SysTick_Handler crashes when incrementing static variable using ++

[drdrone]

Hi all,

I'm working on an STM32F411 project using STM32CubeIDE (bare-metal, no HAL). I'm setting up a 1ms tick using SysTick_Config() and defining my own SysTick_Handler() to keep a millisecond counter. Here's the issue:

When I increment a static volatile global counter inside the handler using '++', the system crashes and ends up in Default_Handler. But strangely, if I assign a constant (e.g. millis = 123;), it works perfectly fine. Moreover, if I increment a local temp variable and increment millis, it'll take it just fine. Here's my code snippet. I'm not using the HAL library for this. Would you know why this would be? I'm using an STM32F411CE6 black pill.

Thanks!

static volatile uint32_t millis = 0;

void SysTick_Handler(void)
{
// This works:
// millis = 123;

// This works as well:
uint32_t tmp = millis;
tmp++;
millis = tmp;

// This causes a crash into a forever loop in default handler:
millis++;

(void)(SysTick->CTRL); // Clear COUNTFLAG
}



// SysTick is initialized like this in main, got all the clocks, etc, working. Running at 75MHz PLL, 25MHZ external crystal.
SysTick_Config(SystemCoreClock / 1000); // 1ms tick

[mbarg.1]

Problem is in 

static volatile

declaration.

If you want more help, post the assembler code generated, the memory address of millis variable, and (if you do not see yourself ) we will show the problem.

[Ozone]

> ...post the assembler code generated, the memory address of millis variable, ...

Switching debugging to assembler level (AFAIK "instruction stepping" in Eclipse lingo) and making a screenshot including the line where it crashes should do the trick.

I think there is something else, outside the code you have shown.

[Andrew Neil]

---

@drdrone wrote:

 a static volatile global counter 

---

If it's static, then it's not global - at most, it's file-scope.

---

@drdrone wrote:

the system crashes and ends up in Default_Handler.

---

On debugging Hard Faults:

https://community.st.com/t5/community-guidelines/how-to-write-your-question-to-maximize-your-chances-to-find-a/tac-p/708193/highlight/true#M51

[Saket_Om]

Hello @drdrone 

Could you try the config below please:

void SysTick_Handler(void) {
    __disable_irq(); // Disable interrupts
    millis++;
    __enable_irq();  // Enable interrupts
}

[mbarg.1]

This is not a solution nor an explanation !

Solution is in generated assembler ...

[Ozone]

> If it's static, then it's not global - at most, it's file-scope.

That should be ok, I often use similiar definitions without problems. Although not with theCube IDE / toolchain.
A common example is a wait / delay routine based upon such a counter, implemented in the same module.

[Andrew Neil]

---

@Ozone wrote:

> If it's static, then it's not global - at most, it's file-scope.

That should be ok,

---

But the point is it's not a global variable - as the OP said - so checking if that was just a typo...

@drdrone - does it make any difference if you make it truly global (ie, remove static)?

Also if you remove volatile?

And what if you use:

millis += 1;

What optimisation level are you using?

[Andrew Neil]

---

@Ozone wrote:

I think there is something else, outside the code you have shown.

---

Indeed.

@drdrone please post a minimum but complete example which demonstrates the issue.

[Ozone]

I think as a quasi-"global" variable (non-auto), it should show up in the map file.
I would at least check.