Hello everyone. Why the function HAL_GetTick() does not disable interrupts before accessing uwTick? What happens if SysTick interrupt occurs while reading uwTick?

[Nikolay A. Neudobnoff]

..

[Tesla DeLorean]

What if it does increment? The granularity here is 1ms, next time you read the value it will have incremented. Not sure I understand where you think the harm/damage would occur.

You only need protection if you have two functions reading-and-writing the value in two different threads of execution. The SysTick interrupt can't reenter itself based on priority/preemption, only tail-chain (run a second time, back-to-back)

[Danish1]

Maybe Nickolay is carrying over experience from 8-bit microcontrollers, and doesn't realise that a read of a 32-bit value on an arm platform happens all at once. There's no possibility of the value being incremented part-way through a read.

[Nikolay A. Neudobnoff]

User 6.xx and Danish, thank you very much for your replies. Yes, I think exactly about possible corruption of the variable to which uwTick is reading. I am very new in ARM (all my previous experience is about PIC and MSP430). So, I got some further questions: Is reading of uwTick a real atomic operation which cannot be interrupted? What is happens if I due to some reasons change uwTick from uint32_t to uint64_t? Must I do interrupts disabling in this case?

[Pavel A.]

>What is happens if I due to some reasons change uwTick from uint32_t to uint64_t? 

Then, get an AARCH64 chip.

-- pa

[Bob S]

Or use multiple reads of the 64-bit value to detect and avoid the rollover. Pseudo code like this:

uint64_t HAL_GetTick64( void )
{
 uint64_t first = uwTick; // Presumes uwTick is now 64 bits
 uint64_t second = uwTick;
 
 if ( upper32bits_of_second != upper32bits_of_first ) {
 // Got rollover from lower 32 bits to upper 32 bits.
 // Read again to get guaranteed valid 64 bit value.
 // At 1ms tick rate, cannot get another 32-bit rollover for 49 days
 first = uwTick;
 }
 
 return( first );
}

[Nikolay A. Neudobnoff]

Thanks to all for very polite and detailed explanations. Will use double read.