DWT_CYCCNT does not seems to count cycle

[leonardo]

I'm using follow code:

volatile unsigned int *DWT_CYCCNT  = (volatile unsigned int *)0xE0001004; //address of the register
volatile unsigned int *DWT_CONTROL = (volatile unsigned int *)0xE0001000; //address of the register
volatile unsigned int *DWT_LAR   = (volatile unsigned int *)0xE0001FB0; //address of the register
volatile unsigned int *SCB_DEMCR  = (volatile unsigned int *)0xE000EDFC; //address of the register
  
  *SCB_DEMCR |= 0x01000000;
  *DWT_LAR = 0xC5ACCE55; // unlock
  *DWT_CYCCNT = 0; // reset the counter
  *DWT_CONTROL |= 1 ; // enable the counter
  uint32_t start=0, end=0;
  uint32_t array[100];
  uint32_t idx=0;
 
  *DWT_CYCCNT = 0;
  start = *DWT_CYCCNT;
  idx=1;
  end = *DWT_CYCCNT;
  idx=0;

I put a breackpoint on line 18 and I get:

start=9

end=22

*DWT_CYCCNT = 29

I'm expecting that start could be something like 2 and end something like 4 or 6. Why I get this results?

Is DWT working in an stm32f722?

thank

[TDK]

These are C statements, not assembly. Examine the disassembly to see what instructions are between start and end. Cache and flash wait states can also have an effect.

[Tesla DeLorean]

Time 100 or 1000 iterations of the code to benchmark, subtract out overhead of empty vs test loop

[leonardo]

The problem I see is a difference between cortex M4 (stm32f411) and M7 (stm32f722)

On M4 I can run the code posted and I get exactly 2 cycle, I can run this other code:

	*DWT_CYCCNT = 0;
	uint_test=1;	// 2 ciclos
	*DWT_CYCCNT = 0;
	float_test=1;	// 2 ciclos
	*DWT_CYCCNT = 0;
	float_test=3.4f * 1.000054f;
	*DWT_CYCCNT = 0;
	float_test=3.4 * float_test;

and get exactly what I expect on debug viewer.

But on M7 I get very wrong values.

I can't find on stm32f722 reference manual the implementation of DWT_CYCCNT. So I'm wondering about the working of this module on M7

In fact, DWT_CYCNT is changing ther values (counting) without code line 6 to 9 on first post. It seems like the DWT is configured and enabled by default? My project is generated using cubemx tool.

[leonardo]

I'm very confused.

I create a new empty project using SystemWorkbench IDE. I don't include HAL library, just startup file.

Then create a main.c file and write:

volatile unsigned int *DWT_CYCCNT  = (volatile unsigned int *)0xE0001004; //address of the register
volatile unsigned int *DWT_CONTROL = (volatile unsigned int *)0xE0001000; //address of the register
volatile unsigned int *DWT_LAR   = (volatile unsigned int *)0xE0001FB0; //address of the register
volatile unsigned int *SCB_DEMCR  = (volatile unsigned int *)0xE000EDFC; //address of the register
 
int main(void)
{
//	*SCB_DEMCR |= 0x01000000;
//	*DWT_LAR = 0xC5ACCE55; // unlock
//	*DWT_CYCCNT = 0; // reset the counter
//	*DWT_CONTROL |= 1 ; // enable the counter
 
	int uint_test=0;
	float float_test =0.0f;
	float f1=3.14f, f2=3.15f;
 
	*DWT_CYCCNT = 0;
	uint_test=1;	// 2 ciclos
	*DWT_CYCCNT = 0;
	float_test=1;	// 2 ciclos
	*DWT_CYCCNT = 0;
	float_test=3.4f * 1.000054f;
	*DWT_CYCCNT = 0;
	float_test=3.4 * float_test;
	*DWT_CYCCNT = 0;
	float_test=f1 * f2;	
	*DWT_CYCCNT = 0;
}

I get wrong values on expression viewer (debug and step by step) and get HardFault on line 26! what am I missing????

Same code on a fresh project running on stm32f411 (with lines 8..11 uncommented) get right values for DWT_CYCCNT but HardFault on line 26.