UC/OS-II基础知识之时钟

UC/OS-II基础知识之时钟

与大多数计算机系统一样，UC/OS-II用硬件定时器产生一个毫秒级的周期性中断来实现系统时钟，最小的时钟单位就是两次中断之间的间隔时间，这个最小时钟单位就叫做时钟节拍。硬件定时器以时钟节拍为周期定时的产生中断，该中断的中断服务程序叫做OSTickISR（），中断服务程序通过调用函数OSTimeTick（）来完成系统在每个时钟节拍时需要做的工作。OSTickISR（）是用汇编语言来编写的，其源码如下：

void OSTickISR()
{
    OSIntEnter();
    OSTimeTick();
    OSIntExit();
}

OSTimeTick（）的源码如下：

void  OSTimeTick (void)
{
    OS_TCB    *ptcb;
#if OS_TICK_STEP_EN > 0
    BOOLEAN    step;
#endif
#if OS_CRITICAL_METHOD == 3                                /* Allocate storage for CPU status register     */
    OS_CPU_SR  cpu_sr = 0;
#endif

#if OS_TIME_TICK_HOOK_EN > 0
    OSTimeTickHook();                                      /* Call user definable hook                     */
#endif
#if OS_TIME_GET_SET_EN > 0
    OS_ENTER_CRITICAL();                                   /* Update the 32-bit tick counter               */
    OSTime++;
    OS_EXIT_CRITICAL();
#endif
    if (OSRunning == OS_TRUE) {
#if OS_TICK_STEP_EN > 0
        switch (OSTickStepState) {                         /* Determine whether we need to process a tick  */
            case OS_TICK_STEP_DIS:                         /* Yes, stepping is disabled                    */
                 step = OS_TRUE;
                 break;

            case OS_TICK_STEP_WAIT:                        /* No,  waiting for uC/OS-View to set ...       */
                 step = OS_FALSE;                          /*      .. OSTickStepState to OS_TICK_STEP_ONCE */
                 break;

            case OS_TICK_STEP_ONCE:                        /* Yes, process tick once and wait for next ... */
                 step            = OS_TRUE;                /*      ... step command from uC/OS-View        */
                 OSTickStepState = OS_TICK_STEP_WAIT;
                 break;

            default:                                       /* Invalid case, correct situation              */
                 step            = OS_TRUE;
                 OSTickStepState = OS_TICK_STEP_DIS;
                 break;
        }
        if (step == OS_FALSE) {                            /* Return if waiting for step command           */
            return;
        }
#endif
        ptcb = OSTCBList;                                  /* Point at first TCB in TCB list               */
        while (ptcb->OSTCBPrio != OS_TASK_IDLE_PRIO) {     /* Go through all TCBs in TCB list              */
            OS_ENTER_CRITICAL();
            if (ptcb->OSTCBDly != 0) {                     /* No, Delayed or waiting for event with TO     */
                if (--ptcb->OSTCBDly == 0) {               /* Decrement nbr of ticks to end of delay       */
                                                           /* Check for timeout                            */
                    if ((ptcb->OSTCBStat & OS_STAT_PEND_ANY) != OS_STAT_RDY) {
                        ptcb->OSTCBStat   &= ~OS_STAT_PEND_ANY;                /* Yes, Clear status flag   */
                        ptcb->OSTCBPendTO  = OS_TRUE;                          /* Indicate PEND timeout    */
                    } else {
                        ptcb->OSTCBPendTO  = OS_FALSE;
                    }

                    if ((ptcb->OSTCBStat & OS_STAT_SUSPEND) == OS_STAT_RDY) {  /* Is task suspended?       */
                        OSRdyGrp               |= ptcb->OSTCBBitY;             /* No,  Make ready          */
                        OSRdyTbl[ptcb->OSTCBY] |= ptcb->OSTCBBitX;
                    }
                }
            }
            ptcb = ptcb->OSTCBNext;                        /* Point at next TCB in TCB list                */
            OS_EXIT_CRITICAL();
        }
    }
}

UC/OS-II在每次响应定时中断时调用OSTimeTick（）做了两件事，一是给计数器OSTime加1，二是了解每个任务的延时状态，使其中已经到延时时限的非挂起任务进入就绪状态。