ARM_DWT_CYCCNT

[sophiemor]

Hello everyone!

I am a beginner coder and I need to figure out how to use ARM_DWT_CYCCNT. Would someone be able to give me a detailed rundown? Thanks!

 

[h4yn0nnym0u5e]

It’s super simple, just an unsigned 32-bit counter that increments on every CPU cycle (hence the CYCCNT). On Teensy 4.x at the default 600MHz it thus rolls over to zero about every 7 seconds.

I really can’t think of anything else to say about it…

 

[PaulStoffregen]

Ordinary usage looks something like this.

uint32_t begin_cycles = ARM_DWT_CYCCNT;
// do something that takes time
uint32_t end_cycles = ARM_DWT_CYCCNT;
Serial.print("cycles = ");
Serial.println(end_cycles - begin_cycles);

Subtracting this way automatically handles the roll over case, so it's really very simple to use.

 

[WMXZ]

Subtracting this way automatically handles the roll over case, so it's really very simple to use.

as long as the "do something that takes time" does not takes longer than the roll-over time

 

[joepasquariello]

as long as the "do something that takes time" does not takes longer than the roll-over time

which is about 7 seconds at 600 MHz

 

[h4yn0nnym0u5e]

On Teensy 4.x at the default 600MHz it thus rolls over to zero about every 7 seconds.

as long as the "do something that takes time" does not takes longer than the roll-over time

which is about 7 seconds at 600 MHz

 

[Angelo]

Just a dumb question: Which kind of microcontroller application would require to measure period bigger than 7s, with 1.6ns resolution ?

 

[defragster]

measure period bigger than 7s, with 1.6ns resolution

The problem remembering the 7 second limit. It is easy/efficient to use ARM_DWT_CYCCNT 'all the time' - but keeping in mind that after 32 bit rollover the results are bad.

@luni did extend a way to use ARM_DWT_CYCCNT to 64 bits.

Reading ARM_DWT_CYCCNT takes 3 cycles for the data transfer and micros() takes 36(+) cycles for the function/calculation. And it relies on millis() sys_tick for reference.