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?"
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
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.
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.