Timer Services

Concepts

A microkernel timer allows a task to determine whether or not a specified time limit has been reached while the task is busy performing other work. The timer uses the kernel’s system clock, measured in ticks, to monitor the passage of time.

Any number of microkernel timers can be defined in a microkernel system. Each timer has a unique identifier, which allows it to be distinguished from other timers.

A task that wants to use a timer must first allocate an unused timer from the set of microkernel timers. A task can allocate more than one timer when it needs to monitor multiple time intervals simultaneously.

A timer is started by specifying:

A duration is the number of ticks the timer counts before it expires for the first time.
A period is the number of ticks the timer counts before it expires each time thereafter.
The microkernel semaphore identifier is what the timer gives each time the semaphore expires.

The semaphore’s state can be examined by the task any time the task needs to determine whether or not the given time limit has been reached.

When the timer’s period is set to zero, the timer stops automatically after reaching the duration and giving the semaphore. When the period is set to any number of ticks other than zero, the timer restarts automatically with a new duration that is equal to its period. When this new duration has elapsed, the timer gives the semaphore again and restarts. For example, a timer can be set to expire after 5 ticks, and to then re-expire every 20 ticks thereafter, resulting in the semaphore being given 3 times after 45 ticks have elapsed.

Note

Care must be taken when specifying the duration of a microkernel timer. The first tick measured by the timer after it is started will be less than a full-tick interval. For example, when the system clock period is 10 milliseconds, starting a timer that expires after 1 tick will result in the semaphore being given anywhere from a fraction of a millisecond later to just slightly less than 10 milliseconds later. To ensure that a timer doesn’t expire for at least N ticks, it is necessary to specify a duration of N+1 ticks. This adjustment is not required when specifying the period of a timer, which always corresponds to full-tick intervals.

A running microkernel timer can be cancelled or restarted by a task prior to its expiration. Cancelling a timer that has already expired does not affect the state of the associated semaphore. Likewise, restarting a timer that has already expired is equivalent to stopping the timer and starting it afresh.

When a task no longer needs a timer it should free the timer. This makes the timer available for reallocation.

Purpose

Use a microkernel timer to determine whether or not a specified number of system clock ticks have elapsed while the task is busy performing other work.

Note

If a task has no other work to perform while waiting for time to pass it can simply call task_sleep().

Note

The microkernel provides additional APIs that allow a task to monitor both the system clock and the higher-precision hardware clock, without using a microkernel timer.

Usage

Configuring Microkernel Timers

Set the CONFIG_NUM_TIMER_PACKETS configuration option to specify the number of timer-related command packets available in the application. This value should be equal to or greater than the sum of the following quantities:

The number of microkernel timers.
The number of tasks.

Note

Unlike most other microkernel object types, microkernel timers are defined as a group using a configuration option, rather than as individual public objects in an MDEF or private objects in a source file.

Example: Allocating a Microkernel Timer

This code allocates an unused timer.

ktimer_t timer_id;

timer_id = task_timer_alloc();

Example: Starting a One Shot Microkernel Timer

This code uses a timer to limit the amount of time a task spends on gathering data. It works by monitoring the status of a microkernel semaphore that is set when the timer expires. Since the timer is started with a period of zero, it stops automatically once it expires.

ktimer_t timer_id;
ksem_t my_sem;

...

/* set timer to expire in 10 ticks */
task_timer_start(timer_id, 10, 0, my_sem);

/* gather data until timer expires */
do {
    ...
} while (task_sem_take(my_sem, TICKS_NONE) != RC_OK);

/* process the new data */
...

Example: Starting a Periodic Microkernel Timer

This code is similar to the previous example, except that the timer automatically restarts every time it expires. This approach eliminates the overhead of having the task explicitly issue a request to reactivate the timer.

ktimer_t timer_id;
ksem_t my_sem;

...

/* set timer to expire every 10 ticks */
task_timer_start(timer_id, 10, 10, my_sem);

while (1) {
    /* gather data until timer expires */
    do {
        ...
    } while (task_sem_take(my_sem, TICKS_NONE) != RC_OK);

    /* process the new data, then loop around to get more */
    ...
}

Example: Cancelling a Microkernel Timer

This code illustrates how an active timer can be stopped prematurely.

ktimer_t timer_id;
ksem_t my_sem;

...

/* set timer to expire in 10 ticks */
task_timer_start(timer_id, 10, 0, my_sem);

/* do work while waiting for input to arrive */
...

/* now have input, so stop the timer if it is still running */
task_timer_stop(timer_id);

/* check to see if the timer expired before it was stopped */
if (task_sem_take(my_sem, TICKS_NONE) == RC_OK) {
    printf("Warning: Input took too long to arrive!");
}

Example: Freeing a Microkernel Timer

This code allows a task to relinquish a previously-allocated timer so it can be used by other tasks.

task_timer_free(timer_id);

APIs

The following microkernel timer APIs are provided by microkernel.h:

task_timer_alloc(): Allocates an unused timer.
task_timer_start(): Starts a timer.
task_timer_restart(): Restarts a timer.
task_timer_stop(): Cancels a timer.
task_timer_free(): Marks timer as unused.