From: bernard.fou...@kuantic.com
[...re: volatile use cases...]
> fourth case: nasty situations where 'volatile' is only a part of the
> solution but does not insure a correct result:
>
> For instance if ISR1 and ISR2 are *nested* ISRs, IsrCounter does not
> correctly hold the count of interrupts:
>
> volatile uint8_t IsrCounter;
>
> ISR1()
> {
> IsrCounter++;
> }
>
> ISR2()
> {
> IsrCounter++;
> }
>
> Consider also memory locations larger than the MCU data processing unit
> size (8 bits for AVR).
>
> The following code gives bad results on an AVR but works on a i386:
>
> volatile uint32_t MilliSeconds;
>
> ISR()
> {
> MilliSeconds++;
> }
>
> main()
> {
> while(MilliSeconds < SOME_LIMIT) ...
> }
>
In both of these cases, "volatile" is insufficient because it does not imply
"atomic". In the first case, you need an atomic read-modify-write, and in the
second you need an atomic read of a wide value. In both cases you can achieve
that by enclosing the the variable access in a cli-sei pair. For example, the
latter case can be fixed by changing the routine to something like
main() // or "int main(void)", yada yada
{
uint32_t ms; // does not need to be volatile
do{ cli(); ms = Milliseconds; sei(); } while (ms < SOME_LIMIT);
}
Regards,
-=Dave
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