Lesson PROGRAMMING 6 - Devi Ahilya … of Lesson-6 Chapter 9 on µ C/OS-II Semaphore Functions...

2008 Chapter-9 L6: "Embedded Systems - Architecture, Programming and Design" , Raj Kamal, Publs.: McGraw-Hill, Inc.

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REAL TIME OPERATING SYSTEM REAL TIME OPERATING SYSTEM PROGRAMMINGPROGRAMMING--I: I: µµµµµµµµC/OSC/OS--II and II and

VxWorks VxWorks

LessonLesson--6:6:µµµµµµµµC/OS-II Semaphore Functions


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1. Semaphore Functions 1. Semaphore Functions


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� Provides for using same semaphore functions as an event signaling flag or mutex or counting semaphore.

µC/OS-II Semaphore functions


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� When a semaphore created is used as a an event signaling flag or as counting semaphore, semaphore value at start = 0, which means event yet to occur and 1 will mean event occurred.

Semaphore for event flag functionsSemaphore for event flag functions


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Semaphore as mutex functions Semaphore as mutex functions � When a semaphore created is used as a

resource acquiring key, Semaphore value at start = 1, which means resource available and 0 will mean not available.


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• OS_Event OSSemCreate (unsigned short semVal)−To create and initialize semaphores

Refer examples 9.16 to 9.18 Steps7, 2 and 6 for event-flag, key and count, respectively.

OSSemCreate (semVal)


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• void OSSemPend (OS_Event *eventPointer, unsigned short timeOut, unsigned byte *SemErrPointer)To check whether semaphore is pending or not pending (0 or >0). If pending (=0), then suspend the task till >0 (released). If >0, decrement the value of semaphore and run the waiting codes (Example 9.16 Step 31)

OSSemPendOSSemPend (*(*eventPointereventPointer, , timeOuttimeOut, , **SemErrPointerSemErrPointer))


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OSSemAcceptOSSemAccept (*(*eventPointereventPointer))

• unsigned short OSSemAccept(OS_EVENT *eventPointer)

− To check whether semaphore value > 0 and if yes, then retrieve and decrement. Used when there is no need to suspend a task, only decrease it to 0 if value is not already zero


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• unsigned byte OSSemPost(OS_EVENT*eventPointer)− SemVal if 0 or more, increment. Increment makes the semaphore again not pending for the waiting tasks. (Examples 9.16, 9.17 and 9.18 Steps 19, 10, 16, respectively)

OSSemPostOSSemPost (*(*eventPointereventPointer))


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unsigned byte OSSemQuery (OS_EVENT *eventPointer, OS_SEM_DATA *SemData) −To get semaphore information

OSSemOSSemQuery (*(*eventPointereventPointer))


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2. Macros for semaphore functions to find 2. Macros for semaphore functions to find status after execution of OS status after execution of OS semaphore semaphore

FunctionsFunctions


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• OS_NO_ERR, if semaphore signaling succeeded. [SemVal > 0

or 0.] or when when querying succeeds; • OS_ERR_EVENT_TYPE, if *eventPointer is not pointing to the semaphore. • OS_SEM_OVF, when semVal overflows (cannot increment and is

already 65535.)

Macros for semaphore functionsMacros for semaphore functions


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3. Example of semaphore function 3. Example of semaphore function Applications as event signaling flag Applications as event signaling flag

2008

Chapter-9 L6: "Embedded Systems -Architecture, Programming and

Design" , Raj Kamal, Publs.: McGraw-Hill, Inc.

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•• Programming Example of two Programming Example of two application tasks in a chocolate vending application tasks in a chocolate vending machine. One task is to Read and get machine. One task is to Read and get coins (coins (ReadTaskReadTask) and other to deliver the ) and other to deliver the chocolate (chocolate (DeliveryTask)

Example of semaphore function Example of semaphore function ApplicationsApplications


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#define OS_MAX_EVENTS 8 /*When total number of IPCs needed in an application = 8*/ #define OS_SEM_EN 1 /*When the use of semaphores is contemplated */

Step Step ii: Initiating the Semaphores: Initiating the Semaphores


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OS_EVENT *SemFlagChocolate;/* When Sem is to be used as flag for the delivery of chocolate, initial value = 0 (means not released) */SemFlagChocolate = OSSemCreate (0);

Step j: Global IPC functions and their Step j: Global IPC functions and their parameters declarationsparameters declarations


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void main (void) {OSInit ();/* Create First task */OSTaskCreate (FirstTask, void (*) 0,(void *)&FirstTaskStack[ FirstTaskStackSize], FirstTaskPriority);OSStart ( );}

Step k: Main functionStep k: Main function


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static void FirstTask (void *taskPointer) {/*System clock time set *//* Create apllication related highest prio task */OSTaskCreate (ReadTask, void (*) 0,(void *)&ReadTaskStack [ReadTaskStackSize], ReadTaskPriority);

Step l: First taskStep l: First task


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OSTaskCreate (DeliveryTask, void (*) 0,(void *)&DeliveryTaskStack[DeliveryTaskStackSize], DeliveryTaskPriority);OSTimeSet (presetTime);OSTickInit (); /* Initiate system timer ticking*/

Step l: First taskStep l: First task


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static void FirstTask (void *taskPointer) {...while (1) {OSTaskSuspend (FirstTaskPriority);}

Step l: First task suspend indefinitely after Step l: First task suspend indefinitely after creating application tasks and initiating creating application tasks and initiating

system timer ticks… system timer ticks…


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static void ReadTask (void *taskPointer) {..while (1) {...; ...; ...;.OSSemPost (SemFlagChocolate);/* after this instruction executes SemFlagChocolate = 1 (released state)*/

Step m: Step m: ReadTaskReadTask releasing semaphore on releasing semaphore on finding appropriate coinsfinding appropriate coins


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OSTimeDelay (3000)/* Write delay function 3000 ticks to let the control transfer to deliver task (next priority task)*/....; }}

Step m: Step m: ReadTaskReadTask delaying to enable delaying to enable lower priority lower priority DeliveryTaskDeliveryTask to runto run


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static void DeliveryTask (void *taskPointer) {.while (1) {OSSemTake (SemFlagChocolate);/* after this instruction executes SemFlagChocolate = 0 (taken state)*/...; ...; ...; /*Deliver Chocolate*/

Step n: Chocolate delivery task Step n: Chocolate delivery task waiting to take the required semaphorewaiting to take the required semaphore


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...; ...; ...; OSTimeDlyResume (ReadTaskPriority);/ * Resume ReadTask for next read*/}; }

Step n: Chocolate delivery task Step n: Chocolate delivery task resuming delayed resuming delayed TaskReadTaskRead ……


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4. Example of semaphore function 4. Example of semaphore function Applications as mutex Applications as mutex


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Programming ExampleProgramming Example

� Mutex m used to let a running task critical section 1 using a resource (for example, print buffer) not used by another task critical section 2 waiting to take same m


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#define OS_MAX_EVENTS 8 /*When total number of IPCs needed in an application = 8*/ #define OS_SEM_EN 1/*When the use of semaphores is

contemplated */

Step A: Creating a Mutex SemaphoreStep A: Creating a Mutex Semaphore


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OS_EVENT *Sem_mCoin;/* Declare a pointer to data structure for semaphore and other IPC events */

Step B: Global IPC functions and their Step B: Global IPC functions and their parameters declarationsparameters declarations


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SemFlagChocolate = OSSemCreate (0);/* When Sem is to be used as a event

signaling flag, initial value = 0 (means not yet released) */Sem _mCoin = OSSemCreate (1);/* When Sem is to be used as a mutex initial value = 1 (means is in released state) */

Step B: Global IPC functions and their Step B: Global IPC functions and their parameters declarations…parameters declarations…


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void main (void) {OSInit ();/* Create First task */OSTaskCreate (FirstTask, void (*) 0,(void *)&FirstTaskStack[ FirstTaskStackSize], FirstTaskPriority);OSStart ( );}

Step C: Main functionStep C: Main function


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static void FirstTask (void *taskPointer) {/* Create apllication related highest prio task */OSTaskCreate (ReadTask, void (*) 0,(void *)&ReadTaskStack [ReadTaskStackSize], ReadTaskPriority);

Step D: First taskStep D: First task


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OSTaskCreate (DeliveryTask, void (*) 0,(void *)&DeliveryTaskStack[DeliveryTaskStackSize], DeliveryTaskPriority); /*System clock time set */OSTimeSet (presetTime);OSTickInit (); /* Initiate system timer ticking*/

Step D: First task…Step D: First task…


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static void FirstTask (void *taskPointer) {...while (1) {OSTaskSuspend (FirstTaskPriority);}

Step D: First task…Step D: First task…


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static void ReadTask (void *taskPointer) {...while (1) {...OSSemtake (Sem_mCoin); /* after this instruction executes the amount can be incremented by ReadTask as Sem_mCoin = 0 (not in released state)*/.....

Step E: Read task taking the mutexStep E: Read task taking the mutex--semaphore before critical section startssemaphore before critical section starts


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/* Critical section/* Critical section run for collecting the coins for collecting the coins and releasing the semaphore for permitting a and releasing the semaphore for permitting a reset later on the coinreset later on the coin--amount after delivery of amount after delivery of chocolate by other task critical section*/chocolate by other task critical section*/• OSSemPost (Sem_mCoin); /* after this instruction executes the next task section can reset the amount */• OSSemPost (SemFlagChocolate); •....... } }

Step E: Read task posting the mutexStep E: Read task posting the mutex--semaphore…semaphore…


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static void DeliveryTask (void *taskPointer) {..while (1) {OSSemTake (SemFlagChocolate);...; ...; ...; OSSemTake (Sem_mCoin);/* after these instruction executes Sem_mCoin

and SemFlagChocolate = 0 (taken state) Critical Critical section 2 starts section 2 starts */

Step F: Delivery task taking the required Step F: Delivery task taking the required semaphoressemaphores


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.../*Reset amount by setting amount = 0 after the chocolate delivery*/OSSemPost(Sem_mCoin); /* Release mutex to let read task section increment the amounts */...;/* code for resuming Delayed ReadTask*/ }}

Step F: Delivery task taking the required Step F: Delivery task taking the required semaphoressemaphores


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SummarySummary


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We learntWe learnt• Use of the OS Functions • Initial setting of system-time, starting

system ticks, • Initiating OS Event semaphore IPC,

initialing semaphore, taking and releasing a semaphore when using it as event flag.


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We learntWe learnt• Initiating OS Event semaphore IPC,

initializing semaphore, taking and releasing a semaphore, when using it for a mutex fro exclusive access to run one of the two task sections .


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We learntWe learnt• A simplicity feature of µC/OS-II is that

same semaphore functions are used for binary semaphore, for event signaling flag, resource key (mutex) and counting.


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End of Lesson-6 Chapter 9 onµµC/OS-II Semaphore Functions

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Lesson PROGRAMMING 6 - Devi Ahilya … of Lesson-6 Chapter 9 on µ C/OS-II Semaphore Functions...

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