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两个Delay函数
有两个延时函数
vTaskDelay：至少等待指定个数的Tick Interrupt才能变为就绪态
xTaskDelayUtil：等待到指定的绝对时刻，才能变为就绪态
个人感觉这两个延时函数就是，比如一个我等3个小时，一个是我等到下午3点的区别。
两个函数的原型如下：
vTaskDelay:
void vTaskDelay( const TickType_t xTicksToDelay ){BaseType_t xAlreadyYielded = pdFALSE;/* A delay time of zero just forces a reschedule. */if( xTicksToDelay > ( TickType_t ) 0U ){configASSERT( uxSchedulerSuspended == 0 );vTaskSuspendAll();{traceTASK_DELAY();/* A task that is removed from the event list while the* scheduler is suspended will not get placed in the ready* list or removed from the blocked list until the scheduler* is resumed.** This task cannot be in an event list as it is the currently* executing task. */prvAddCurrentTaskToDelayedList( xTicksToDelay, pdFALSE );}xAlreadyYielded = xTaskResumeAll();}else{mtCOVERAGE_TEST_MARKER();}/* Force a reschedule if xTaskResumeAll has not already done so, we may* have put ourselves to sleep. */if( xAlreadyYielded == pdFALSE ){portYIELD_WITHIN_API();}else{mtCOVERAGE_TEST_MARKER();}}
xTaskDelayUtil
BaseType_t xTaskDelayUntil( TickType_t * const pxPreviousWakeTime,const TickType_t xTimeIncrement ){TickType_t xTimeToWake;BaseType_t xAlreadyYielded, xShouldDelay = pdFALSE;configASSERT( pxPreviousWakeTime );configASSERT( ( xTimeIncrement > 0U ) );configASSERT( uxSchedulerSuspended == 0 );vTaskSuspendAll();{/* Minor optimisation. The tick count cannot change in this* block. */const TickType_t xConstTickCount = xTickCount;/* Generate the tick time at which the task wants to wake. */xTimeToWake = *pxPreviousWakeTime + xTimeIncrement;if( xConstTickCount < *pxPreviousWakeTime ){/* The tick count has overflowed since this function was* lasted called. In this case the only time we should ever* actually delay is if the wake time has also overflowed,* and the wake time is greater than the tick time. When this* is the case it is as if neither time had overflowed. */if( ( xTimeToWake < *pxPreviousWakeTime ) && ( xTimeToWake > xConstTickCount ) ){xShouldDelay = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}}else{/* The tick time has not overflowed. In this case we will* delay if either the wake time has overflowed, and/or the* tick time is less than the wake time. */if( ( xTimeToWake < *pxPreviousWakeTime ) || ( xTimeToWake > xConstTickCount ) ){xShouldDelay = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}}/* Update the wake time ready for the next call. */*pxPreviousWakeTime = xTimeToWake;if( xShouldDelay != pdFALSE ){traceTASK_DELAY_UNTIL( xTimeToWake );/* prvAddCurrentTaskToDelayedList() needs the block time, not* the time to wake, so subtract the current tick count. */prvAddCurrentTaskToDelayedList( xTimeToWake - xConstTickCount, pdFALSE );}else{mtCOVERAGE_TEST_MARKER();}}xAlreadyYielded = xTaskResumeAll();/* Force a reschedule if xTaskResumeAll has not already done so, we may* have put ourselves to sleep. */if( xAlreadyYielded == pdFALSE ){portYIELD_WITHIN_API();}else{mtCOVERAGE_TEST_MARKER();}return xShouldDelay;}
下面是图示：
使用vTaskDelay(n)时，进入，退出vTaskDelay的时间间隔至少是n个Tick中断
使用xTaskDelayUtil(&Pre,n)时，前后两次退出xTaskDelayUntil的时间至少是n个Tick中断
退出xTaskDelayUntil时任务就进入就绪态，一般都能得到执行机会
所以可以使用xTaskDelayUntil来让任务周期性的运行
实验证明
程序创建2个任务
Task1：
高优先级
设置变量flag为1，然后调用vTaskDelay(xDelay50ms)或vTaskDelayUntil(&xLastWakeTime,xDelay50ms)
Task2:
低优先级
设置变量flag=0
main函数代码如下：
int main(void)
{prvSetupHardware();/*Task1的优先级更高，Task1先执行*/xTaskCreate(vTask1,"Task1",1000,NULL,2,NULL);xTaskCreate(vTask2,"Task2",1000,NULL,1,NULL);/*启动调度器*/vTaskStartScheduler();/*如果程序运行到这里，就表示出错了，一般是内存不足*/return 0;}Task1的代码中使用条件开关来选择Delay函数，把#if 1 改为 #if 0 就可以使用vTaskDelayUntil，代码如下：
void vTask1(void *pvParameters)
{const TickType_t xDelay50ms = pdMS_TO_TICKS(50UL);TickType_t xLastWakeTime;int i;/*获得当前的Tick Count*/xLastWakeTime = xTaskGetTickCount();for(;;){flag =1;/*故意加入多个循环，让程序运行时间长一点*/for(i=0;i<5;i++)printf("Task1 is running\r\n");
#if 1vTaskDelay(xDelay50ms);
#else vTaskDelayUntil(&PreWakeTime,xDelay50ms);}
}使用MDK的逻辑分析仪，可以观察flag变量的bit波形，如下：
flag为1时表示，Task1正在运行，flag为0时表示Task2正在运行，也就是Task1处于阻塞状态
vTaskDelay：指定的是阻塞时间
vTaskDelayUntil：指定的是任务执行的间隔，周期
