FreeRTOS中Systick的问题

硬件花园

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在Cortex-M内核中，系统节拍由Systick时钟提供，当配置好系统滴答时钟后，每次时钟中断就会触发中断处理数xPortSysTickHandler()。

void xPortSysTickHandler( void ){    /* The SysTick runs at the lowest interrupt priority, so when this interrupt     * executes all interrupts must be unmasked.  There is therefore no need to     * save and then restore the interrupt mask value as its value is already     * known - therefore the slightly faster vPortRaiseBASEPRI() function is used     * in place of portSET_INTERRUPT_MASK_FROM_ISR(). */    vPortRaiseBASEPRI();//屏蔽归属FreeRTOS的中断优先级    {        /* Increment the RTOS tick. */        if( xTaskIncrementTick() != pdFALSE )//时钟计数处理        {            /* A context switch is required.  Context switching is performed in             * the PendSV interrupt.  Pend the PendSV interrupt. */            portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;//如果需要切换上下文操作，PendSV标记置位        }    }
) O# \" D2 Q, t( c$ G4 C    vPortClearBASEPRIFromISR();}
% v. H. ~3 X) x- f: g$ @$ \这部分主要是依靠 xTaskIncrementTick()，来判断任务切换是否在此次系统时钟中断时被需要。如果是，则PendSV标记置位，等待触发PendSV中断。来看看 xTaskIncrementTick()。

BaseType_t xTaskIncrementTick( void ){    TCB_t * pxTCB;    TickType_t xItemValue;    BaseType_t xSwitchRequired = pdFALSE;
    /* Called by the portable layer each time a tick interrupt occurs.     * Increments the tick then checks to see if the new tick value will cause any     * tasks to be unblocked. */    traceTASK_INCREMENT_TICK( xTickCount );
2 h2 M0 [/ N& Y! M    if( uxSchedulerSuspended == ( UBaseType_t ) pdFALSE ) //调度是否被挂起,默认为否    {        /* Minor optimisation.  The tick count cannot change in this         * block. */        const TickType_t xConstTickCount = xTickCount + ( TickType_t ) 1;
        /* Increment the RTOS tick, switching the delayed and overflowed         * delayed lists if it wraps to 0. */        xTickCount = xConstTickCount;
0 g: Z, g% s; {$ `) L" a6 U        if( xConstTickCount == ( TickType_t ) 0U ) /*lint !e774 'if' does not always evaluate to false as it is looking for an overflow. 如果xConstTickCount为0，说明溢出了*/        {            taskSWITCH_DELAYED_LISTS();/*切换延迟列表*/        }        else        {            mtCOVERAGE_TEST_MARKER();        }
3 v5 i1 k6 ~! [, Q1 `        /* See if this tick has made a timeout expire.  Tasks are stored in         * the  queue in the order of their wake time - meaning once one task         * has been found whose block time has not expired there is no need to         * look any further down the list. */        if( xConstTickCount >= xNextTaskUnblockTime )        {            for( ; ; )            {                if( listLIST_IS_EMPTY( pxDelayedTaskList ) != pdFALSE )                {                    /* The delayed list is empty.  Set xNextTaskUnblockTime                     * to the maximum possible value so it is extremely                     * unlikely that the                     * if( xTickCount >= xNextTaskUnblockTime ) test will pass                     * next time through. */                    xNextTaskUnblockTime = portMAX_DELAY; /*lint !e961 MISRA exception as the casts are only redundant for some ports. */                    break;                }                else                {                    /* The delayed list is not empty, get the value of the                     * item at the head of the delayed list.  This is the time                     * at which the task at the head of the delayed list must                     * be removed from the Blocked state. */                    pxTCB = listGET_OWNER_OF_HEAD_ENTRY( pxDelayedTaskList ); /*lint !e9079 void * is used as this macro is used with timers and co-routines too.  Alignment is known to be fine as the type of the pointer stored and retrieved is the same. */                    xItemValue = listGET_LIST_ITEM_VALUE( &( pxTCB->xStateListItem ) );
3 @+ V, e3 p% b' c; a. j关键问题是，这个函数使用到了 pxDelayedTaskList, 这定义在本文件
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PRIVILEGED_DATA static List_t * volatile pxDelayedTaskList;
6 M  j* z( h3 l# b4 E  b' \该变量初始化为0，该变量正常初始化位置在创建 Task 对象等的函数中， 也就是说，如果在Tick中断到来时，如果还没有任务被创建，就会导致不可预期的结果，中断服务函数会使用这个野指针，执行任务切换。这会导致触发栈溢出钩子函数，或者是直接Hardfault。有些硬件初始化需要借助delay功能，不得不在初始化之前配置SysTick。而又不希望在硬件初始化阶段触发这个Bug。所以在配置SysTick之前，先创建一个初始化任务，初始化 pxDelayedTaskList 这个指针，在初始化任务里配置SysTick，和其他初始化，这样能够避免此类问题。或者是在配置SysTick的时候屏蔽中断，一切准备就绪后，开启中断。执行vTaskStartScheduler()默认创建一个空闲任务。==========
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5 ?4 A+ V. n, Y% e  Q3 G==========
原文链接：点击阅读原文
; }: ~& Z1 [0 m; J平台：博客园
作者：ficusdxx

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