Micro-Controller Operating Systems

Micro C/OS-II The Real Time Kernel

Agenda

Task Control Blocks Ready Lists Task Scheduling An Example - CPU Usage

Critical Sections Disables the interrupts to access critical sections of code and reenables interrupts when finished with Interrupt disable time depends on the processor architecture and the quality of the code generated by the compiler MicroC/OS-II provides two macros: OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL() Tasks Typically an infinite loop

      void YourTask (void *pdata)
     {
            for (;;)  {
                  /*USER CODE*/
              Call one of mC/OS-II’s services:

OSMboxPend(); OSQPend();

                   OSSemPend();
                   OSTaskDel(OS_PRIO_SELF);
                   OSTaskSuspend 		  	      (OS_PRIO_SELF);
                   OSTimeDlyHMSM();
                 /*USER CODE*/
                          }
                    } 

Void YourTask (void *pdata) { /*USER CODE*/ OSTaskDel (OS_PRIO_SELF); } Task States Task Control Blocks When a task is created, a Task Control Block, a data structure OS_TCB,is assigned All OS_TCBs reside in RAM OS_TCBs contain the information on the current top of the task bottom of the stack size of the stack in bytes an identifier for the task task priority state of the task

Task Control Blocks The max number of tasks that an application can have is in OS_CFG.H and it can be changed

All OS_TCBs are placed in OSTCBTbl[]

When OS is initialized,all OS_TCBs in the table are linked in a singly linked list of free OS_TCBs

The creation of a task assigns OS_TCB pointed to by OSTCBFreeList to itself and OSTCBFreeList now points to the next OS_TCB in the chain Ready List Each task is assigned a unique priority level between 0 and OS_LOWEST_PRIO which is assigned to an idle task

Each task that is ready to run is placed in a ready list consisting of two variables, OSRdyGrp and OSRdyTbl[]

The scheduler determines the lowest priority number that has its bit set in OSRdyTbl[] to run the next task

mC/OS-II Ready list Contents of OSMapTbl[] Making a task ready OSRdyGrp |= OSMaptbl [prio>>3]; OSRdyTbl [prio>>3] |= OSMapTbl [prio & 0x07]; e.g: Say priority of a task is 12 and the initial contents of OSRdyGrp is 01000010 and

              OSRdyTbl [1] is = 10100000

Then, tbl OSMap [12>>3 =00000001]=00000010

   OSRdyGrp = 01000010 OR 00000010 =01000010;

OSMapTbl [prio & 07=00000100]=00010000 OSRdyTbl [prio>>3=00000001]=10100000 OR 00010000 = 10110000 indicating that task priority 12 has been set to 1. More on ready list Removing a task from the ready list

    if ((OSRdyTbl [prio>>3]&=~OSMaptbl [prio & 0x07]) ==0)
    OSRdyGrp &= ~OSMapTbl[prio>>3];

E.g: say initial conditions are OSRdyGrp =00000010

       and OSRdyTbl[1] =00010000 and we want to remove  the task
        whose priority is 12 from the ready list,then

~OSMapTbl [12 & 07 =04]=111011111 OSRdyTbl [12>>3=01] =00010000 & 11101111=00000000; Therefore, OSRdyGrp = 00000010 & 11111101 =00000000

More on ready list Finding the highest priority task ready to run

    y =OSUnMaptbl [OSRdyGrp];
    x =OSUnMapTbl [OSRdyTbl[y]];
    prio = (y<<3) +x;

E.g: say initial conditions are OSRdyGrp =01101000

       and OSRdyTbl[3] =11100100,then 

y = returns the least significant bit that is set ;that is 3 x = OSunMapTbl [ 11100100]= returns the least significant bit that is set ;that is 2

      prio = 00000110<<3 +x =00011000 + 00000010=00011010
                  =26;
 

OSUnMapTbl[] Task Scheduling OSSched() performs task level scheduling ISR level scheduling is handled by OSIntExit() OSSched() exits if called from an ISR or if scheduling has been disabled Or else, OSSched() determines the priority of the highest priority task that is ready to run If the highest priority task is not the current one,then context switch occurs

Locking the Scheduler The OSSchedLock and OSSchedUnlock functions are used to disable and reenable task scheduling void OSSchedLock (void) {

   		 if (OSRunning ==TRUE ) {
        		 OS_ENTER_CRITICAL();
        		 OSLockNesting++;
         		 OS_EXIT_CRITICAL();
       		 }
  		 }

Idle Task

OSTaskIdle() increments a 32 bit counter called OSIdleCtr to determine the percent CPU time actually being consumed by the application software Statistics Task

Called the OSTaskStat() provides run time statistics - executes every second and computes the percent CPU usage

It is created when the configuration constant OS_TASK_STAT_EN in OS_CFG.H is set to 1 If the application is to use the statistic task, then OSStatInit() should be called from the first task created in the application during initialization

Statistics Task Void main (void) {

   OSInit();

/*install uc/OS-II’s context switch vector ,create your startup task,say TaskStart*/

   OSStart();

}

void TaskStart (void *pdata) { /* install and initialze uCOS-II ‘s ticker*/

  OSStatInit();

/*create your application task(s)*/

  for (;;)   {

/* Code for TaskStart() goes here*/

    }

}

Statistic Task Initialization