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Task Sync

Run code of downstream function-call subsystem or Stateflow chart by spawning an example RTOS (VxWorks) task

  • Task Sync block

Simulink Coder / Asynchronous / Interrupt Templates


The Task Sync block spawns an example RTOS (VxWorks) task that calls a function-call subsystem or Stateflow® chart. Typically, you place the Task Sync block between an Async Interrupt block and a function-call subsystem block or Stateflow chart. Alternatively, you could connect the Task Sync block to the output port of a Stateflow diagram that has an event, Output to Simulink, configured as a function call.

The Task Sync block:

  • Uses the RTOS (VxWorks) system call taskSpawn to spawn an independent task. When the task is activated, it calls the downstream function-call subsystem code or Stateflow chart. The block calls taskDelete to delete the task during model termination.

  • Creates a semaphore to synchronize the connected subsystem with execution of the block.

  • Wraps the spawned task in an infinite for loop. In the loop, the spawned task listens for the semaphore by using semTake. The first call to semTake specifies NO_WAIT. This setting lets the task determine whether a second semGive has occurred before the completion of the function-call subsystem or chart. This sequence indicates that the interrupt rate is too fast or the task priority is too low.

  • Generates synchronization code (for example, semGive()). This code lets the spawned task run. The task in turn calls the connected function-call subsystem code. The synchronization code can run at interrupt level. The connection between the Async Interrupt and Task Sync blocks accomplishes this operation and triggers execution of the Task Sync block within an ISR.

  • Supplies absolute time if blocks in the downstream algorithmic code require it. The time comes from the timer maintained by the Async Interrupt block or comes from an independent timer maintained by the task associated with the Task Sync block.

When you design your application, consider when timer and signal input values could be taken for the downstream function-call subsystem that is connected to the Task Sync block. By default, the time and input data are read when the RTOS (VxWorks) activates the task. For this case, the data (input and time) are synchronized to the task itself. If you select the Synchronize the data transfer of this task with the caller task option and the Task Sync block driver is an Async Interrupt block, the time and input data are read when the interrupt occurs (that is, within the ISR). For this case, data is synchronized with the caller of the Task Sync block.


Use the blocks in the Interrupt Templates block library (Async Interrupt and Task Sync) for simulation and code generation. These blocks provide starting point examples to help you develop custom blocks for a target environment.



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A call from an Async Interrupt block.

Output Arguments

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A call to a function-call subsystem.


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The first argument passed to the taskSpawn system call in the RTOS. The RTOS (VxWorks) uses this name as the task function name. This name also serves as a debugging aid. Routines use the task name to identify the task from which they are called.

The RTOS task priority assigned to the function-call subsystem task when spawned. RTOS (VxWorks) priorities range from 0 to 255, with 0 representing the highest priority.


The Simulink® software does not simulate asynchronous task behavior. The task priority of an asynchronous task is for code generation purposes only and is not honored during simulation.

Maximum size to which the stack of the task can grow. The stack size is allocated when the RTOS (VxWorks) spawns the task. Choose a stack size based on the number of local variables in the task. Determine the size by examining the generated code for the task (and functions that are called from the generated code).

If not selected (the default),

  • The block maintains a timer that provides absolute time values required by the computations of downstream blocks. The timer is independent of the timer maintained by the Async Interrupt block that calls the Task Sync block.

  • A Timer resolution option appears.

  • The Timer size option specifies the word size of the time counter.

If selected,

  • The block does not maintain an independent timer and does not display the Timer resolution field.

  • Downstream blocks that require timers use the timer maintained by the Async Interrupt block that calls the Task Sync block (see Timers in Asynchronous Tasks). The timer value is read at the time the asynchronous interrupt is serviced. Data transfers to blocks called by the Task Sync block execute within the task associated with the Async Interrupt block. Therefore, data transfers are synchronized with the caller.

The resolution of the timer of the block in seconds. This parameter appears only if Synchronize the data transfer of this task with the caller task is cleared. By default, the block gets the timer value by calling the tickGet function in the RTOS (VxWorks). The default resolution is 1/60 second.

The number of bits to store the clock tick for a hardware timer. The size can be 32bits (the default), 16bits, 8bits, or auto. If you select auto, the code generator determines the timer size based on the settings of model configuration parameter Application lifespan (days) and block parameterTimer resolution.

By default, timer values are stored as 32-bit integers. When Timer size is auto, you can indirectly control the word size of the counters by setting the Application lifespan (days) parameter. If you set Application lifespan (days) to a value that is too large for the code generator to handle as a 32-bit integer of the specified resolution, it uses a second 32-bit integer to address overflows.

For more information, see Optimize Memory Usage and Prevent Overflows for Time Counters. See also Timers in Asynchronous Tasks.

Version History

Introduced in R2006a