Electronic – High resolution system timer in STM32

armmicrocontrollerstm32timer

I have some software running on an STM32F103 (I'd also like to be able to use the F4) where I'd like to be able to 'timestamp' events (such as interrupts on external pins) to the nearest microsecond (if not better).

As the software will be running for some time I want to use a 64 bit timer, and to me it made sense to use the built in SysTick timer. I have created SysTickMajor (a 64 bit counter) which increments every time SysTick overflows, and a function getSystemTime which combines this counter and the current value of SysTick to get me an accurate 64 bit time:

#define SYSTICK_RANGE 0x1000000 // 24 bit
volatile long long SysTickMajor = SYSTICK_RANGE;

voif onInit(void) {
  SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8);
  SysTick_Config(SYSTICK_RANGE-1);
  /* Super priority for SysTick - is done over absolutely everything. */
  NVIC_InitStructure.NVIC_IRQChannel = SysTick_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);
}

void SysTick_Handler(void) {
  SysTickMajor+=SYSTICK_RANGE;
}

long long getSystemTime() {
  long long t1 = SysTickMajor;
  long long time = (long long)SysTick->VAL;
  long long t2 = SysTickMajor;
  // times are different and systick has rolled over while reading
  if (t1!=t2 && time > (SYSTICK_RANGE>>1)) 
    return t2 - time;
  return t1-time;
}

The only problem is this doesn't appear to be 100% accurate – for instance, if I do the following:

bool toggle = false;
while (true) {
  toggle = !toggle;
  LED1.write(toggle);
  long long t = getSystemTime()() + 1000000;
  while (getSystemTime() < t);
}

I won't get a good square wave – occasionally there will be glitches (even though the SysTick interrupt is finished very quickly), because at points the time returned by getSystemTime is completely wrong.

UPDATE: When this happens (I am recording the time value on an interrupt triggered by an external event) I dump the last 3 values to serial. Repeatably I get things like: 

>0x278-E2281A 
 0x278-E9999A 
 0x278-0001E5

>0x5BE-F11F51
 0x5BE-F89038
 0x5BE-0000FB

I've stuck dashes in to depict which bits come from SysTick.

Now just to avoid doubt, I've used the code from starblue below. I've also changed it to use a 32 bit value for SysTickMajor, and I've ANDed out SysTick->VAL, just in case.

It appears that the SysTick_Handler is not preempting the other interrupt!

Now I've checked over this and I did have the Preemption priorities wrong previously, but now I set NVIC_PriorityGroupConfig(NVIC_PriorityGroup_4); then set up the interrupts – SysTick preemption priority 0, and the other interrupt is 7.

Is there something else I have to do to get preemption to work?

UPDATE2: I created a tiny test case for this exact issue (SysTick not preempting other interrupts), and have put it here:

STM32 Interrupt Priority (preemption) Problems

So what's wrong with the code above? Is there a better way to get a good high resolution timer? It seems like something obvious that people would want to do but I'm struggling to find examples.

Best Answer

As you've discovered, handling counter overflow with an interrupt handler is tricky and race-prone. Even with your priority boost, you'll get the wrong answer if interrupts are disabled while you read the counter. There's a much simpler technique which works if you're not reading the clock from interrupt context (note that I'm using the DWT cycle counter here, which is built into the processor thus fast and simple):

uint64_t last_cycle_count_64 = 0;

// Call at least every 2^32 cycles (every 59.6 seconds @ 72 MHz).
// Do not call from interrupt context!
uint64_t GetCycleCount64() {
  last_cycle_count_64 += DWT->CYCCNT - (uint32_t)(last_cycle_count_64);
  return last_cycle_count_64;
}

If you need to call it from interrupt context (or with a preemptive OS):

volatile uint64_t last_cycle_count_64 = 0;

// Call at least every 2^32 cycles (every 59.6 seconds @ 72 MHz).
uint64_t GetCycleCount64() {
  uint32_t primask;
  asm volatile ("mrs %0, PRIMASK" : "=r"(primask));
  asm volatile ("cpsid i");  // Disable interrupts.
  int64_t r = last_cycle_count_64;
  r += DWT->CYCCNT - (uint32_t)(r);
  last_cycle_count_64 = r;
  asm volatile ("msr PRIMASK, %0" : : "r"(primask));  // Restore interrupts.
  return r;
}

You may need to enable the cycle counter first:

DWT->CYCCNT |= DWT_CTRL_CYCCNTENA_Msk;  // Set bit 0.
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