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FEERTOS的TICKLESS模式下最大低功耗的时间只能是349tick的原因分析

原因

开启TICKLESS模式后,在PreSleepProcessing和PostSleepProcessing中打印ulExpectedIdleTime变量,即空闲时间.输出如下

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代码如下

void PreSleepProcessing(uint32_t *ulExpectedIdleTime)
{
    log_i("ulExpectedIdleTime %lu", *ulExpectedIdleTime);
}

void PostSleepProcessing(uint32_t *ulExpectedIdleTime)
{

}

分析

为什么是349tick进入一次空闲线程?
分析FREERTOS源码,在portTASK_FUNCTION函数中调用接口,写入空闲时间值
在赋值后打印变量

if( xExpectedIdleTime >= configEXPECTED_IDLE_TIME_BEFORE_SLEEP )
{
    vTaskSuspendAll();
    {
        configASSERT( xNextTaskUnblockTime >= xTickCount );
        xExpectedIdleTime = prvGetExpectedIdleTime();
//这里打印一下时间
printf("xExpectedIdleTime = %lu", xExpectedIdleTime);

        if( xExpectedIdleTime >= configEXPECTED_IDLE_TIME_BEFORE_SLEEP )
        {
            traceLOW_POWER_IDLE_BEGIN();
            portSUPPRESS_TICKS_AND_SLEEP( xExpectedIdleTime );
            traceLOW_POWER_IDLE_END();
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }
    }
    ( void ) xTaskResumeAll();
}

输出如下

1 2	xExpectedIdleTime = FFFF FD23 I/NO_TAG [ms:0000000732] ulExpectedIdleTime 349

空闲执行时间没问题,传入函数后有问题.
进入vPortSuppressTicksAndSleep分析,找到如下代码

/* Make sure the SysTick reload value does not overflow the counter. */
/*确保 SysTick 重载值不会溢出计数器。*/
if( xExpectedIdleTime > xMaximumPossibleSuppressedTicks )
{
    xExpectedIdleTime = xMaximumPossibleSuppressedTicks;
}

查找变量xMaximumPossibleSuppressedTicks赋值

/* The systick is a 24-bit counter. */
#define portMAX_24_BIT_NUMBER				( 0xffffffUL )
/* 48MHZ / 1000hz调度频率 */
ulTimerCountsForOneTick = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ );
/*xMaximumPossibleSuppressedTicks = 0xffffffU / 48000 ≈ 349 */
xMaximumPossibleSuppressedTicks = portMAX_24_BIT_NUMBER / ulTimerCountsForOneTick;