title: Move an interrupt handler and variables from main to a include file
body: <p>I've a newbie question about how to correctly move an interrupt handler to an include file. I've a SysTick handler in my main.c file defined in this way:</p>
  
  <pre><code>volatile int systick_timer_counter = 0;
  
  void SysTick_Handler(void) {
      systick_timer_counter++;
  }
  </code></pre>
  
  <p>I use this <code>systick_timer_counter</code>, that is increased every 10 ms, to trigger some checks after some time has passed, then reset it.</p>
  
  <p>What I want is to use this <code>systick_timer_counter</code> also inside some functions that aren't in my <code>main.c</code> file, but in another function's <code>.c</code> file. So I want to create a file called <code>systick_counter</code> to include where I need to use it.</p>
  
  <p>What I'm asking is, how should I do this in correct way? I can just create a <code>.c</code> file and place variable and interrupt header inside it, or should I add something more or change variable definition?</p>