Programming Embedded Systems, 2nd Edition

Semaphore Task Synchronization

The eCos semaphore example is similar to a push button light switch using an LED for the light. The following example has two tasks: a producer and a consumer. The producer task, producerTask, monitors the button labeled SW0 on the Arcom board’s add-on module. Figure 11-1 shows the button used in this example.

Figure 11-1. Arcom board add-on module’s SW0 button

When the SW0 button is pressed, the producer task signals the consumer task using a semaphore. The consumer, consumerTask, waits for the semaphore signal from the producer task. Upon receiving the signal, the consumer task outputs a message and toggles the green LED.

The main function, cyg_user_start, starts by initializing the LED by calling ledInit. Next, the semaphore is initialized with a call to cyg_semaphore_init. The initial value of the semaphore, semButton, is set to zero so that the consumer task that is waiting does not execute until the semaphore is signaled by the producer task. Lastly, the two tasks are created and resumed, as in the prior example, and then a message is output signifying the start of the program.

#include <cyg/kernel/kapi.h> #include <cyg/infra/diag.h> #include "led.h" cyg_sem_t semButton; /********************************************************************** * * Function: cyg_user_start * * Description: Main routine for the eCos semaphore program. This * function ...