How to Determine If Cortex-M Processor Is Executing Interrupt Function?

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Author | strongerHuang

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Today, I want to share some knowledge I encountered while debugging code: the usage of __get_CONTROL, and the differences between xQueueSend and xQueueSendFromISR;

Source of the Problem

I previously ported some code written by others on the FreeRTOS system. After carefully reviewing the source code before the porting, confirming there were no issues, I compiled, downloaded, and ran it, only to suddenly find it “frozen”······

So, I confirmed the ported code again and found no bugs. At this point, I enabled online debugging and found that the program was stuck in the assertion statement in the vPortEnterCritical function. As follows:

Process of Solving the Problem

I solved the problem by following a conventional thought process, tracking step by step. Many issues are actually similar in principle and can be traced.

1. Check what configASSERT does?

Tracking the code:

#define configASSERT( x ) if( ( x ) == 0 ) { taskDISABLE_INTERRUPTS(); for( ;; ); }

Here, taskDISABLE_INTERRUPTS(); means to disable interrupts. Right after that, for( ;; ); is executed.

At this point, I understood that the program was stuck in the for( ;; ); loop.

2. Further Investigate the Problem

I began to think, why did it execute to this point? Why did it execute:

portDISABLE_INTERRUPTS(); uxCriticalNesting++;   if( uxCriticalNesting == 1 )

these statements?

This is what we commonly refer to as a “critical section.” I understood this when I studied RTOS, and this function must have been called. Therefore, I focused on the parameter portNVIC_INT_CTRL_REG:

#define portNVIC_INT_CTRL_REG     ( * ( ( volatile uint32_t * ) 0xe000ed04 ) )

0xe000ed04? This address, as those familiar with NVIC would know, is the Interrupt Control State Register.

3. Identify the Problem Point

From the above analysis, the problem has already emerged. I then checked relevant content in the Cortex-M3 authoritative guide. (PS: This manual can really solve many problems, translating it into Chinese is a good thing for most friends)

Actually, there is a register: Control Register (CONTROL), which explains this very clearly:

As shown in the figure, the general idea is: in interrupt mode, CONTROL[1] is 0. So, I turned my attention to the core_cm3.c file’s source code:

__ASM uint32_t __get_CONTROL(void)
{
  mrs r0, control
  bx lr
}

Those who understand a bit of assembly should already understand that this roughly means accessing the control register state, which is what I mentioned at the beginning, to let everyone understand __get_CONTROL.

4. Online Debugging, Analysis Conclusion

From the analysis above regarding the control register CONTROL, we need to verify whether it meets our expected effect. Through online debugging, we can conclude, as shown in the following two images:

a. Value 0x02 in non-interrupt condition

b. Value 0x00 in interrupt condition

At this point, the problem has been identified as CONTROL.

Application of get_CONTROL

In general, in RTOS real-time operating systems, queues are often used to handle our data, which is commonly referred to as FIFO (First In, First Out).

For example: in the FreeRTOS system, to add data from UART to the queue, we add to the queue in interrupts and in non-interrupts. At this point, we need to use get_CONTROL to determine whether we are currently in an interrupt function.

Of course, there are many similar situations, such as CAN, I2C, SPI, and the same principle applies.

For example, sending data to the queue via CAN bus:

This is the content for today, I hope it helps you.

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