FPGA Implementation

From HDL to FPGA

The process of converting hardware design language (HDL) files into a configuration bitstream which can be used to program the FPGA, is done several steps.

First, the HDL files are synthesized. Synthesis is the process of converting behavioral HDL descriptions into a network of logic gates. The synthesis engine takes as input the HDL design files and a library of primitives. Primitives are not necessarily just simple logic gates like AND and OR gates and D-registers, but can also include more complicated things such as shift registers and arithmetic units. Primitives also include specialized circuits such as DLLs that cannot be inferred by behavioral HDL code and must be explicitly instantiated. The libraries guide in the Xilinx documentation provides an complete description of every primitive available in the Xilinx library. (Note that, while there are occasions when it is helpful or even necessary to explicitly instantiate primitives, it is much better design practice to write behavioral code whenever possible.)

Say, we use the Xilinx supplied synthesis engine known as XST. XST takes as input a verilog (.v) file and generates a .ngc file. A synthesis report file (.srp) is also generated, which describes the logic inferred for each part of the HDL file, and often includes helpful warning messages.

The .ngc file is then converted to an .ngd file. (This step mostly seems to be necessary to accommodate different design entry methods, such as third-part synthesis tools or direct schematic entry. Whatever the design entry method, the result is an .ngd file.)

The .ngd file is essentially a netlist of primitive gates, which could be implemented on any one of a number of types of FPGA devices Xilinx manufacturers. The next step is to map the primitives onto the types of resources (logic cells, I/O cells, etc.) available in the specific FPGA being targeted. The output of the Xilinx map tool is an .ncd file.

The design is then placed and routed, meaning that the resources described in the .ncd file are assigned specific locations on the FPGA, and the connections between the resources are mapped into the FPGAs interconnect network. The delays associated with interconnect on a large FPGA can be quite significant, so the place and route process has a large impact on the speed of the design. The place and route engine attempts to honor timing constraints that have been added to the design, but if the constraints are too tight, the engine will give up and generate an implementation that is functional, but not capable of operating as fast as desired. Be careful not to assume that just because a design was successfully placed and routed, that it will operate at the desired clock rate.

The output of the place and route engine is an updated .ncd file, which contains all the information necessary to implement the design on the chosen FPGA. All that remains is to translate the .ncd file into a configuration bitstream in the format recognized by the FPGA programming tools. Then the programmer is used to download the design into the FPGA, or write the appropriate files to a compact flash card, which is then used to configure the FPGA.