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diff --git a/c-user/timer/background.rst b/c-user/timer/background.rst new file mode 100644 index 0000000..20cf202 --- /dev/null +++ b/c-user/timer/background.rst @@ -0,0 +1,71 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Background +========== + +Required Support +---------------- + +A clock tick is required to support the functionality provided by this manager. + +Timers +------ + +A timer is an RTEMS object which allows the application to schedule operations +to occur at specific times in the future. User supplied timer service routines +are invoked by either a clock tick directive or a special Timer +Server task when the timer fires. Timer service routines may perform any +operations or directives which normally would be performed by the application +code which invoked a clock tick directive. + +The timer can be used to implement watchdog routines which only fire to denote +that an application error has occurred. The timer is reset at specific points +in the application to ensure that the watchdog does not fire. Thus, if the +application does not reset the watchdog timer, then the timer service routine +will fire to indicate that the application has failed to reach a reset point. +This use of a timer is sometimes referred to as a "keep alive" or a "deadman" +timer. + +Timer Server +------------ + +The Timer Server task is responsible for executing the timer service routines +associated with all task-based timers. This task executes at a priority +specified by :ref:`rtems_timer_initiate_server() <rtems_timer_initiate_server>` +and it may have a priority of zero (the highest priority). In uniprocessor +configurations, it is created non-preemptible. + +By providing a mechanism where timer service routines execute in task rather +than interrupt space, the application is allowed a bit more flexibility in what +operations a timer service routine can perform. For example, the Timer Server +can be configured to have a floating point context in which case it would be +safe to perform floating point operations from a task-based timer. Most of the +time, executing floating point instructions from an interrupt service routine +is not considered safe. The timer service routines invoked by the Timer Server +may block, however, since this blocks the Timer Server itself, other timer +service routines that are already pending do not run until the blocked timer +service routine finished its work. + +The Timer Server is designed to remain blocked until a task-based timer fires. +This reduces the execution overhead of the Timer Server. + +.. index:: rtems_timer_service_routine + +Timer Service Routines +---------------------- + +The timer service routine should adhere to C calling conventions and have a +prototype similar to the following: + +.. code-block:: c + + rtems_timer_service_routine user_routine( + rtems_id timer_id, + void *user_data + ); + +Where the timer_id parameter is the RTEMS object ID of the timer which is being +fired and user_data is a pointer to user-defined information which may be +utilized by the timer service routine. The argument user_data may be NULL. |