=============================== 博客点滴积累，部分话语和知识点来源于网络，感谢网络资源的提供者======
freertos的定时器实现比较简单，创建一个守护进程，创建一个定时器队列，当创建一个定时器时，就会将该定时器添加到定时器链表中，定时器守护进程循环读取定时器链表的定时器，判断是否到期，到期就执行。
vTaskStartScheduler( ) //tasks.c
xTimerCreateTimerTask(); //timers.c
/*创建一个后台守护任务*/
xTaskCreate( prvTimerTask, "Tmr Svc", ( uint16_t ) configTIMER_TASK_STACK_DEPTH, NULL, ( ( UBaseType_t ) configTIMER_TASK_PRIORITY ) | portPRIVILEGE_BIT, &xTimerTaskHandle );
分析这个定时器task
for( ;; )
{
/*获取下一个要执行的地上那个定时器*/
xNextExpireTime = prvGetNextExpireTime( &xListWasEmpty );
/ *如果计时器过期了,处理它。否则,阻塞这一任务直到一个计时器到期,或收到一个命令。* /
prvProcessTimerOrBlockTask( xNextExpireTime, xListWasEmpty );
/*处理接收到的命令*/
prvProcessReceivedCommands();
}
static TickType_t prvGetNextExpireTime( BaseType_t * const pxListWasEmpty )
{
TickType_t xNextExpireTime;
/* 定时器链表，先执行的会放在链表 头，查看定时器链表 是否为空，不为空则获取链表 头*/
*pxListWasEmpty = listLIST_IS_EMPTY( pxCurrentTimerList );
if( *pxListWasEmpty == pdFALSE )
{
xNextExpireTime = listGET_ITEM_VALUE_OF_HEAD_ENTRY( pxCurrentTimerList );
}
else
{
/* Ensure the task unblocks when the tick count rolls over. */
xNextExpireTime = ( TickType_t ) 0U;
}
return xNextExpireTime;
}
---------------------------------------------------------------------
这两个函数在文章的最后面
prvProcessTimerOrBlockTask( xNextExpireTime, xListWasEmpty );
prvProcessReceivedCommands();
--------------------------------------
static TickType_t prvSampleTimeNow( BaseType_t * const pxTimerListsWereSwitched )
{
TickType_t xTimeNow;
PRIVILEGED_DATA static TickType_t xLastTime = ( TickType_t ) 0U; /*lint !e956 Variable is only accessible to one task. */
/*获取当前tick值*/
xTimeNow = xTaskGetTickCount();
/*判断是否溢出，溢出就切换定时器list*/
if( xTimeNow < xLastTime )
{
prvSwitchTimerLists();
*pxTimerListsWereSwitched = pdTRUE;
}
else
{
*pxTimerListsWereSwitched = pdFALSE;
}
/*更新tick值*/
xLastTime = xTimeNow;
return xTimeNow;
}
/*向定时器队列xTimerQueue发消息*/
BaseType_t xTimerGenericCommand( TimerHandle_t xTimer, const BaseType_t xCommandID, const TickType_t xOptionalValue, BaseType_t * const pxHigherPriorityTaskWoken, const TickType_t xTicksToWait )
{
BaseType_t xReturn = pdFAIL;
DaemonTaskMessage_t xMessage;
if( xTimerQueue != NULL )
{
/* Send a command to the timer service task to start the xTimer timer. */
xMessage.xMessageID = xCommandID;
xMessage.u.xTimerParameters.xMessageValue = xOptionalValue;
xMessage.u.xTimerParameters.pxTimer = ( Timer_t * ) xTimer;
if( xCommandID < tmrFIRST_FROM_ISR_COMMAND )
{
if( xTaskGetSchedulerState() == taskSCHEDULER_RUNNING )
{
xReturn = xQueueSendToBack( xTimerQueue, &xMessage, xTicksToWait );
}
else
{
xReturn = xQueueSendToBack( xTimerQueue, &xMessage, tmrNO_DELAY );
}
}
else
{
xReturn = xQueueSendToBackFromISR( xTimerQueue, &xMessage, pxHigherPriorityTaskWoken );
}
traceTIMER_COMMAND_SEND( xTimer, xCommandID, xOptionalValue, xReturn );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
return xReturn;
}
/*将定时器插入活跃的链表中*/
static BaseType_t prvInsertTimerInActiveList( Timer_t * const pxTimer, const TickType_t xNextExpiryTime, const TickType_t xTimeNow, const TickType_t xCommandTime )
{
BaseType_t xProcessTimerNow = pdFALSE;
listSET_LIST_ITEM_VALUE( &( pxTimer->xTimerListItem ), xNextExpiryTime );
listSET_LIST_ITEM_OWNER( &( pxTimer->xTimerListItem ), pxTimer );
if( xNextExpiryTime <= xTimeNow )
{/*定时器溢出了*/
if( ( xTimeNow - xCommandTime ) >= pxTimer->xTimerPeriodInTicks )
{/*当前的时间与要执行的时间已经超过PeriodInTicks,要立即执行*/
xProcessTimerNow = pdTRUE;
}
else
{
vListInsert( pxOverflowTimerList, &( pxTimer->xTimerListItem ) );
}
}
else
{
if( ( xTimeNow < xCommandTime ) && ( xNextExpiryTime >= xCommandTime ) )
{
xProcessTimerNow = pdTRUE;
}
else
{
vListInsert( pxCurrentTimerList, &( pxTimer->xTimerListItem ) );
}
}
return xProcessTimerNow;
}
/*-----------------------------------------------------------*/
static void prvProcessExpiredTimer( const TickType_t xNextExpireTime, const TickType_t xTimeNow )
{
BaseType_t xResult;/*获取要执行的定时器list head*/
Timer_t * const pxTimer = ( Timer_t * ) listGET_OWNER_OF_HEAD_ENTRY( pxCurrentTimerList );
/*删除要执行的定时器list item*/
( void ) uxListRemove( &( pxTimer->xTimerListItem ) );
traceTIMER_EXPIRED( pxTimer );
/*判断是否是否自动重载的周期定时器*/
if( pxTimer->uxAutoReload == ( UBaseType_t ) pdTRUE )
{
if( prvInsertTimerInActiveList( pxTimer, ( xNextExpireTime + pxTimer->xTimerPeriodInTicks ), xTimeNow, xNextExpireTime ) == pdTRUE )
{
xResult = xTimerGenericCommand( pxTimer, tmrCOMMAND_START_DONT_TRACE, xNextExpireTime, NULL, tmrNO_DELAY );
configASSERT( xResult );
( void ) xResult;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* Call the timer callback. */
pxTimer->pxCallbackFunction( ( TimerHandle_t ) pxTimer );
}
static void prvProcessTimerOrBlockTask( const TickType_t xNextExpireTime, const BaseType_t xListWasEmpty )
{
TickType_t xTimeNow;BaseType_t xTimerListsWereSwitched;
vTaskSuspendAll();
{
/*判断是否溢出，溢出就切换定时器list*/
xTimeNow = prvSampleTimeNow( &xTimerListsWereSwitched );
if( xTimerListsWereSwitched == pdFALSE )
{
/* 定时器没有溢出，没有切换定时器链表，判断定时器是否过期，过期立即执行 */
if( ( xListWasEmpty == pdFALSE ) && ( xNextExpireTime <= xTimeNow ) )
{
( void ) xTaskResumeAll();
prvProcessExpiredTimer( xNextExpireTime, xTimeNow );
}
else
{
vQueueWaitForMessageRestricted( xTimerQueue, ( xNextExpireTime - xTimeNow ) );
if( xTaskResumeAll() == pdFALSE )
{
portYIELD_WITHIN_API();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
}
else
{
( void ) xTaskResumeAll();
}
}
}
static void prvProcessReceivedCommands( void )
{
DaemonTaskMessage_t xMessage;Timer_t *pxTimer;BaseType_t xTimerListsWereSwitched, xResult;TickType_t xTimeNow;
while( xQueueReceive( xTimerQueue, &xMessage, tmrNO_DELAY ) != pdFAIL ) /*lint !e603 xMessage does not have to be initialised as it is passed out, not in, and it is not used unless xQueueReceive() returns pdTRUE. */
{
#if ( INCLUDE_xTimerPendFunctionCall == 1 )
{
/* Negative commands are pended function calls rather than timer
commands. */
if( xMessage.xMessageID < ( BaseType_t ) 0 )
{
const CallbackParameters_t * const pxCallback = &( xMessage.u.xCallbackParameters );
/* The timer uses the xCallbackParameters member to request a
callback be executed. Check the callback is not NULL. */
configASSERT( pxCallback );
/* Call the function. */
pxCallback->pxCallbackFunction( pxCallback->pvParameter1, pxCallback->ulParameter2 );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* INCLUDE_xTimerPendFunctionCall */
/* Commands that are positive are timer commands rather than pended
function calls. */
if( xMessage.xMessageID >= ( BaseType_t ) 0 )
{
/* The messages uses the xTimerParameters member to work on a
software timer. */
pxTimer = xMessage.u.xTimerParameters.pxTimer;
if( listIS_CONTAINED_WITHIN( NULL, &( pxTimer->xTimerListItem ) ) == pdFALSE )
{
/* The timer is in a list, remove it. */
( void ) uxListRemove( &( pxTimer->xTimerListItem ) );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
traceTIMER_COMMAND_RECEIVED( pxTimer, xMessage.xMessageID, xMessage.u.xTimerParameters.xMessageValue );
xTimeNow = prvSampleTimeNow( &xTimerListsWereSwitched );
switch( xMessage.xMessageID )
{
case tmrCOMMAND_START :
case tmrCOMMAND_START_FROM_ISR :
case tmrCOMMAND_RESET :
case tmrCOMMAND_RESET_FROM_ISR :
case tmrCOMMAND_START_DONT_TRACE :
/* Start or restart a timer. */
if( prvInsertTimerInActiveList( pxTimer, xMessage.u.xTimerParameters.xMessageValue + pxTimer->xTimerPeriodInTicks, xTimeNow, xMessage.u.xTimerParameters.xMessageValue ) == pdTRUE )
{
pxTimer->pxCallbackFunction( ( TimerHandle_t ) pxTimer );
traceTIMER_EXPIRED( pxTimer );
if( pxTimer->uxAutoReload == ( UBaseType_t ) pdTRUE )
{
xResult = xTimerGenericCommand( pxTimer, tmrCOMMAND_START_DONT_TRACE, xMessage.u.xTimerParameters.xMessageValue + pxTimer->xTimerPeriodInTicks, NULL, tmrNO_DELAY );
configASSERT( xResult );
( void ) xResult;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
break;
case tmrCOMMAND_STOP :
case tmrCOMMAND_STOP_FROM_ISR :
/* The timer has already been removed from the active list.
There is nothing to do here. */
break;
case tmrCOMMAND_CHANGE_PERIOD :
case tmrCOMMAND_CHANGE_PERIOD_FROM_ISR :
pxTimer->xTimerPeriodInTicks = xMessage.u.xTimerParameters.xMessageValue;
configASSERT( ( pxTimer->xTimerPeriodInTicks > 0 ) );
( void ) prvInsertTimerInActiveList( pxTimer, ( xTimeNow + pxTimer->xTimerPeriodInTicks ), xTimeNow, xTimeNow );
break;
case tmrCOMMAND_DELETE :
/* The timer has already been removed from the active list, just free up the memory. */
vPortFree( pxTimer );
break;
default :
/* Don't expect to get here. */
break;
}
}
}
}
/*创建定时器的函数*/
TimerHandle_t xTimerCreate( const char * const pcTimerName, const TickType_t xTimerPeriodInTicks, const UBaseType_t uxAutoReload, void * const pvTimerID, TimerCallbackFunction_t pxCallbackFunction ) /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
{
Timer_t *pxNewTimer;
/* Allocate the timer structure. */
if( xTimerPeriodInTicks == ( TickType_t ) 0U )
{
pxNewTimer = NULL;
}
else
{
pxNewTimer = ( Timer_t * ) pvPortMalloc( sizeof( Timer_t ) );
if( pxNewTimer != NULL )
{
/* Ensure the infrastructure used by the timer service task has been
created/initialised. */
prvCheckForValidListAndQueue();
/* Initialise the timer structure members using the function parameters. */
pxNewTimer->pcTimerName = pcTimerName;
pxNewTimer->xTimerPeriodInTicks = xTimerPeriodInTicks;
pxNewTimer->uxAutoReload = uxAutoReload;
pxNewTimer->pvTimerID = pvTimerID;
pxNewTimer->pxCallbackFunction = pxCallbackFunction;
vListInitialiseItem( &( pxNewTimer->xTimerListItem ) );
traceTIMER_CREATE( pxNewTimer );
}
else
{
traceTIMER_CREATE_FAILED();
}
}
/* 0 is not a valid value for xTimerPeriodInTicks. */
configASSERT( ( xTimerPeriodInTicks > 0 ) );
return ( TimerHandle_t ) pxNewTimer;
}
/*--------------------------------------------