Real-Time Workshop

Blocks That Depend on Absolute Time

Some Simulink blocks use the value of absolute time (i.e., the time from the beginning of the program to the present time) to calculate their outputs. If you are designing a program that is intended to run indefinitely, then you cannot use blocks that have a dependency on absolute time.

The problem arises when the value of time reaches the largest value that can be represented by a double precision number. At that point, time is no longer incremented and the output of the block is no longer correct.

Note    In addition to the blocks listed below, logging Time (in the Workspace  I/O page of the Simulation Parameters dialog box) also requires absolute time.

The following Simulink blocks depend on absolute time:

• Continuous Blocks
• Derivative
• Transport Delay
• Variable Transport Delay
• Discrete Blocks
• Discrete-Time Integrator (when used in triggered subsystems)
• Nonlinear Blocks
• Rate Limiter
• Sinks
• Scope
• To File
• To Workspace (only if logging to StructureWithTime format)
• Sources

The following Simulink blocks depend on absolute time:

• Chirp Signal
• Clock
• Digital Clock
• From File
• From Workspace
• Pulse Generator
• Ramp
• Repeating Sequence
• Signal Generator
• SineWave
• Step

Note    The Sine Wave block is dependent on absolute time only when the Sine type parameter is set to Time-based. Set this parameter to Sample-based to avoid absolute time computations.

In addition to the Simulink block above:

• Blocks in other Blocksets may reference absolute time. Please refer to the documentation for the Blocksets that you use.
• Stateflow charts that use time are dependent on absolute time.

Glossary

Targeting DOS for Real-Time Applications