Real-Time Workshop®

Using Timers in Asynchronous Tasks

An ISR can set a source for absolute time. This is done with the function ssSetTimeSource, which has the following three options:

• SS_TIMESOURCE_SELF: Each generated ISR maintains its own absolute time counter, which is distinct from any periodic base rate or subrate counters in the system. The counter value and the timer resolution value (specified in the Timer resolution (seconds) parameter of the Async Interrupt block) are used by downstream blocks to determine absolute time values required by block computations.

• SS_TIMESOURCE_CALLER: The ISR reads time from a counter maintained by its caller. Time resolution is thus the same as its caller's resolution.

• SS_TIMESOURCE_BASERATE: The ISR can read absolute time from the model's periodic base rate. Time resolution is thus the same as its base rate resolution.

By default, the counter is implemented as a 32-bit unsigned integer member of the Timing substructure of the real-time model structure. For any target that supports the rtModel data structure, when the time data type is not set by using ssSetAsyncTimeDataType, the counter word size is determined by the Application lifespan (days) model parameter. As an example (from ERT target code),

/* Real-time Model Data Structure */
struct _RT_MODEL_elapseTime_exp_Tag {
   const char *errorStatus;
  
  /*
   * Timing:
   * The following substructure contains information regarding
   * the timing information for the model.
   */
  struct {
    uint32_T clockTick1;
    uint32_T clockTick2;
  } Timing;
};

The example omits unused fields in the Timing data structure (a feature of ERT target code not found in GRT). For any target that supports the rtModel data structure, the counter word size is determined by the Application lifespan (days) model parameter.

By default, the library blocks for the Wind River Systems VxWorks RTOS set the timer source to SS_TIMESOURCE_SELF and update their counters by using the system call tickGet. tickGet returns a timer value maintained by the VxWorks kernel. The maximum word size for the timer is UINT32. The following VxWorks example for the shows a generated call to tickGet.

/* VxWorks Interrupt Block: '<Root>/Async Interrupt' */
void isr_num2_vec193(void)
{

 /* Use tickGet() as a portable tick counter example. A much
    higher resolution can be achieved with a hardware counter */
 rtM->Timing.clockTick2 = tickGet();
. . .

The tickGet call is supplied only as an example. It can (and in many instances should) be replaced by a timing source that has better resolution. If you are targeting the VxWorks RTOS, you can obtain better timer resolution by replacing the tickGet call and accessing a hardware timer by using your BSP instead.

If you are implementing a custom asynchronous block for an RTOS other than the VxWorks RTOS, you should either generate an equivalent call to the target RTOS, or generate code to read the appropriate timer register on the target hardware.

The default Timer resolution (seconds) parameter of your Async Interrupt block implementation should be changed to match the resolution of your target's timing source.

The counter is updated at interrupt level. Its value represents the tick value of the timing source at the most recent execution of the ISR. The rate of this timing source is unrelated to sample rates in the model. In fact, typically it is faster than the model's base rate. Select the timer source and set its rate and resolution based on the expected rate of interrupts to be serviced by the Async Interrupt block.

For an example of timer code generation, see Async Interrupt Block Implementation.

Rate Transitions and Asynchronous Blocks

Creating a Customized Asynchronous Library