Logic devices can be divided into two categories - fixed logic devices and programmable logic devices. As it is named, the circuits in fixed logic devices are permanent, and they complete one or a group of functions - once manufactured, they cannot be changed. On the other hand, programmable logic devices (PLD) are standard finished components that can provide customers with a wide range of logic capacity, characteristics, speed and voltage parameters - and such devices can be changed at any time to complete many different functions. Two types of programmable logic devices are field programmable gate array (FPGA) and complex programmable logic device (CPLD). In these two kinds of programmable logic devices, FPGA provides the highest logic density, the richest characteristics and the highest performance. Now the latest FPGA devices, such as Xilinx virtex? Some devices in the series can provide 8million "system gates" (relative logic density).