Timer Services
Concepts
The nanokernel’s timer object type uses the kernel’s system clock to monitor the passage of time, as measured in ticks. It is mainly intended for use by fibers.
A nanokernel timer allows a fiber or task to determine whether or not a specified time limit has been reached while the thread itself is busy performing other work. A thread can use more than one timer when it needs to monitor multiple time intervals simultaneously.
A nanokernel timer points to a user data structure that is supplied by the thread that uses it; this pointer is returned when the timer expires. The user data structure must be at least 4 bytes long and aligned on a 4-byte boundary, as the kernel reserves the first 32 bits of this area for its own use. Any remaining bytes of this area can be used to hold data that is helpful to the thread that uses the timer.
Any number of nanokernel timers can be defined. Each timer is a distinct
variable of type struct nano_timer, and is referenced using a pointer
to that variable. A timer must be initialized with its user data structure
before it can be used.
A nanokernel timer is started by specifying a duration, which is the number of ticks the timer counts before it expires.
Note
Care must be taken when specifying the duration of a nanokernel timer,
since 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 than expires after 1 tick will result
in the timer expiring 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.
Once started, a nanokernel timer can be tested in either a non-blocking or
blocking manner to allow a thread to determine if the timer has expired.
If the timer has expired, the kernel returns the pointer to the user data
structure. If the timer has not expired, the kernel either returns
NULL (for a non-blocking test), or it waits for the timer to expire
(for a blocking test).
Note
The nanokernel does not allow more than one thread to wait on a nanokernel timer at any given time. If a second thread starts waiting, only the first waiting thread wakes up when the timer expires. The second thread continues waiting.
A task that waits on a nanokernel timer does a busy wait. This is
not an issue for a nanokernel application’s background task; however, in
a microkernel application, a task that waits on a nanokernel timer remains
the current task and prevents other tasks of equal or lower priority
from doing useful work.
A nanokernel timer can be cancelled after it has been started. Cancelling a timer while it is still running causes the timer to expire immediately, thereby unblocking any thread waiting on the timer. Cancelling a timer that has already expired has no effect on the timer.
A nanokernel timer can be reused once it has expired, but must not be restarted while it is still running. If desired, a timer can be re-initialized with a different user data structure before it is started again.
Purpose
Use a nanokernel timer to determine whether or not a specified number of system clock ticks have elapsed while a fiber or task is busy performing other work.
Note
If a fiber or task has no other work to perform while waiting
for time to pass, it can simply call fiber_sleep()
or task_sleep(), respectively.
Note
The kernel provides additional APIs that allow a fiber or task to monitor the system clock, as well as the higher precision hardware clock, without using a nanokernel timer.
Usage
Example: Initializing a Nanokernel Timer
This code initializes a nanokernel timer.
struct nano_timer my_timer;
uint32_t data_area[3] = { 0, 1111, 2222 };
nano_timer_init(&my_timer, data_area);
Example: Starting a Nanokernel Timer
This code uses the above nanokernel timer to limit the amount of time a fiber spends gathering data before processing it.
/* set timer to expire in 10 ticks */
nano_fiber_timer_start(&my_timer, 10);
/* gather data until timer expires */
do {
...
} while (nano_fiber_timer_test(&my_timer, TICKS_NONE) == NULL);
/* process the data */
...
Example: Cancelling a Nanokernel Timer
This code illustrates how an active nanokernel timer can be stopped prematurely.
struct nano_timer my_timer;
uint32_t dummy;
...
/* set timer to expire in 10 ticks */
nano_timer_init(&my_timer, &dummy);
nano_fiber_timer_start(&my_timer, 10);
/* do work while waiting for an input signal to arrive */
...
/* now have input signal, so stop the timer if it is still running */
nano_fiber_timer_stop(&my_timer);
/* check to see if the timer expired before it was stopped */
if (nano_fiber_timer_test(&my_timer, TICKS_NONE) != NULL) {
printf("Warning: Input signal took too long to arrive!");
}
APIs
APIs for a nanokernel timer provided by nanokernel.h
Initialize a timer.
nano_task_timer_start(), nano_fiber_timer_start(),
nano_isr_timer_start(), nano_timer_start()
Start a timer.
nano_task_timer_test(), nano_fiber_timer_test(),
nano_isr_timer_test(), nano_timer_test()
Wait or test for timer expiration.
nano_task_timer_stop(), nano_fiber_timer_stop(),
nano_isr_timer_stop(), nano_timer_stop()
Force timer expiration, if not already expired.
Return timer ticks before timer expiration.