IPC@CHIP Documentation - SC12 @CHIP-RTOS V1.10

ackground data base search in the event that the user presses the escape key.
In more involved applications, there might be a complete spectrum of concurrent activity at different
priorities. Just for an example, the set of tasks within a real-time application might be as follows.
Listed in priority order:
■ 50 Hz Data Acquisition Loop (top priority)
■ 10 Hz Data Pre-processing Loop
■ 1 Hz Data Processing Loop
■ 0.1 Hz Kalman Filter
■ Background Loop for Integrity Self-test
Event Triggered Actions
Sometimes an activity is required only upon an event such as some data becoming available. For
these situations a task could be made to block, awaiting the event or data arrival. For example, a task
could make a blocking call on a TCP/IP port with the recv() library function. The task sleeps until
some data arrives at the respective socket, or until a prescribed time-out period expires.
Note that polling can usually be an alternative software design approach, as opposed to applying a
dedicated task to each event or data source.
Reasons not to use Multitasking
While multitasking can be an ideal solution for some applications, it also has its disadvantages. Some of
these disadvantages are noted here. The point is that you should not complicate an application with
multiple tasks unless to do so truly makes your application simpler.
Resources Expended per Task
Each task requires its own stack space. This stack must be made large enough to support the
system's interrupt handlers, some of which operate without a switch to a system stack. A minimum
stack space of 1024 bytes is recommended.
Critical Sections
Objects (memory or devices) shared between tasks can lead to critical sections. A critical section is a
section of code in a task which is sensitive to order of execution relative to code in some other task,
such that there is a possible (prior to adding the necessary protection) firing order of the two
instruction streams which leads to incorrect results.
When you design your application with only a single task, it is safe to say you have no critical
sections. When more than one task is used, you must beware. Due to the importance of
understanding and recognizing critical sections in multitasking systems, this topic is covered in more
detail below on chapter Critical Sections in Programs.
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