Sommerville-Software-Engineering-10ed

21.3 ■ Timing analysis 629
Figure 21.16
Timing requirements
for the burglar
alarm system
Stimulus/Response
Audible alarm
Communications
Door alarm
Lights switch
Movement detector
Power failure
Voice synthesizer
Window alarm
Timing requirements
The audible alarm should be switched on within half a
second of an alarm being raised by a sensor.
The call to the police should be started within
2 seconds of an alarm being raised by a sensor.
Each door alarm should be polled twice per second.
The lights should be switched on within half a second
of an alarm being raised by a sensor.
Each movement detector should be polled twice per
second.
The switch to backup power must be completed within
a deadline of 50 ms.
A synthesized message should be available within
2 seconds of an alarm being raised by a sensor.
Each window alarm should be polled twice per second.
The starting point for timing analysis in a real-time system is the timing requirements,
which should set out the deadlines for each required response in the system.
Figure 21.16 shows possible timing requirements for the office building burglar
alarm system discussed in Section 21.2.1. To simplify this example, let us ignore
stimuli generated by system testing procedures and external signals to reset the system
in the event of a false alarm. This means there are only two types of stimulus
processed by the system:
1. Power failure is detected by observing a voltage drop of more than 20%. The
required response is to switch the circuit to backup power by signaling an electronic
power-switching device that switches the mains power to battery backup.
2. Intruder alarm is a stimulus generated by one of the system sensors. The
response to this stimulus is to compute the room number of the active sensor, set
up a call to the police, initiate the voice synthesizer to manage the call, and
switch on the audible intruder alarm and building lights in the area.
As shown in Figure 21.16, you should list the timing constraints for each class of
sensor separately, even when (as in this case) they are the same. By considering
them separately, you leave scope for future change and make it easier to compute the
number of times the controlling process has to be executed each second.
Allocating the system functions to concurrent processes is the next design stage.
Four types of sensors must be polled periodically, each with an associated process:
the voltage sensor, door sensors, window sensors, and movement detectors.
Normally, the processes associated with the sensor will execute very quickly as all