Session 5: FPGAs
FPGAs are devices that implement programmable digital designs, allowing designers to run their synthesized RTL at speeds that appear native compared to simulation. They’re immensely useful tools during the chip design process, both before and after tape-out.
Note
A note on FPGA terminology: the chip that contains the logic cells and programmable routing is referred to as the FPGA. However, these alone are of little practicality and must be mounted on a development board to be used. The development boards provide external interfaces like USB and SPI flash and are responsible for regulating power to the board as well as providing a crystal for clocking.
This distinction is important; FPGA vendors rarely make their own dev boards and they have different concerns to the user of the FPGA dev board.
As with any circuit component, it’s paramount to know what’s inside in order to best make use of its resources. Internally, FPGAs consist of a sea of look-up tables (LUT), each representing some logic function like ‘x OR y AND z’ and some programmable routing. A LUT is characterized by its number of inputs, so we say 4-LUT or 6-LUT to describe a 4 or 6 input look-up table. LUTs store the truth table of a logic function and by chaining together LUTs in specific arrangements, we can compute vastly large combinational and sequential logic.
Just like with regular ASICs, we use a hardware description language (HDL) like SystemVerilog to can describe the behavior we want our FPGA to ‘execute’ and the FPGA synthesis tool will convert this into thousands of logic functions, which we can then program onto the LUTs in the FPGA. Note that execute is in quotation marks, HDL cannot be ‘run’ in the same sense as code in C or Python can. The FPGA is simply a target for our hardware implementation as are ASICs. ASICs are the ‘real deal’; they compute logic functions using logic gates, while FPGAs compute logic functions by connecting together LUTs containing pre-computed values representing our design that produce equivalent (nearly) behavior.
The logic cell
There are a large number of FPGA varieties and families out there, each targeting its own segment of the market. At their core, though, they all share a good number of fundamental components, the most important of which is the logic cell.
FPGAs contain logic cells in the many thousands and they’re behind the “Gate Array” of FPGA. A logic cell contains a LUT, a bypass mux and a D-flop. Together, these components allow the logic cell to implement sequential or combinational logic. The inputs to the LUT are pre-programmed and come from the bitstream produced by the FPGA synthesis tool.