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Single-task background (main loop) priority scheduler;

 
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Ken Johnson



Joined: 23 Mar 2006
Posts: 197
Location: Lewisburg, WV

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Single-task background (main loop) priority scheduler;
PostPosted: Wed Sep 26, 2007 9:08 am     Reply with quote

A clean, simple-looking main() loop makes your code easier to understand and maintain. Rather than numerous flags and several if-statements, I use the following code, with the added benefit that "tasks" to be executed are prioritized (but NOT pre-emptive). Hope this is self-explanatory. Comments appreciated.

Ken


Code:

//  Filename:   Work.h          Copyright © 2007 K & N Co.
//  Created:    Sep 21, 2007    by Ken Johnson
//  Purpose:    Single-task background (main loop) priority scheduler;
//              Designed for and tested with the CCS PIC Compiler;
//              May be freely used, copied, modified, etc;

#if !defined (WorkHeaderIncluded)
#define WorkHeaderIncluded      //  Include this stuff only once;

typedef unsigned int16  WORK;   //  Bit flags tell the main loop what to do;
//  Lower bit numbers are higher priority;
//  You can make this int8, or even int32 if you need to;

//  Use this macro to set one or more Work flags:
#define SetWork(flags)  Work |=  ((WORK) (flags))
#define ClrWork(flags)  Work &= ~((WORK) (flags))
//  (You probably don't need ClrWork)

//  This macro returns the highest-priority bit (lowest bit number) from x:
#define loBit(x)    ((x) & (~(x)+1))

static  WORK    Work;           //  This is the one and only WORK variable;

WORK    GetWork (void)
//  Returns the highest-priority bit (if any) of Work, after clearing that bit;
//  Returns 0 if no bits are set;
{
    WORK    work;               //  Working copy of Work;

    work = Work;                //  Copy;
    work = loBit (work);        //  Pull off the lo-bit (if any);
    if ( work ) {               //  If we pulled off a bit, clear it:
        //  Depending on Compiler/Processor, you may need to disable interrupts here;
        Work ^= work;           //  This is "atomic" where I've tested it (several PIC18xxxx);
        //  Re-enable interrupts if disabled;
    }
    return work;
}

#if 0                           //  Sample pseudo-code:

#define W_ZEROWORK  0x0000      //  There's nothing to do;
#define W_TIMER     0x0001      //  Highest priority work to be done;
#define W_RXDATA    0x0002
#define W_DEBOUNCE  0x0004      //  Lowest priority work to be done;

void    RxISR (void)
{
    //  read uart and stuff byte in buffer
    SetWork (RXDATA);           //  Work bits may be set in an ISR;
}

void    TimerISR (void)
{
    //  Clear timer interrupt;
    SetWork (W_TIMER);
}

void    DoTimer (void)
{
    //  Maybe every n calls:
    SetWork (W_DEBOUNCE);       //  Work bits may be set in background loop;
}

void    main (void)             //  My main() code always looks like this:
{
    Initialize ();              //  Initialize PIC, etc;

    for ( ;; ) switch (GetWork) ) {
        //  These are alphabetized (one of my habits),
        //  but *processed* by priority:
        case W_DEBOUNCE:    DoDebounce ();      break;
        case W_RXDATA:      DoRxData ();        break;
        case W_TIMER:       DoTimer ();         break;
        case W_ZEROWORK:    MaybeSleep ();      break;
    }
}

#endif                          //  end of sample code

#endif                          //  WorkHeaderIncluded;
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