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
so it assigns the Sine Wave block a sample time of
You can verify this sample time setting by selecting Information Overlays > Colors from the Debug tab of the Simulink toolstrip 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 in the model below, causes the Sine Wave and Gain blocks to change to continuous blocks. You can test this change by, on the Modeling tab, selecting Update Model to update the colors. Both blocks now appear black.
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.