USB(即Universal Serial Bus)是一種通用串行總線。隨著技術水平的提高，計算機的廣泛應用，人們對串行通信提出了更高的要求。開發一種兼容低速和高速的技術，從而為廣大用戶提供一種可共享的。可擴容的。使用方便的串行總線成為眾多廠商的共同目標。為了實現整個計算機系統中總線的一致性，USB已發布了一種成為通用計算機串行技術規范，簡稱為USB。
VHDL實現SPI功能源代碼
-- The SPI bus is a 3 wire bus that in effect links a serial shift
-- register between the "master" and the "slave". Typically both the
-- master and slave have an 8 bit shift register so the combined
-- register is 16 bits. When an SPI transfer takes place, the master and
-- slave shift their shift registers 8 bits and thus exchange their 8
-- bit register values.
Universal Serial Bus (USB) is a communications architecture that gives a personal
computer (PC) the ability to interconnect a variety of devices using a simple four-
wire cable. The USB is actually a two-wire serial communication link that runs at
either 1.5 or 12 megabits per second (mbs). USB protocols can configure devices
at startup or when they are plugged in at run time. These devices are broken into
various device classes. Each device class defines the common behavior and
protocols for devices that serve similar functions. Some examples of USB device
classes are shown in the following table
