User`s guide

7 Models with Multiple Sample Rates

7-12

In transitioning from slower to faster blocks, you must add Unit Delay blocks

between slow to fast transitions and set the sample rate of the Unit Delay to

that of the slower block:

Figure 7-7: Transitioning from Slower to Faster Blocks (T = Sample Period)

The next four sections describe the theory and reasons why Unit Delay and

Zero-Order Hold blocks are necessary for sample time transitions.

Faster to Slower Transitions in Simulink

In a model where a faster block drives a slower block having direct

feedthrough, the outputs of the faster block are always computed first. In

simulation intervals where the slower block does not execute, the simulation

progresses more rapidly because there are fewer blocks to execute. The

following diagram illustrates this situation:

becomes

FasterSlower

Block Block

T=1secT=2sec

Slower

Faster

Block

T=2sec

T=1sec

Unit Delay

Block

T=2sec

t2 t3

Time

Faster Slower

BlockBlock

T=1sec T=2sec

T=2s T=1s

T=2s

T=1sT=1s T=1s

T=2sT=1s