LeafLabs Garden » Topic: Is there something like "atomic read" for accessing a volatile variable?

LeafLabs Garden » Topic: Is there something like "atomic read" for accessing a volatile variable? http://forums.leaflabs.com/topic.php?id=1112 A place to share, learn, and grow... en-US Fri, 22 Jan 2016 00:13:06 +0000 http://bbpress.org/?v=1.0.2 <![CDATA[Search]]> q http://forums.leaflabs.com/search.php poslathian on "Is there something like "atomic read" for accessing a volatile variable?" http://forums.leaflabs.com/topic.php?id=1112#post-6861 Thu, 13 Oct 2011 10:29:09 +0000 poslathian 6861@http://forums.leaflabs.com/ darth - perry is right, you and I are both living on the edge. Darth Maker on "Is there something like "atomic read" for accessing a volatile variable?" http://forums.leaflabs.com/topic.php?id=1112#post-6856 Wed, 12 Oct 2011 17:18:47 +0000 Darth Maker 6856@http://forums.leaflabs.com/ 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. perry on "Is there something like "atomic read" for accessing a volatile variable?" http://forums.leaflabs.com/topic.php?id=1112#post-6850 Wed, 12 Oct 2011 16:08:48 +0000 perry 6850@http://forums.leaflabs.com/ 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. poslathian on "Is there something like "atomic read" for accessing a volatile variable?" http://forums.leaflabs.com/topic.php?id=1112#post-6847 Wed, 12 Oct 2011 15:48:09 +0000 poslathian 6847@http://forums.leaflabs.com/ 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. frpr on "Is there something like "atomic read" for accessing a volatile variable?" http://forums.leaflabs.com/topic.php?id=1112#post-6835 Wed, 12 Oct 2011 10:55:56 +0000 frpr 6835@http://forums.leaflabs.com/ 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]