Solution to the problem of incorrect clock in FreeRTOS kernel

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problem

In the use of RTOS when , Suddenly found that RTOS There is something wrong with your clock , Specific performance in the use of vTaskDelay or vTaskDelayUntil The delay time is not correct , Roughly targeted 10 About times .

Problem location

After checking for a long time, it was found that the problem was configSYSTICK_CLOCK_HZ, By this definition . The original understanding of this macro was , First, only for Cortex-M You can use , And then if Systick Like the system clock, there is no need to define , When different, it is defined as Systick The clock frequency of , Then the following methods are simply used when transplanting ：

#define configCPU_CLOCK_HZ                      ( ( unsigned long ) 72000000 )
#define configSYSTICK_CLOCK_HZ                  configCPU_CLOCK_HZ

Leading to the final problem .

Problem analysis

Why didn't you find any problems at the beginning , Because I only saw the following picture at the beginning , There are no settings for the whole Systick understand ,

Look at , I think the setting value is correct , Then I didn't read the next sentence , I didn't pay attention to configSYSTICK_CLOCK_HZ This

The difference between macro definition and undefined . Two questions need to be understood when identifying the source of the problem ：

1．Systick Is your clock the system kernel clock , Whether the clock frequency can be selected

2．Systick How to switch the system clock

The first question is

Systick Your clock is not necessarily a system clock （AHB）, find CM The kernel can see Systick register , in 2 Bit type , It is clear that , There are two situations 1 Is the kernel clock ,0 Is the kernel clock 8 frequency division

The second question is

Obviously, the second question , Come out naturally , By changing the bit 2 Can change

good , After clarifying the above two questions , Let's ask RTOS How to help us set up Systick Of

/* Configure SysTick to interrupt at the requested rate. */
portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );

Two words , No problem , The first sentence is set in the numerical value , The second step is to set the control, that is, the status register , good , First translate it according to my original problem ：

/* Configure SysTick to interrupt at the requested rate. */
0xE000_E014 = ( 72000000/ 1000) - 1UL;
0xE000_E010 = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );

Only pay attention to the correctness of reloading register data , The configuration of control and status registers is ignored , When you pay attention to this problem, it will be solved naturally , First find portNVIC_SYSTICK_CLK_BIT Definition

#ifndef configSYSTICK_CLOCK_HZ
 #define configSYSTICK_CLOCK_HZ configCPU_CLOCK_HZ
 /* Ensure the SysTick is clocked at the same frequency as the core. */
 #define portNVIC_SYSTICK_CLK_BIT ( 1UL << 2UL )
#else
 /* The way the SysTick is clocked is not modified in case it is not the same
 as the core. */
 #define portNVIC_SYSTICK_CLK_BIT ( 0 )
#endif

See? , If you define configSYSTICK_CLOCK_HZ It will change portNVIC_SYSTICK_CLK_BIT The numerical , Even if you promise configSYSTICK_CLOCK_HZ It can't be the same as the system clock frequency , This will cause the clock source of the clock source to change , Leading to the final problem .

terms of settlement

How to solve this problem perfectly? Let's go to the official website to see the official explanation of macro itself ：

Undefine configSYSTICK_CLOCK_HZ

Optional parameter for ARM Cortex-M ports only.

By default ARM Cortex-M ports generate the RTOS tick interrupt from the Cortex-M SysTick timer. Most Cortex-M MCUs run the SysTick timer at the same frequency as the MCU itself - when that is the case configSYSTICK_CLOCK_HZ is not needed and should be left undefined. If the SysTick timer is clocked at a different frequency to the MCU core then set configCPU_CLOCK_HZ to the MCU clock frequency, as normal, and configSYSTICK_CLOCK_HZ to the SysTick clock frequency.

The general meaning is only to Cortex-M The kernel is valid , The second is if Systick The system clock is the same as the kernel, so don't define this macro , On the contrary, define the macro , At the same time, the actual clock frequency after dividing its value .

That's easy , Just remove the definition .

summary

stay RTOS If RTOS The kernel clock is wrong , First position Systick Configuration problem , Check whether the configuration of each step changes the relevant clock frequency , There is no need to focus on other issues .

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