What is an FPGA and how does it differ from an ASIC?

An FPGA is a chip with programmable logic that can implement a digital circuit without custom fabrication. Unlike an ASIC it can be reprogrammed over and over, is available off-the-shelf cheaply, and is faster to deploy. However it is slower than an ASIC, has no analog capability, and becomes more expensive if millions of units are needed.

How does an FPGA implement logic?

An FPGA consists of a huge number of repeated logic elements, each typically containing a lookup table (LUT), a flip-flop, and carry logic. The LUT can be programmed to model any type of gate, such as an inverter or a 4-input NAND.

What tools does FPGA design share with ASIC design?

FPGAs require synthesis, place and route, simulation, and verification tools — the same categories needed for ASIC design. The main additional tool needed for FPGAs is one that programs the finished bitstream onto the device.

FPGA

Suggest an improvement

November 13, 2020

An FPGA, or field programmable gate array, is a way to implement a digital circuit on a chip without fabricating it from scratch. This allows the designer to get a result more quickly, which can be useful for prototyping, and is much cheaper if only a small number of the chips need to be produced. If millions of devices need to be made, then an ASIC is probably going to be much cheaper than using FPGAs for the same function, and will have higher performance.

An FPGA is quite like an ASIC, but has some major differences.

Available to buy blank chips cheaply, off-the-shelf
Can program your digital design onto them, over and over again
Have a very limited set of standard cells - at the least lookup tables, flip-flops, memories. Could include multipliers, memory controllers or other high speed interfaces.
No analog capability
Slower than an ASIC

They have a reasonable overlap of the tools required to design for them. FPGAs need synthesis, place and route, simulation and verification tools. Additionally you need a tool that can take the finished bitstream and send that to the FPGA for programming.

An FPGA consists of a huge amount of repeated logic elements/blocks/slices. These pictures are of the Lattice iCE40 FPGA.

FPGA overview

These usually contain a lookup table, a flip-flop and some carry-logic (speeds up addition). The lookup table can be programmed to model any type of gate - for example an inverter or a 4 input NAND.

FPGA LUT

FPGA boards

An FPGA board can help you test your digital design at speed. For example I can simulate my vga clock, and even simulate the screen. But with an FPGA I can actually plug it into a real screen and see it works.

If you want to buy an FPGA board to play with and help you learn digital design, I recommend going for one with an active, supportive community:

Learn more

If you’re interested in learning how the Open Source FPGA tool flow works from end to end, check this video series I did for Symbiotic EDA.

Course feedback

The Zero To ASIC course took me on a fantastic journey from drawing and simulating a MOSFET, formal verification leading up to implementing a custom design with an open PDK and completely open source tools. The course is crammed full of interesting material with great pacing and support from Matt, and it's been a fantastic opportunity to meet other folks with shared interests and different backgrounds. The course has left me excited with opportunities for new projects and optimism for some working silicon!

Jamie Iles (digital course)