title: STM32F4 GPIO IDR
body: <p>I am learning STM32F407 MCU but confused with its Input Data Register. This register is 32 bits which 0-15 bits are read only and the remain bits are reserved. My question is that the register is defined with defaulf value of 0x0000XXXX in data sheet. XXXX are "undefined" in datasheet. What does really mean "undefined" term ? 
  Here is an example:<br>
  if (GPIOA->IDR &amp; 0x0001)
        GPIOD-ODR |= 0xF000;</p>
  
  <p>So undefined register bits can cause turning the LEDS on while I do not pressing the button ?</p>
  
  <p>Thanks....</p>