Real-Time Workshop User's Guide

Task Synchronization Block

The VxWorks Task Synchronization block is a function-call subsystem that spawns, as an independent VxWorks task, the function-call subsystem connected to its output. Typically it would be placed between the VxWorks Asynchronous Interrupt block and a function-call subsystem block or a Stateflow chart. Another example would be to place the Task Synchronization block at the output of a Stateflow diagram that has an Event, "Output to Simulink," configured as a function-call.

The VxWorks Task Synchronization block performs the following functions:

• The downstream function-call subsystem is spawned as an independent task using the VxWorks system call taskSpawn(). The task is deleted using taskDelete() during model termination.
• A semaphore is created to synchronize the downstream system to the execution of the Task Synchronization block.
• Code is added to this spawned function-call subsystem to wrap it in an infinite while loop.
• Code is added to the top of the infinite while loop of the spawned task to wait on a the semaphore, using semTake(). When semTake() is first called, NO_WAIT is specified. This allows the task to determine if a second semGive() has occurred prior to the completion of the function-call subsystem. This would indicate the interrupt rate is too fast or the task priority is too low.
• Synchronization code, i.e., semgive(), is generated for the Task Synchronization block (a masked function-call subsystem). This allows the output function-call subsystem to run. As an example, if you connect the Task Synchronization block to the output of a VxWorks Asynchronous Interrupt block, only a semGive() would occur inside an ISR.

Task Synchronization Parameters

The picture below shows the VxWorks Task Synchronization block dialog box.

Parameters associated with the Task Synchronization block are:

• Task Name -- An optional name, which if provided, is used as the first argument to the taskSpawn() system call. This name is used by VxWorks routines to identify the task they are called from to aid in debugging.
• Task Priority -- The task priority is the VxWorks priority that the function-call subsystem task is given when it is spawned. The priority can be a very important consideration in relation to other tasks priorities in the VxWorks system. In particular, the default priority of the model code is 30 and, when multitasking is enabled, the priority of the each subrate task increases by one from the default model base rate. Other task priorities in the system should also be considered when choosing a task priority. VxWorks priorities range from 0 to 255 where a lower number is a higher priority.
• Stack Size -- The function-call subsystem is spawned with the stack size specified. This is maximum size to which the task's stack can grow. The value should be chosen based on the number of local variables in the task.

By default, Real-Time Workshop limits the number of bytes for local variables in all of the generated code to 8192 bytes (see assignment of MaxStackSize in
matlabroot/rtw/c/tornado/tornado.tlc). As a rule, providing twice 8192 bytes (16384) for the one function that is being spawned as a task should be sufficient.

Task Synchronization Block Example

This example shows a Task Synchronization block as a simple ISR.

The Task Synchronization block inserts this code during the Target Language Compiler phase of code generation:

• In MdlStart, the Task Synchronization block is registered by the Asynchronous Interrupt block as an ISR. The Task Synchronization block creates and initializes the synchronization semaphore. It also spawns the function-call subsystem as an independent task.

/* Create and spawn task: <Root>/Faster Rate(.015) */
if ((*(SEM_ID *)rtPWork.s6_S_Function.SemID =
  semBCreate(SEM_Q_PRIORITY, SEM_EMPTY)) == NULL) 
  ssSetErrorStatus(rtS,"semBCreate call failed "
                   "for block <Root>/Faster Rate(.015).\n ");
}
if ((rtIWork.s6_S_Function.TaskID = taskSpawn("root_Faster_", 20, VX_FP_TASK, 
    1024, (FUNCPTR)Sys_root_Faster__OutputUpdate,
    (int_T)rtS, 0, 0, 0, 0, 0, 0, 0, 0, 0)) == ERROR) {
      ssSetErrorStatus(rtS,"taskSpawn call failed for block <Root>/ Faster Rate 
"                            "(.015).\n");
  }

• The Task Synchronization block modifies the downstream function-call subsystem by wrapping it inside an infinite loop and adding semaphore synchronization code.

/* Output and update for function-call system: <Root>/Faster    Rate(.015) */
void Sys_root_Faster__OutputUpdate(void *reserved, int_T 
                                   controlPortIdx, int_T tid)
{
  /* Wait for semaphore to be released by system: <Root>/Task Synchronization */
  for(;;) {
    if (semTake(*(SEM_ID *)rtPWork.s6_S_Function.SemID,NO_WAIT) != ERROR) {
      logMsg("Rate for function-call subsystem"
             "Sys_root_Faster__OutputUpdate() fast.\n",0,0,0,0,0,0);
#if STOPONOVERRUN
      logMsg("Aborting real-time simulation.\n",0,0,0,0,0,0);
      semGive(stopSem);
      return(ERROR);
#endif
    } else {

      semTake(*(SEM_ID *)rtPWork.s6_S_Function.SemID, WAIT_FOREVER);
    }
    /* UniformRandomNumber Block: <S3>/Uniform Random Number */
    rtB.s3_Uniform_Random_Number =
    rtRWork.s3_Uniform_Random_Number.NextOutput;
   .
   .
   .

}

Asynchronous Interrupt Block

Asynchronous Buffer Block