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[oss-web.git] / in / rgblamp / task.md

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-title: RGB Lamp Tasks
-linktitle: rgb-task
-parent: rgb-code
-ctime: 2012-03-24
-
-# Tasks
-
-A task implements an application behavior in response to an event.  Tasks are
-executed to completion without pre-emption.  Task start requests are queued if
-another task is already executing.  A task start request when that task is
-already queued to start is effectively a null operation.  However, a task start
-request for a task that is not queued but currently running will queue the task
-for a subsequent execution.  This very simple task design is more than
-sufficient for the needs of the lamp application.
-
-One key decision under such a task architecture is which task to run next if
-multiple tasks are queued.  This code selects the task with the highest priority
-in such a case.  Application code must be cognizant of this fact, as a
-perpetually queueing high priority task will prevent execution of a queued lower
-priority task.  This behavior is similar to hardware interrupt dispatch in many
-processor architectures, so most developers should already be familiar with its
-pitfalls.
-
-# Task implementation
-
-Tasks are implemented in the task.c, task.h and task_defs.h files,
-as found in the [repository](http://oss/gitweb?p=rgblamp.git;a=summary).
-
-Task identifiers are defind in the task_defs.h file.  Each task id is an
-enumeration, with the highest priority task given the value of zero, and
-successively lower priority tasks having incrementally larger numbers.  Since
-the task queue is implemented as bits in a variable, the width of the task_id_t
-structure defines the maximum number of supported tasks.
-
-Before using, initialize the task subsystem:
-
-    task_init();
-
-To queue a task for execution, post it.  Tasks can be posted from within an ISR
-or from without.  A task can post itself as well.
-
-    task_post(TASK_ID);
-
-Task dispatch is constructed from the task_get() function, which atomically gets
-and clears the next task (bit).  The user application code then constructs a
-switch statement or similar structure to call a function associated with each
-task id.    The lamp code implements this behavior in the user_tasks() function
-in main.c.  Check it out in the
-[repository](http://oss/gitweb?p=rgblamp.git;a=summary).  A short version:
-
-    void user_tasks(unsigned char block)
-    {
-      task_id_t tid;
-
-      while ((tid = task_get(block)) >= 0) {
-        switch (tid) {
-          case TASK_BTN_PB:
-            pb_task();
-            break;
-          ...
-        }
-      }
-    }
-