Interrupts changing variables as they are being used

AnthonyF · Guest

Hi,

I am doing a comparison using an if statement on a 32-bit variable. There is a hardware interrupt that will change this variable. Is it possible for the interrupt to happen in the middle of the comparison? Could this be a problem?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Here's a thread with several explanations of it.
http://www.ccsinfo.com/forum/viewtopic.php?t=17352

This post has an example. Look in the while() loop in main().
It copies the interrupt variable to a local variable, and it disables
interrupts while doing that. Then the local variable is used by
the rest of the code in main().
http://www.ccsinfo.com/forum/viewtopic.php?t=31470&start=6

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Yes, that is a common problem with multi-byte comparisons and interrupts. The solution is to disable relevant interrupts while the comparison is made.
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Ken Johnson · Joined: 23 Mar 2006 Posts: 197 Location: Lewisburg, WV

Better yet, disable ints, copy the variable to a temp, enable ints, then do the compare (in the interest of keeping ints off as short a time as possible)

Ken