Adam Bryant wrote:
> 
> Katherine,
> To set the configuration bits for a PIC (including enabling the WDT), I use
> the "__CONFIG"
> directive in my application as follows:

Thanks for listing these. I was a bit lazy...

> instruction within 128 cycles, the WDT will reset the PIC.  So your main
> application
> loop may look like the following:
> 
> Main
>      CLRWDT
>      CALL      subroutine1
>      CALL      subroutine2
>      CALL      subroutine3
>      GOTO      Main
> 
> If any of the subroutines "hang", the WDT will not get cleared and the PIC
> will be reset.

This is true for most cases, but not all cases. What if 
one of your subroutines caused another to stop working? 
For instance, you have a few routines that process 
serial data. One waits for <CR> (carriage return) before 
setting a flag to start the processing of the data. For 
some reason, it missed the <CR>. The main loop will still 
run perfectly, and theoretically, the program is working 
as it should, but your automatic garage door still closes 
on your kid. For *certain* people with more kids than the 
average high school (Hi Andy!) this may be a bigger problem <GRIN>.

Say the data looks like this :
Garage Door : Open<CR>
or
Garage Door : Close<CR>
or 
Garage Door : Check Switch<CR>

Say the 'check switch' stream is transmitted every 500ms.

You know that if this stops there is a problem, right?

Your program looks like this :

/********* Interrup service routine *********/

/** 100ms Timed interrupt **/
If (Watch_Handle_Data_Thing<255) then 
        increase Watch_Handle_Data_Thing
If (Watch_Some_Other_Routine<255) then 
        increase Watch_Some_Other_Routine
Watch_Interrupt_Operation = 0

/** Serial data handling Stuff **/
If ((Char received) and (buffer pointer<28)), place in buffer
If (Char=<CR) then 
        set the flag called _Got_The_CR
        Reset the buffer pointer to zero

/***** End of Interrup service routine *****/

/******** Definitions of Procedures ********/
Procedure Do_The_Watch_Dog_Thing
       if (Watch_Interrupt_Operation<255) then
          increase Watch_Interrupt_Operation
       if ((Watch_Interrupt_Operation < 100)
       and (Watch_Handle_Data_Thing < 100)
       and (Watch_Some_Other_Routine < 100)) then
            Clear Watchdog      

Procedure Some_Other_Important_Routine
       blah
       blah
       Watch_Some_Other_Routine = 0

Procedure Handle_Data_Thing
       blah
       blah
       Watch_Handle_Data_Thing = 0

Procedure Open_Door
Procedure Close_Door

/******End of Definitions of Procedures ****/


/**************** Main *********************/
Initialize()
Do this forever :
        Do_The_Watch_Dog_Thing()
        if _Got_The_CR is set then
                                  Handle_Data_Thing()
                                  Clear _Got_The_CR
        Some_Other_Routine()

/************ End of Main ******************/
                                   
See how it works? Not only can you guarantee that 
your Garage Door Opener won't go into Window-mode, 
but you can also guarantee that it will always  
recover, even if the data it received could cause 
it to work in mysterious ways. 

If you sell 100000 of these products, you don't 
want to go and reset them evey time there is a 
comms error.

-- 
Friendly Regards          /"\
                          \ /
Tjaart van der Walt        X  ASCII RIBBON CAMPAIGN
mailto:tjaart@wasp.co.za  / \ AGAINST HTML MAIL

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