92 lines
3.5 KiB
Plaintext
92 lines
3.5 KiB
Plaintext
/*
|
|
ChibiOS/RT - Copyright (C) 2006,2007,2008,2009,2010 Giovanni Di Sirio.
|
|
|
|
This file is part of ChibiOS/RT.
|
|
|
|
ChibiOS/RT is free software; you can redistribute it and/or modify
|
|
it under the terms of the GNU General Public License as published by
|
|
the Free Software Foundation; either version 3 of the License, or
|
|
(at your option) any later version.
|
|
|
|
ChibiOS/RT is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
GNU General Public License for more details.
|
|
|
|
You should have received a copy of the GNU General Public License
|
|
along with this program. If not, see <http://www.gnu.org/licenses/>.
|
|
*/
|
|
|
|
/**
|
|
* @page article_timing Reliable timings using Threads
|
|
* One common task is to have threads do something at regular, scheduled,
|
|
* intervals.
|
|
* An obvious solution is to write something like this:
|
|
* @code
|
|
msg_t my_thread(void *param) {
|
|
|
|
while (TRUE) {
|
|
do_something();
|
|
chThdSleepMilliseconds(1000); // Fixed interval
|
|
}
|
|
}
|
|
* @endcode
|
|
* This example works well assuming that the @p do_something() execution time
|
|
* is well below the system tick period and that @p my_thread() is not
|
|
* preempted by other threads that could insert long intervals.<br>
|
|
* If the above conditions are not satisfied you may have @p do_something()
|
|
* executed at irregular intervals, as example:<br><br>
|
|
* T0...T0+1000...T0+2002...T0+3002...T0+4005...etc.<br><br>
|
|
* Also note that the error increases over time and this kind of behavior can
|
|
* lead to anomalies really hard to debug.
|
|
* <h2>A better solution</h2>
|
|
* It is possible to rewrite the above code using absolute deadlines rather
|
|
* than fixed intervals:
|
|
* @code
|
|
msg_t my_thread(void *param) {
|
|
|
|
systick_t time = chTimeNow(); // T0
|
|
while (TRUE) {
|
|
time += MS2ST(1000); // Next deadline
|
|
do_something();
|
|
chThdSleepUntil(time);
|
|
}
|
|
}
|
|
* @endcode
|
|
* Using this code @p do_something() will always be executed at an absolute
|
|
* deadline time and the error will not accumulate over time regardless of
|
|
* the execution time and delays inserted by other threads.<br>
|
|
* Note that this solution requires that the @p do_something() execution
|
|
* time must not exceed the deadline or the thread would stay sleeping into
|
|
* @p chThdSleepUntil().
|
|
*
|
|
* <h2>A different way</h2>
|
|
* Another way to perform activities at regular intervals is the use of a
|
|
* virtual timer. Virtual timers are able to generate callbacks at scheduled
|
|
* intervals. Virtual timers are one shot timers so you need to restart them
|
|
* from within the callback if you need a periodic timer like in this case.
|
|
* @code
|
|
VirtualTimer vt;
|
|
|
|
void do_something(void *p) {
|
|
|
|
chVTSetI(&vt, MS2ST(1000), do_something, p); // Restarts the timer.
|
|
// Periodic code here.
|
|
}
|
|
|
|
int main(int argc, char **argv) {
|
|
|
|
chSysLock();
|
|
chVTSetI(&vt, MS2ST(1000), do_something, NULL); // Starts the timer.
|
|
chSysUnlock();
|
|
...
|
|
}
|
|
* @endcode
|
|
* Note that the callback code is executed from within the I-Locked state (see
|
|
* @ref system_states) so you can only execute I-Class APIs from there (see
|
|
* @ref api_suffixes).<br>
|
|
* This solution has the advantage to not require a dedicated thread and
|
|
* thus uses much less RAM but the periodic code must have a very short
|
|
* execution time or it would degrade the overall system response time.
|
|
*/
|