure.
14.6.7 Compatibility
This is applicable especially for accessories and add-on units, which are available to the users for a
longer time. Any change in the interfacing bus-type and similar modi cations might result in non-
compatibility with already existing devices and may not be a good design strategy.
The reader may note that in real-life situations, there arise many more design issues other than those
that are presented above and several of which might have to be solved quickly, while in some other
cases the related decisions of system designer might have a long-term effect on the performance of the
embedded system.
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