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Context switching without RTOS

Michael_V
Prodigy 250 points

Hi.

This is a general question, but for examples lets use C2000 (because that's what I'm working on right now :) )

Suppose I have two functions. One of them is for "general purposes" (like communication) and the other is for real time events response.

void FuncRT()
{
  // response to RT
}

void FuncGeneral()
{
  // do general stuff
}

void ISR()
{
  // switch context to FuncRT
}

void main()
{
  while(1)
  {
    FuncGeneral();
  }
}

Suppose I want the ISR to be very short and its purpose is only to "switch the context" to the RT function, which will do whatever it's doing, and after its return the General function will continue.

I know how to implement this with TI-RTOS. My question is how to do it without it.

Thanks.

  • 0
    TI__Expert 8175 points
    Hi Michael,
    One way you can do this is to create 2 tasks one for funcRT and the other for funcGeneral. Then have your funcRT task pend on semaphore (say SemaphoreA) in a loop and your general function pend on another semaphore (Semaphore B) in a loop as well. In your ISR function, you post SemaphoreA which will cause funcRT to unblock and run the code you have in there and after that you post Semaphore B and go back to pending on Semaphore A a the beginning of the loop. This will cause funcRT to block again and release funcGeneral to run its code and after which it pends again on SemaphoreB at the beginning of its loop. Now both tasks will be blocked till the next ISR happens and the process repeats itself.
    Let me know if this helps.

    Thanks,
    Moses
  • 0 in reply to Moses Isang
    Prodigy 250 points

    Hi Moses.

    Thank you for the response.

    In order to use semaphores, I need TI-RTOS, and that's what I'm trying to avoid.

    Maybe the answer for this is POPing and PUSHing some registers inside the ISR? This is how context switch is generally done, isn't it?

  • 0 in reply to Michael_V
    TI__Expert 8175 points

    Hi Michael,

       If you're not using TI-RTOS you can post directly to the C2000 forum here, they should be able to help you. I don't know of any mechanism of "popping /pushing registers". Every time you call a function things are pushed on the stack and when you return the previous contents are popped back on the stack. Is this what you mean?

    Thanks,

    Moses

  • 0 in reply to Moses Isang
    Prodigy 250 points
    Yes, Moses, this is what I mean. But you're referring to the automatic PUSH/POP whenever a function is called. I'm referring to a "manual" PUSH/POP in order to switch to a different function.
    I posted the question here because it's kind of related to RTOS. Is there a way to move this thread to the C2000 forum, or I need to repost it?
    Thanks.
  • 0 in reply to Michael_V
    TI__Expert 8175 points

    Hi Michael,

           It looks like your trying to reinvent your own RTOS. To do your own context switch you'll need to write your own dispatcher and save the state of a bunch of registers before the context switch and restore them after. What you'll end up with will look like the typical dispatcher and context switching code of any RTOS. I don't see any significant performance gain in writing one yourself.

    Regards,

    Moses 

  • 0 in reply to Moses Isang
    Prodigy 250 points

    I'm not trying to reinvent anything.

    I have only 2 tasks. One runs all the time in the While loop, and the other only reacts to interrupts. So I what to switch between them without the overheard of RTOS.

  • 0 in reply to Michael_V
    TI__Expert 8175 points
    Could you simply have funcRT be called from your ISR function? Is the code you have in there suitable to be executed in an ISR context?

    Moses
  • 0 in reply to Moses Isang
    Prodigy 250 points
    Right now there is no FuncRT, as this is a theoretical question. But I think of implementing this in the future.
    The idea behind the question is making ISR as short as possible, especially because there is no interrupt nesting in C2000.
  • 0 in reply to Michael_V
    TI__Expert 8175 points

    Okay I see. Making your ISR short as possible is the best route in this case in my opinion. Trying to write your own context switch mechanism is pretty much trying to do the work of an RTOS and isn't that what you're trying to avoid in the first place.

    Regards,
    Moses