11.FreeRTOS延時函式
阿新 • • 發佈：2020-12-22
FreeRTOS延時簡易分析
- 架構：Cortex-M3
- 版本：FreeRTOS V9.0.0
- 前言：在很多情況，需要使用延時來達到一些效果，那麼FreeRTOS的延時是怎麼實現的
1.相對延時vTaskDelay
只有一個引數就是等待時間
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(); } }
具體呼叫prvAddCurrentTaskToDelayedList
static void prvAddCurrentTaskToDelayedList( TickType_t xTicksToWait, const BaseType_t xCanBlockIndefinitely ) { TickType_t xTimeToWake; const TickType_t xConstTickCount = xTickCount; #if( INCLUDE_xTaskAbortDelay == 1 ) { /* About to enter a delayed list, so ensure the ucDelayAborted flag is reset to pdFALSE so it can be detected as having been set to pdTRUE when the task leaves the Blocked state. */ pxCurrentTCB->ucDelayAborted = pdFALSE; } #endif /* Remove the task from the ready list before adding it to the blocked list as the same list item is used for both lists. */ if( uxListRemove( &( pxCurrentTCB->xStateListItem ) ) == ( UBaseType_t ) 0 ) { /* The current task must be in a ready list, so there is no need to check, and the port reset macro can be called directly. */ portRESET_READY_PRIORITY( pxCurrentTCB->uxPriority, uxTopReadyPriority ); } else { mtCOVERAGE_TEST_MARKER(); } #if ( INCLUDE_vTaskSuspend == 1 ) { if( ( xTicksToWait == portMAX_DELAY ) && ( xCanBlockIndefinitely != pdFALSE ) ) { /* Add the task to the suspended task list instead of a delayed task list to ensure it is not woken by a timing event. It will block indefinitely. */ vListInsertEnd( &xSuspendedTaskList, &( pxCurrentTCB->xStateListItem ) ); } else { /* Calculate the time at which the task should be woken if the event does not occur. This may overflow but this doesn't matter, the kernel will manage it correctly. */ xTimeToWake = xConstTickCount + xTicksToWait; /* The list item will be inserted in wake time order. */ listSET_LIST_ITEM_VALUE( &( pxCurrentTCB->xStateListItem ), xTimeToWake ); if( xTimeToWake < xConstTickCount ) { /* Wake time has overflowed. Place this item in the overflow list. */ vListInsert( pxOverflowDelayedTaskList, &( pxCurrentTCB->xStateListItem ) ); } else { /* The wake time has not overflowed, so the current block list is used. */ vListInsert( pxDelayedTaskList, &( pxCurrentTCB->xStateListItem ) ); /* If the task entering the blocked state was placed at the head of the list of blocked tasks then xNextTaskUnblockTime needs to be updated too. */ if( xTimeToWake < xNextTaskUnblockTime ) { xNextTaskUnblockTime = xTimeToWake; } else { mtCOVERAGE_TEST_MARKER(); } } } } #else /* INCLUDE_vTaskSuspend */ { /* Calculate the time at which the task should be woken if the event does not occur. This may overflow but this doesn't matter, the kernel will manage it correctly. */ xTimeToWake = xConstTickCount + xTicksToWait; /* The list item will be inserted in wake time order. */ listSET_LIST_ITEM_VALUE( &( pxCurrentTCB->xStateListItem ), xTimeToWake ); if( xTimeToWake < xConstTickCount ) { /* Wake time has overflowed. Place this item in the overflow list. */ vListInsert( pxOverflowDelayedTaskList, &( pxCurrentTCB->xStateListItem ) ); } else { /* The wake time has not overflowed, so the current block list is used. */ vListInsert( pxDelayedTaskList, &( pxCurrentTCB->xStateListItem ) ); /* If the task entering the blocked state was placed at the head of the list of blocked tasks then xNextTaskUnblockTime needs to be updated too. */ if( xTimeToWake < xNextTaskUnblockTime ) { xNextTaskUnblockTime = xTimeToWake; } else { mtCOVERAGE_TEST_MARKER(); } } /* Avoid compiler warning when INCLUDE_vTaskSuspend is not 1. */ ( void ) xCanBlockIndefinitely; } #endif /* INCLUDE_vTaskSuspend */ }
首先掛起全部任務，把當前任務從狀態連結串列中移除。
如果等待的時間為portMAX_DELAY，則把當前任務插入到xSuspendedTaskList的尾部。
如果等待時間小於portMAX_DELAY，設定等待的時間（把當前時間xConstTickCount加上等待時間xTicksToWait），如果xTimeToWake < xConstTickCount意味著時間溢位了，會把當前任務掛載到另一條delayList上（FreeRTOS維護著兩條delayList，就是為了解決溢位的問題：當前時間溢位時，FreeRTOS就會選擇另一條delayList作為延時List）。如果沒有溢位就正常掛載到當前delayList上。
2.vTaskDelayUntil
void vTaskDelayUntil( 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();
}
}
傳入的引數1其實是會上次任務結束阻塞的時間，引數2是週期性等待時間。
首先把所有任務掛起，xTimeToWake就是下次結束阻塞的時間，如果xConstTickCount < *pxPreviousWakeTime說明溢位了，如果沒有溢位的話，檢查是否到結束阻塞的時間，如果沒到，就把當前任務掛載到DelayedList。
3 為什麼說vTaskDelay是相對延時？而vTaskDelayUntil是絕對延時
有一個任務A，它首先會執行2S的某個程式後，呼叫vTaskDelay延時10s。
void TaskA(void * pvParameters)
{
whlie(1)
{
dosomething(2);
vTaskDelay(10);
}
}
這10s內會全部執行其它任務，具體如圖：
而如果任務A，呼叫vTaskDelayUntil延時10s，
void TaskA(void * pvParameters)
{
static portTickType s_SystickCount;
s_SystickCount = xTaskGetTickCount();
whlie(1)
{
dosomething(2);
vTaskDelayUntil(&s_SystickCount,10);
}
}
那麼只有8秒會去執行其它程式，如圖：
這就是區別。