How to express time constraints on ChangeEvent(s) in UML?

Question

I am currently working to create a UML model of an embedded system to aid the description of the related software requirements specification.

I have a physical pressure sensor which outputs a voltage with a determined relationship between the voltage value and the pressure measured. Since the model is used to elicit SwRS, I have decided model the pressure sensor through a class that abstracts the voltage-pressure relationship and exposes the currently measured pressure through its pressure property.

I used a property rather than an operation like getPressure():Real, because the pressure sensor measures that pressure continuously, not only when required.

The pressure sensor is considered broken if the pressure goes negative.

I would like to use UML to express a time constraint between this event and the time taken by the software to show an alarm message.

I believe that the event that I am after is the ChangeEvent, namely when(PressureSensor.pressure < 0).

I believe that the diagram type that can be used for this is the Sequence Diagram, but I am not able to express what I would like and complying with the UML 2.5 specs.

For example, if I use a sequence diagram I can express the concept in this way.

The problem is that pag. 572 of the UML 2.5 spec states that

The signature of a Message refers to either an Operation or a Signal.

This means that the signature cannot refer to a ChangeEvent, hence my diagram does not respect the UML 2.5 spec.

I also though of using an activity diagram, but from I could not figure out a proper way to use an activity diagram for my purpose.

Answer 1

UML allows to include timing details in a SD like this:

I don't know whether Papyrus allows to depict it this ways (I used EA).

Answer 2

You have a functional and a non-functional requirement:

1. "The UserInterface shall show a warning message if the pressure drops below 0."
2. "The pressure warning message shall be shown within 10ms after the pressure has dropped below 0."

Both can be expressed in a state diagram.

Here, PressureSensor.Pressure < 0 is a change event and UserInterface.showMessage("WARNING: Pressure below 0") is the effect. The timing requirement is modeled as a constraint.