426 Computer Architecture and Organization
14.7 EXAMPLE OF EMBEDDED SYSTEM
To conclude our discussions on embedded systems, we discuss about an example case, which may be
designated as a programmable audio alarm for educational institutions. In all educational institutions
(irrespective of its level), some audio indications are periodically necessary to denote the beginning or
end of classes, recess time and nally the end of the day. There may be different timings for examina-
tion days with respect to normal instruction days. In this section, we discuss about an embedded system,
which is designed to take care of all these responsibilities. It may be mentioned here that the embedded
system is not a ctitious one and was really designed, fabricated and installed by one academician in an
educational institution of our country.
14.7.1 User Requirements
From the user’s point of view, the following requirements were visualized:
R The alarm must be audible from all corners of the multi-winged four-storied academic
building.
R During examination days, the timings would be different from regular working days. All
these timings would be prede ned and would be supplied to the designer before the system
design.
R For safety reasons, all A/C supply would be switched off everyday at the end of the working hours.
After initial survey, it was decided that six electric bells at six strategic places of the academic building
would be suf cient to make the alarm audible from all corners of the entire academic building. It was
agreed to provide a simple switch setting to distinguish between normal working days from examination
days. As the system demands a real-time clock, considering the third requirements of the user, it was
decided to provide a battery backup for the system.
14.7.2 System Specifications
Schematically, the overall system is illustrated through Figure 14.11 . Six heavy-duty electric bells were
placed in parallel connection, driven simultaneously by a heavy-duty solid-state relay with optocou-
pler. The heart of the system, one ATmega8 microcontroller, controlled the relay through a driver. A
six-digit hour-minute-second display was provided to display the clock-time in 24-hour format. For
pattern selection (normal/exam) one input key was provided. The system had a battery backup so that
even if the power is turned off, the microcontroller would be operating uninterruptedly and the internal
counting and time-keeping would not be disturbed. As it was observed that the rst alarm bell of the
day would never start before 9 AM, the normal/exam function select key must be operated before that
time. This input would change an internal ag of the system and would display ‘n’ of ‘E’ through the
left-most display digit for a second. This would be a toggle-type function, i.e., whenever it is pressed, it
would change from old function to new function.
The remaining part of the software would keep track of real time and at every second it would go
through an appropriate table of prede ned time-settings. There would be two such time settings, one for
regular operation and another for operation during examination days, selectable by the current state
of the internal ag. If the current time matches with the preset time then the relay would be operated
for 5 seconds and then it would be turned off.
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