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Written by Mandapati Krishna Chaitanya   
Page 1 of 5
FPGA contains a two dimensional arrays of logic blocks and interconnections between logic blocks. Both the logic blocks and interconnects are programmable. Logic blocks are programmed to implement a desired function and the interconnects are programmed using the switch boxes to connect the logic blocks.
To be more clear, if we want to implement a complex design (CPU for instance), then the design is divided into small sub functions and each sub function is implemented using one logic block. Now, to get our desired design (CPU), all the sub functions implemented in logic blocks must be connected and this is done by programming the interconnects.
Internal structure of an FPGA is depicted in the following figure.

 

FPGA Architecture
FPGA Architecture

FPGAs, alternative to the custom ICs, can be used to implement an entire System On one Chip (SOC). The main advantage of FPGA is ability to reprogram. User can reprogram an FPGA to implement a design and this is done after the FPGA is manufactured. This brings the name “Field Programmable.”
Custom ICs are expensive and takes long time to design so they are useful when produced in bulk amounts. But FPGAs are easy to implement with in a short time with the help of Computer Aided Designing (CAD) tools (because there is no physical layout process, no mask making, and no IC manufacturing).
Some disadvantages of FPGAs are, they are slow compared to custom ICs as they can’t handle vary complex designs and also they draw more power.
Xilinx logic block consists of one Look Up Table (LUT) and one FlipFlop. An LUT is used to implement number of different functionality. The input lines to the logic block go into the LUT and enable it. The output of the LUT gives the result of the logic function that it implements and the output of logic block is registered or unregistered out put from the LUT.
SRAM is used to implement a LUT.A k-input logic function is implemented using 2^k * 1 size SRAM. Number of different possible functions for k input LUT is 2^2^k. Advantage of such an architecture is that it supports implementation of so many logic functions, however the disadvantage is unusually large number of memory cells required to implement such a logic block in case number of inputs is large.
Figure below shows a 4-input LUT based implementation of logic block.
Xilinx LUT
Xilinx LUT


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