Weddington, Eric wrote:
IIRC it said volatile must be used to keep the compiler from
optimizing
away access to a variable that he thinks might be pointless. Like
writing a variable that is never read back.
The way volatile was explained to me many years ago, is that a variable must be declared
as "volatile" if it's value can be changed by something outside of the mainline
code. This boils down to two use cases:
- A memory location that can be changed by hardware (i.e. a register)
- A memory location that can be changed by an interrupt service routine (ISR).
third case: if you use a multi-threaded operating system and a memory
location is modified by at least one thread and read by other threads
(one may also need a lock mechanism in such a case)
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) ...
}
Bernard
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