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5.4 Behavioral models 137

State Description

Waiting The oven is waiting for input. The display shows the current time.

Half power The oven power is set to 300 watts. The display shows ‘Half power’.
Full power The oven power is set to 600 watts. The display shows ‘Full power’.

Set time The cooking time is set to the user’s input value. The display shows
the cooking time selected and is updated as the time is set.
Disabled Oven operation is disabled for safety. Interior oven light is on.
Display shows ‘Not ready’.
Enabled Oven operation is enabled. Interior oven light is off. Display shows
‘Ready to cook’.
Operation Oven in operation. Interior oven light is on. Display shows the timer
countdown. On completion of cooking, the buzzer is sounded for
five seconds. Oven light is on. Display shows ‘Cooking complete’
while buzzer is sounding.
Stimulus Description

Half power The user has pressed the half-power button.
Full power The user has pressed the full-power button.

Timer The user has pressed one of the timer buttons.
Number The user has pressed a numeric key.

Door open The oven door switch is not closed.
Door closed The oven door switch is closed.

Start The user has pressed the Start button.
Figure 5.17 States
and stimuli for the Cancel The user has pressed the Cancel button.
microwave oven

their mind after selecting one of these and press the other button. The time is set and,
if the door is closed, the Start button is enabled. Pushing this button starts the oven
operation and cooking takes place for the specified time. This is the end of the cook-
ing cycle and the system returns to the waiting state.
The UML notation lets you indicate the activity that takes place in a state. In a
detailed system specification you have to provide more detail about both the stimuli
and the system states. I illustrate this in Figure 5.17, which shows a tabular descrip-
tion of each state and how the stimuli that force state transitions are generated.
The problem with state-based modeling is that the number of possible states
increases rapidly. For large system models, therefore, you need to hide detail in the