variable shared with an interrupt

Projects General Guidance
1

Hi

I am wondering how shall I declare a global variable that can be changed within an interrupt.

Any advice or warnings regarding this?

2

How will the interrupt change the variable? increment/decrement it?

3
4

HazardsMind:
How will the interrupt change the variable? increment/decrement it?

//ICR interrupt vector 
   ISR(TIMER1_CAPT_vect){  
      
   isr_increment++;
       if (isr_increment > 5)
       {
         isr_increment = 0;
         frequency++
       }

Coding badly solution may be the most logical

void loop()

cli();
frequency=frequency1;
sei();

frequency1...

...
5

most important hints in a nutshell

volatile unsigned long counter = 0;

ISR()
{
counter++;
}

loop()
{
  cli();
  unsigned long workcopy = counter;
  sei();
  ...
}
6

Coding badly Nick Gammon's solution may be the most logical