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Real time operating systems (RTOSes) achieve multitasking using these same principals -
but their objectives are very different to those of non real time systems. The different
objective is reflected in the scheduling policy. Real time / embedded systems are designed
to provide a timely response to real world events. Events occurring in the real world
can have deadlines before which the real time / embedded system must respond and the
RTOS scheduling policy must ensure these deadlines are met.
To achieve this objective the software engineer must first assign a priority to each task. The scheduling policy of the RTOS is
then to simply ensure that the highest priority task that is able to execute is the task given processing time. This may
require sharing processing time "fairly" between tasks of equal priority if they are ready to run simultaneously.
Example:
The most basic example of this is a real time system that incorporates a keypad and LCD. A user must get visual feedback of each
key press within a reasonable period - if the user cannot see that the key press has been accepted within this period the software product will at best be awkward to use. If the longest acceptable period was 100ms - any response between 0 and 100ms would be acceptable. This functionality could be implemented as an autonomous task with the following structure:
void vKeyHandlerTask( void *pvParameters )
{
// Key handling is a continuous process and as such the task
// is implemented using an infinite loop (as most real time
// tasks are).
for( ;; )
{
[Suspend waiting for a key press]
[Process the key press]
}
}
Now assume the real time system is also performing a control function that relies on a digitally filtered input. The input must be sampled, filtered and the control cycle executed every 2ms. For correct operation of the filter the temporal regularity of the sample must be accurate to 0.5ms. This functionality could be implemented as an autonomous task with the following structure: void vControlTask( void *pvParameters )
{
for( ;; )
{
[Suspend waiting for 2ms since the start of the previous
cycle]
[Sample the input]
[Filter the sampled input]
[Perform control algorithm]
[Output result]
}
}
The software engineer must assign the control task the highest priority as:
- The deadline for the control task is stricter than that of the key handling task.
- The consequence of a missed deadline is greater for the control task than for the key handler task.
The next page demonstrates how these tasks would be scheduled by a real time operating system.
Next: RTOS Fundamentals - Real Time Scheduling
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