title: STM32 STM32CubeF4 USB CDC operation
body: <p>I built the code from the STM32CubeF4 for the USB CDC example. I added the missing receive code for CDC_Receive_FS() in usbd_cdc_if.c.
  I loaded this into my STM32F4 Discovery and it works. A character typed on Tera Term returns and is displayed on Tera Term.</p>
  
  <p>I am hoping that someone here, could give me some knowledge about how this USB CDC firmware works, specifically, is this being driven by an interrupt that is generated when there is a level shift in voltage on the USB -D and +D pins, or is there an infinite while loop that was launched somewhere, and it's just polling waiting for some data to appear?
  What prompted my question is that I see that one can blink the LEDs on this board by toggling the state of the GPIO pins within an infinite while loop in main.c. However, there is nothing within this while loop at all within main.c for USB. So how does this USB CDC firmware get and send a character from/to Tera Term.</p>