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Cyclic Tasks (Timer)Tasks that must run in a fixed time frame are frequently encountered in real-time applications. The structure of such a task could be as follows:
void RTKAPI CyclicTask(void * p)
{
#define Cycle 5
RTKTime NextActivation;
NextActivation = RTKGetTime();
while (1)
{
NextActivation += Cycle;
RTKDelayUntil(NextActivation);
/* the task's job is done here */
}
}
This task would run exactly once every 5 timer ticks, provided the task's job takes no longer than 5 timer ticks. The actual cycle time can be adjusted using constant Cycle and the timer interrupt interval. If the cycle time of a task is not an exact multiple of the timer interrupt interval, the following task can be used:
#include <timer.h>
void CyclicTask(void)
{
#define Cycle 0.7 /* seconds */
TISeconds NextActivation;
NextActivation = TITicksToSeconds(RTKGetTime());
while (True)
{
NextActivation += Cycle;
RTKDelayUntil(TISecondsToTicks(NextActivation));
/* the task's job is done here */
}
}
Since RTKernel-32 can only activate tasks waiting in an RTKDelay or RTKDelayUntil through a timer interrupt, these tasks would, on the average, conform to their cycle exactly. The start of a cycle can deviate from the desired point in time up to one timer tick. This round-off error arises from the conversion of a float point number to an integer in function TISecondsToTicks of module Timer. However, the error does not accumulate; it is the same in each cycle. It must be noted that rounding errors can occur when floats are used to store large numbers representing RTKernel-32 times. Unlike the first example, an overflow of RTKernel-32's clock must also be considered (see section Module RTKernel-32, Time).
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