title: Nested Vectored Interrupt Controller
body: <p>I been struggling with NVIC, why it is used? and how it is used? 
  In my project I'm using</p>
  
  <pre><code> NVIC_IntDisable(NVIC_SSP0); 
   NVIC_ClrPend(NVIC_PIO_0);
   NVIC_IntPri(NVIC_PIO_0, 17);    
   NVIC_IntEnable(NVIC_PIO_0);     
  </code></pre>
  
  <p>These are all defined in the <code>system.c</code> can anyone explain me why these are used what the use perticular commands. Please mention any good matrial that I can reffer.</p>
  
  <p>Thank you.</p>