title: ATMEGA168A - F_CPU warning
body: <p>I have written the code below in order to make an ATMEGA168A blink a small led:</p>
  
  <pre><code>#include &lt;avr/io.h&gt;
  #include &lt;util/delay.h&gt;
  
  #define F_CPU 1000000UL
  
  int main(void)
  {
      DDRB = 0b00000010;
      PORTB = 0b00000000;
  
      while(1)
      {
          PORTB ^= 1 &lt;&lt; 1;
          _delay_ms(1000);
      }
  }
  </code></pre>
  
  <p>The compiler is giving me a warning as follows:</p>
  
  <pre><code>Warning     #warning "F_CPU not defined for &lt;util/delay.h&gt;"
  </code></pre>
  
  <p>Here is where this warning comes from (delay.h)</p>
  
  <pre><code>#ifndef F_CPU
  /* prevent compiler error by supplying a default */
  # warning "F_CPU not defined for &lt;util/delay.h&gt;"
  # define F_CPU 1000000UL
  #endif
  </code></pre>
  
  <p>What am I doing wrong here? Is my declaration incorrect?</p>