Posted on September 03, 2013 at 21:35A common problem in realtime systems is when tasks work standalone but not together. You likely have some kind of race condition where global variables are updated at the wrong times, or access is optimized by the compiler to mask updates (check your volatile declarations). Or you may have a simultaneous update problem where both tasks update a global without some type of synchronization, like a semaphore or mutex.
If you are seeing hard, bus or usage faults you may be clobbering a task stack or a shared global with some kind of buffer overrun. Or you are passing data that's on a stack, which is corrupted when you exit the calling routine. Check for data passed between tasks that's on a stack and not statically allocated.
These types of bugs can be very difficult to find, especially if it's someone else's code that wasn't designed for multi-tasking. For a first pass make sure the shared globals are declared as volatile, and you have some type of locking mechanism to manage updates.
Remember, an interrupt and task context change can happen in the middle of a line of code. Between the time you evaluate the assignment on the right side and store the result on the left side of a statement the underlying variables can be altered by another task.
Jack Peacock
