uC/OS-II 内核源码分析（三）---时间管理机制详解



 uC/OS-II时间管理相对比较简单，大体包含定时器创建、启动、删除，systick中断isr，timer线程处理，可能会涉及到任务调度，优先级反转，线程同步相关知识，本部分内容重点讲解定时器机制，其他内容，可在本人博客中的其他文章查询，欢迎提出宝贵意见，共同学习uC/OS-II，本书遵循先总后分的一贯做法，希望大家提出宝贵建议和意见。

一.废话不多讲，直接上图

 上图基本涵盖了定时器的全部内容，首先大概解释下上图的内容，uC/OS中的定时器机制，是靠芯片内部的systick驱动的，当然也可以用其他的timer(已超出本文范围），uC/OS内核的定时机制是一种软定时机制，这样就具有很好的跨平台性，系统在每个systick中断到来时，在中断处理函数中释放定时器线程信号量，同时将各线程的tcb结构体的OSTCBDly成员减1，当减为0时，将对应的线程tcb移动到就绪队列表中（关于任务调度内容会在后续的博客中持续更新），在退出中断处理函数时，触发一次pendsv异常，进行任务上下文切换，此时定时器处理线程从阻塞状态恢复到运行状态（当有更高优先级的任务时例外），开始递减每个定时器的计数值，当发现超时时，执行对应的处理函数。

下面逐条逐个函数进行分析

二. 创建定时器

/*
*********************************************************************************************************
*                                           CREATE A TIMER
*
* Description: This function is called by your application code to create a timer.
*
* Arguments  : dly           Initial delay.
*                            If the timer is configured for ONE-SHOT mode, this is the timeout used.
*                            If the timer is configured for PERIODIC mode, this is the first timeout to
*                               wait for before the timer starts entering periodic mode.
*
*              period        The 'period' being repeated for the timer.
*                               If you specified 'OS_TMR_OPT_PERIODIC' as an option, when the timer
*                               expires, it will automatically restart with the same period.
*
*              opt           Specifies either:
*                               OS_TMR_OPT_ONE_SHOT       The timer counts down only once
*                               OS_TMR_OPT_PERIODIC       The timer counts down and then reloads itself
*
*              callback      Is a pointer to a callback function that will be called when the timer expires.
*                               The callback function must be declared as follows:
*
*                               void MyCallback (OS_TMR *ptmr, void *p_arg);
*
*              callback_arg  Is an argument (a pointer) that is passed to the callback function when it is called.
*
*              pname         Is a pointer to an ASCII string that is used to name the timer.  Names are
*                               useful for debugging.
*
*              perr          Is a pointer to an error code.  '*perr' will contain one of the following:
*                               OS_ERR_NONE
*                               OS_ERR_TMR_INVALID_DLY     you specified an invalid delay
*                               OS_ERR_TMR_INVALID_PERIOD  you specified an invalid period
*                               OS_ERR_TMR_INVALID_OPT     you specified an invalid option
*                               OS_ERR_TMR_ISR             if the call was made from an ISR
*                               OS_ERR_TMR_NON_AVAIL       if there are no free timers from the timer pool
*
* Returns    : A pointer to an OS_TMR data structure.
*              This is the 'handle' that your application will use to reference the timer created.
*********************************************************************************************************
*/

OS_TMR  *OSTmrCreate (INT32U           dly,
                      INT32U           period,
                      INT8U            opt,
                      OS_TMR_CALLBACK  callback,
                      void            *callback_arg,
                      INT8U           *pname,
                      INT8U           *perr)
{
    OS_TMR   *ptmr;

#ifdef OS_SAFETY_CRITICAL
    if (perr == (INT8U *)0) {
        OS_SAFETY_CRITICAL_EXCEPTION();
        return ((OS_TMR *)0);
    }
#endif

#ifdef OS_SAFETY_CRITICAL_IEC61508
    if (OSSafetyCriticalStartFl