I got it! Thanks!

Russell McMahon wrote:
>>> But if interrupt latency is an issue, you can use a different
>>> method: have the ISR put the 32-bit integer into a temporary
>>> holding register and set a flag bit somewhere to indicate to the
>>> main code that a new integer value is available.  The main code
>>> can test the flag and when a new value is available, transfer it
>>> to the working variable and then reset the flag.  The ISR, in
>>> turn, refrains from updating the holding register unless the flag
>>> has been cleared by the main code.
>
>> I have thought of that, but as there is no instruction to
>> test-and-clear a variable in a single cycle a racing-condition could
>> occur. Perhaps a combination of both solutions to keep the
>> disable-interrupt-period as short as possible.
>
> As described it's faily safe as the ISR writes the flag but never
> resets it. The "main" task will never inspect the data until it is
> definitely ready. Also the main task will never clear the flag until
> after it is read. The "semaphore" flag is sepearted in time by the
> reading andwriting of 4 bytes.
>
> If you want a more conventional semaphore then you should be able to
> overcome the lack of an "indivisible instruction" by a multiple read
> of the semaphore flag as follows -
>
> Main task sees the interupts resource_in_use  flag is free, sets its
> own "I am using the resource" flag THEN reads the interupt's
> resource_in_use flag for a secind time. IF the interupt has crept in
> and stolen the resource between the time that the main task saw it
> was free and then set it's in use flag then the second time around it
> will find the interupt's flag is set. It then de-asserts it's own
> flag and tries again.
>
> Main task:
>
>                                                ; own flag is cleared
> at this point
> Do while irq flag is set
>     Clear own flag                  ; for loop back condition
>     Do while irq flag is set     ; second test catchers the irq
> stealing scenario
>         mumble
>     End
>     Assert own flag
> End    ; stolen resource is detected here - loop back if stolen
>
> do task
> clear own flag
>
>
> ____________
>
> Interupt:
>
> Do while main_task flag is set
>     Something else
> End
> Set irq flag
> Do task
> Clear irq flag.

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