Sommerville-Software-Engineering-10ed

628 Chapter 21 ■ Real-time software engineering
Voltage
Normal voltage
level
R1 R2 R3 R4
Critical voltage
level
Power switcher
Battery startup
4ms 8ms 12ms 16ms 20ms 24ms 28ms 32ms 36ms 40ms
Time
Figure 21.15
Power failure timing
analysis
power to a battery backup. Figure 21.15 presents a timeline showing the events in
the system:
1. Assume that, after a mains power failure event, it takes 50 milliseconds (ms)
for the supplied voltage to drop to a level where the equipment may be damaged.
The battery backup must therefore be activated and in operation within
50 ms. Usually, you allow for a margin of error, so you should set a shorter
deadline of 40 ms because of physical variations in the equipment. This
means that all equipment must be running on the battery backup power supply
within 40 ms.
2. However, the battery backup system cannot be instantaneously activated. It
takes 16 ms from starting the backup power supply to the supply being fully
operational. This means that the time available to detect the power failure and
start the battery backup system is 24 ms.
3. There is a process that is scheduled to run 250 times per second, that is, every 4 ms.
This process assumes that there is a power supply problem if a significant drop
in voltage occurs between readings and is sustained for three readings. This time
is allowed so that temporary fluctuations do not cause a switch to the battery
backup system.
4. In the above timeline, the power fails immediately after a reading has been
taken. Therefore, reading R1 is the start reading for the power fail check. The
voltage continues to drop for readings R2–R4, so a power failure is assumed.
This is the worst possible case, where a power failure event occurs immediately
after a sensor check, so 16 ms have elapsed since that event.
5. At this stage, the process that switches to the battery backup is started. Because
the battery backup takes 16 ms to become operational, the worst-case execution
time for this process is 8 ms, so that the 40 ms deadline can be reached.