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1. 

   frpr
   Member

   Hello,
   I'm just a newbie at Maple IDE. Normally, I can use functions: cli(); y=x; sti(); in Arduino IDE. Is there something like "atomic read" for accessing a volatile variable in Maple IDE? My situation is like the code below. Please, is there also some "atomic write" func.?

   [code]
   volatile uint32 my_timeflag;
   uint32 currentValue;

   void isr_TIM2OVF(void){
   my_timeflag++;
   }

   setup(){
   // init TIM2 code
   // ...
   // reset counter
   my_timeflag=0;
   }

   loop(){
   //
   // A PROBLEM HERE.
   // I'd like to do "atomic read" of the
   // "my_timeflag" var.
   //

   currentValue = my_timeflag; //???

   // do something in loop
   }
   [/code]

   Posted 4 years ago #
2. 

   poslathian
   LeafLabs

   perhaps I am missing something but that should just work. By declaring my_timeflag to be volatile, the compiler should expect that that var may be modified by hardware or interrupts and not stash it in a register.

   did this not work for you?

   btw, I do this all the time, where a variable is incremented or set in an ISR and then read or reset from the main loop. Very handy, its also how millis/micros() work.

   Posted 4 years ago #
3. 

   perry
   Administrator

   frpr: This is fine as written provided you only READ and do not modify the volatile
   in loop(). Whether or not you need these atomic operations will depend on the context in
   which you're accessing the variables. If you only ever modify it in interrupt
   context and ensure that no other interrupt (you may get preempted by a higher
   priority interrupt) modifies that counter, then you won't require locks or anything.

   If you want to safely modify the variable, you will need to either disable interrupts
   (nvic_globalirq_disable()), or use some form of strex/ldrex, which are ARM-specific
   instructions that lock and tag the memory bus to allow you to implement things like
   atomic operations, mutexes, semaphores, etc. I'd suggest a google on these instructions
   if you decide to go this route.

   The easiest route is probably to just disable interrupts and reenable them when you're done.

   Posted 4 years ago #
4. 

   Darth Maker
   Member

   I've written programs that have a volatile tick variable that is incremented by a timer interrupt, then cleared the variable in the main program without issue. It's possible that it just never collided, but it did work.

   Posted 4 years ago #
5. 

   poslathian
   LeafLabs

   darth - perry is right, you and I are both living on the edge.

   Posted 4 years ago #

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