title: FreeRTOS Systick Handler
body: <p>I'm studying the FreeRTOS scheduler (port.c) and have a question about Systick handler. I understand <code>xPortSysTickHandler()</code> is supposed to be called at RTOS ticks, but I don't see where it gets specified by saying <code>xPortSysTickHandler()</code> is the Systick timer interrupt callback function.</p>
  <p>Especially, I didn't see that in <code>vPortSetupTimerInterrupt()</code>, which seems a bit strange to me since this function does configure, such as load register value, of the timer.</p>
  <p>Does it gets specified as the callback somewhere else? Or is there something I'm missing?</p>
  <p>Thanks!</p>