## How Propagation Affects Inherited Sample
Times

During
a model update, for example at the beginning of a simulation, Simulink^{®} uses
a process called sample time propagation to determine the sample times
of blocks that inherit their sample times. The figure below illustrates
a Discrete Filter block
with a sample time period `T`_{s}

driving
a Gain block.

Because the output of the Gain block is the input
multiplied by a constant, its output changes at the same rate as the
filter. In other words, the Gain block has an effective
sample rate equal to the sample rate of the filter. The establishment
of such effective rates is the fundamental mechanism behind sample
time propagation in Simulink.

### Process for Sample Time Propagation

Simulink uses the following basic process to assign sample
times to blocks that inherit their sample times:

Propagate known sample time information forward.

Propagate known sample time information backward.

Apply a set of heuristics to determine additional
sample times.

Repeat until all sample times are known.

### Simulink Rules for Assigning Sample Times

A block having a block-based sample time inherits a sample time
based on the sample times of the blocks connected to its inputs, and
in accordance with the following rules:

Rule | Action |

All of the inputs have the same sample time and the block can
accept that sample time | Simulink assigns the sample time to the block |

The inputs have different discrete sample times and all of
the input sample times are integer multiples of the fastest input
sample time | Simulink assigns the sample time of the fastest input
to the block . (This assignment assumes that the block can accept
the fastest sample time.) |

The inputs have different discrete sample times, some of the
input sample times are not integer multiples of the fastest sample
time, and the model uses a variable-step solver | Simulink assigns a fixed-in-minor-step sample time to
the block. |

The inputs have different discrete sample times, some of the
input sample times are not integer multiples of the fastest sample
time, the model uses a fixed-step solver, and Simulink can compute
the greatest common integer divisor (GCD) of the sample times coming
into the block | Simulink assigns the GCD sample time to the block. Otherwise, Simulink assigns
the fixed step size of the model to the block. |

The sample times of some of the inputs are unknown, or if the
block cannot accept the sample time | Simulink determines a sample time for the block based
on a set of heuristics. |

## See Also

Blocks for Which Sample Time Is Not Recommended

## More About