Monitoring Backpropagation in Sample Times
When you update or simulate a
model that specifies the sample time of a source block as inherited
(–1), the sample time of the source block may be backpropagated; Simulink may
set the sample time of the source block to be identical to the sample
time specified by or inherited by the block connected to the source
block. For example, in the model below, the Simulink software
recognizes that the Sine Wave block
is driving a Discrete-Time Integrator block
whose sample time is 1; so it assigns the Sine
Wave block a sample time of 1.
You can verify this sample time setting by selecting Sample Time Display >
Colors from the Simulink Format menu
and noting that both blocks are red. Because the Discrete-Time
Integrator block looks at its input only during its sample
hit times, this change does not affect the results of the simulation,
but does improve the simulation performance.
Now replacing the Discrete-Time
Integrator block with a continuous Integrator block,
as shown below, causes the Sine Wave and Gain blocks
to change to continuous blocks. You can test this change by selecting Update Diagram from the Edit menu
to update the colors; both blocks now appear black.
Note
Backpropagation makes the sample times of model sources dependent
on block connectivity. If you change the connectivity of a model whose
sources inherit sample times, you can inadvertently change the source
sample times. For this reason, when you update or simulate a model,
by default, Simulink displays warnings at the command line if
the model contains sources that inherit their sample times. See Source block specifies -1 sample time for more information. |
 | How Propagation Affects Inherited Sample
Times | | Creating a Model |  |
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