Here is an easy an popular way to start using USB in your designs without going into learning the complicated USB protocol. This circuit converts normal USART signals from any microcontroller into USB compatible signals which can be directly connected to the PC. If u r designing a circuit and u need pc interface, then this is the best way, use USB, as the RS232 ports are disappearing from PCs and laptops very fast.
USART-to-USB converter using FT232BM chip – [Link]
Nowadays, USB is the most popular connection connection between PC and peripherals such as AVR programmers, printers, scanners etc. For that reason I had to modify my old serial AVR In-System-Programmer (ISP) to work with USB connection. You can say, “use a USB to Serial adaptor to connect your AVR ISP with your PC”. Yes, that could be a solution but it would cost me more money than a singe FT232BM chip because I had to include an USB to RS232 adaptor and a power supply for my programmer.
USB AVR In-System-Programmer - [Link]
Today and future the serial and parallel ports being phased out on new computers, hardware designers and hobbyists have no choice but to convert the USB port back to RS232 serial. FTDI have a very nice chip that does just that, the FT232AM. This chip converts USB to a standard high-speed serial port. The bonus with FTDI is the drivers are already written for you. All you have to do is design the hardware and download the serial USB drivers from FTDI’s web site. The schematic is quite simple, however the FT232AM requires a lot of external components to get it working. FTDI now have a FT232BM 2nd generation device. This device requires less external parts. [via]
USB to RS232 Dongle - [Link]