Page 383 - ARM Based Microcontroller Projects Using MBED

15.4 PROJECT 1—DIFFERENT FLASHING A PAIR OF LEDs—USING Mbed THREAD 369
assigned to PC_0 and PC_1, respectively. Two functions named LEDAControl and
LEDBControl are created to flash the two LEDs. LEDAControl flashes LEDA every second,
while LEDBControl flashes LEDB every 0.5s. Inside the main program threads LEDAControl
and LEDBControl are started. The main program loops forever, not doing anything useful. In
actualfact,mainisanotherthreadandoneoftheLEDscouldhavebeenflashedinsidethemain.
Because main is another thread, in this program there are actually three threads, two of them
flashing the two LEDs and the third one just repeating itself in a loop and wasting CPU re-
sources. We can place the main thread in a wait state so that it does not consume any CPU re-
sources. This can be done by declaring a semaphore at the beginning of the program, such as:
Semaphore sema;
And then place the main thread in a forever wait state with the following statement:

sema.wait(osWaitForever);
We can also set the priority of the main to idle (see the following section) so that it does not
consume any CPU resources. Another option is to set the thread to wait forever by the fol-
lowing statement:
Thread::wait(osWaitForever);

15.4.6 Mbed Thread Priorities

The default thread priority is osPriorityNormal. A thread can be set to one of the following
priorities (Mbed OS2 only):

osPriotityIdle lowest priority ( 3)
osPriorityLow low priority ( 2)
osPriorityBelowNormal below normal priority ( 1)
osPriorityNormal normal (default) priority (0)
osPriorityAboveNormal above normal priority (+1)
osPriorityHigh high priority (+2)
osPriorityRealtime real-time (highest) priority (+3)

Table 15.1 gives a list of the important thread related functions with example
usage (further details and complete list can be obtained from the web site:
https://os.mbed.com/handbook/RTOS):

15.4.7 Thread States
A thread can be in any one of the following four states at any time:

RUNNING: this is the currently running thread (only one thread can be in this state
at any time).