Methodology : The Rational Statemate Approach : The Top-Down System Specification : Functional Decomposition - System Level 2

Functional Decomposition - System Level 2

At the next level of functional decomposition, referred to as System Level 2, again some generic guidelines can be applied to the structures at this level.

For I/O functions no further generic approach is possible because of the system-dependent structure (see the Functional Decomposition of the Input Function figure).

For the bus interface it is possible to talk about a generic approach.

In a Functional Model, no further decomposition is required. At this stage the engineers are only interested that a certain message is sent / received via the bus interface with a specific timing/frequency. Although these messages are actually between the bus interface and the Process_Buffer, in the analysis the engineers can stimulate and sense them as if they went to the external world.

In the Implementation Model, the bus interface must be decomposed because now the engineers must consider the details of the message packing/unpacking and the timing of the messages. When developing the bus model, bear in mind that the ultimate goal in analysis is the HW-in-the-loop analysis. Structure the bus model so as to make it easy to switch from the Rational Statemate software model of the bus to using the actual system hardware (see the Generic Bus Interface figure).

A generic approach can also be used for the System Level 2 decomposition of the control algorithms. Whereas on System Level 1 the engineers show the main system functions, System Level 2 shows the main algorithm functions (see the Generic Control Algorithm Approach figure).

At this level, explicitly show each input and output instead of the single information flow used at the previous level. Then at this level you can once again see all the relevant inputs and outputs— inputs on the left and outputs on the right.

Structure this level in functional modules that later help in code generation. Also, arrange the activities to make the system readable from the structure. That is to say, all Activity-charts are read from top to bottom and left to right and the ordering of the activities should be consistent with the calling sequence in the control activity.

For readability reasons, as with System Level 1, no further hierarchical decomposition should be done on this chart.

 
Functional Decomposition of the Input Function

 

Generic Bus Interface
 
 
Generic Control Algorithm Approach