USB and Serial Communication - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 110

the two. Option one is to use a secondary integrated circuit (IC) to facilitate
the conversion between the two (either on or off the Arduino board). This is
the type of interface present on an Uno, where an intermediary IC facilitates
USB-to-serial communication. Option two is to opt for a microcontroller that
has a USB controller built in (such as the Arduino Leonardo's 32U4 MCU).
ArduinoBoardswithanInternalorExternalFTDIUSB-to-
SerialConverter
As just explained, many Arduino boards (and Arduino clones) use a secondary
integrated circuit to facilitate the USB-to-serial conversion. The “FTDI” chip
is a popular chip that has just one function: convert between serial and USB.
When your computer connects to an FTDI chip, it shows up in your computer
as a “Virtual Serial Port” that you can access as if it was a DB9 port wired right
into your computer. Figure 6-2 shows the bottom of an Arduino Nano, which
utilizes an integrated FTDI chip.
FTDI chip
Figure 6-2: Arduino Nano with integrated FTDI chip shown