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The application note suggests to wrap a timer tick variable in a function which disables all interrupts, reads and copies the variable into a temp, re-enables interrupts, then returns the temp. Can the variable be accessed directly outside of the ISR without disabling interrupts if the timer tick variable is declared as "volatile?"
Really? Why? I'm not very familiar with 'volatile' but from what I've read so far, it did not say it had to be a char (or 8 bits). Will you be so kind as to direct me towards a reference for this? I would really appreciate it.
This has nothing to do with volatile, it has to do with the '51 being an 8 bit device. Example: a counter is incremented in the ISR and read in main. The count now is 0x1ff
Main read low byte to a register = 0xff INTERRUPT HAPPEN HERE ISR increment counter to 0x200 interrupt exit main read high byte 0x02
"I'm not very familiar with 'volatile'" See K&R.
IMHO, "volatile" really _should_ play a role here. From the point-of-view of the C language definition, any object whose value may change for reasons not explicitly part of the C program itself (e.g. a counter/timer) should still work as expected, *if* it's declared "volatile". It's in the responsibility of the compiler to ensure that it does. For the sake of the 8051, this means that a large fraction of the SFRs should implicitly be considered volatile. If a variable larger than 1 byte is declared "volatile", the compiler would have to turn off interrupts while accessing it automatically. That's one of the effects of the "volatile" qualifier.
In general, you can't expect a compiler to solve the problem of needing atomic access to a value that's more than a single bus transfer wide. While masking interrupts is sufficient in the example above, since software changes the value, consider a free-running 16-bit counter in hardware. Interrupts have nothing to do with it, and volatile doesn't help. The problem is simply that it takes two bus cycles to read the value, which may straddle a rollover. You'd need hardware to latch a single consistent 16-bit value before you started to transfer it into the processor. Even for all-software cases, you have the problems that you might not want to mask all interrupts just to mask the one with which you have a synchronization issue. And if you have custom interrupt controller logic, the compiler can't know which bits to set. In general, the programmer is going to have to solve the problem anyway. When it comes to the definition of volatile as "any object whose value may change for reasons not explicitly part of the C program itself", keep in mind that the "volatility" is seen from a small, local, code-generator/optimizer-ish view of "the program", rather than any sort of whole-source analysis. "volatile" helps let the compiler know it has to generate a physical read and not just use a value that may be cached in a register from a couple of lines ago. You still need to declare "volatile" values that are shared between two threads of execution, or interrupt handlers, even though the code may be all part of "the C program", or even the same file, all visible to the compiler. For example:
U8 counter; // should be "volatile" void TimerTick (void) interrupt 0 { counter++; } void MainRoutine (void) { for (;;) printf ("%d\n", counter); }
From K&R: "The purpose of volatile is to force an implementation to suppress optimisation that could otherwise occur" So there you are: its sole purpose is to control optimisation - not to guarantee atomic accesses, nor anything else.