User`s guide

Single- Versus Multitasking Environments

7-9

Simulating Models with Simulink

Before Simulink simulates a model, it orders all of the blocks based upon their

topological dependencies. This includes expanding subsystems into the

individual blocks they contain and flattening the entire model into a single list.

Once this step is complete, each block is executed in order.

The key to this process is the proper ordering of blocks. Any block whose output

is directly dependent on its input (i.e., any block with direct feedthrough)

cannot execute until the block driving its input has executed.

Some blocks set their outputs based on values acquired in a previous time step

or from initial conditions specified as a block parameter. The output of such a

blockisdeterminedbyavaluestoredinmemory,whichcanbeupdated

independently of its input. During simulation, all necessary computations are

performed prior to advancing the variable corresponding to time. In essence,

this results in all computations occurring instantaneously (i.e., no

computational delay).

Executing Models in Real-Time

A real-time program differs from a Simulink simulation in that the program

must execute the model code synchronously with real-time. Every calculation

results in some computational delay. This means the sample intervals cannot

be shortened or lengthened (as they can be in Simulink), which leads to less

efficient execution:

Figure 7-5: Unused Time in Sample Interval

Sample interval t1 cannot be compressed to increase execution speed because

sample times must stay in sync with real-time.

Real-Time Workshop application programs are designed to circumvent this

potential inefficiency by using a multitasking scheme. This technique defines

tasks with different priorities to execute parts of the model code that have

different sample rates.

Time