Contents
What should be the last instruction of ISR?
RETI
The last instruction of the ISR is RETI, which causes the microcontroller to pick up running where it left off before the interrupt happened. The microcontroller goes back to the MOV instruction, since it’s after the SUB instruction that was running when the interrupt happened.
Can an ISR return a value?
ISR cannot return any value, also we cannot pass any argument to it.
Can we call other functions inside ISR?
Even the function call overhead might be considered too much in some cases. So if you call functions from inside an ISR, it might be wise to inline those functions. You must also ensure that the called function is either re-entrant or that it isn’t called by other parts of the code except the ISR.
Does Each function has its own stack?
Each function does have its own space in the stack but every function gets its space allocated in the same stack. This is called the Global Call Stack of your program. It is used to store local variables which are used inside the function. However, dynamically allocated space is stored on the heap.
What happens when you put return statement in ISR ( )?
If you put a return anywhere in you code (C code), it will end up being a ‘ret’, with the I bit left unchanged. So if you call a function from within an ISR, the called function will not re-enable interrupts when it returns (because it will use ‘ret’).
What’s the difference between SREG and ISR ( )?
The only difference between the two is ‘reti’ also sets the global interrupt bit I in SREG. The ISR () macro (from interrupt.h) will give the interrupt function the attribute of ‘signal’, which means in addition to other details, the compiler will put a ‘reti’ at the end of the interrupt function.
Can a ISR be allowed to preempt itself?
If an ISR is allowed to preempt itself, the stack will keep on filling (the case where interrupt keeps on occurring and preempting the ISR) and will easily overflow the stack. Avoid nesting interrupts unless absolutely necessary and, in the latter case, ensure that the ISR can never preempt itself.
Why are global variables important in an ISR?
Now we will discuss the implications of the liberal use of global/local variables in an ISR. Global variables need to be carefully handled while used with ISRs, because interrupts are generally asynchronous and if a global variable is being written into by an ISR, it can get modified at any time.