Programmable Logic/FPGAs

FPGA

An FPGA Die.

FPGA stands for "Field Programmable Gate Array", and is a type of VLSI that is programmable (and reprogrammable) in "the field". FPGAs can be programmed by using an HDL, and those programs get downloaded to the FPGA from a host computer system.

A field programmable gate array (FPGA) is a semiconductor device containing programmable logic components and programmable interconnects. The programmable logic components can be programmed to duplicate the functionality of basic logic gates such as AND, OR, XOR, NOT or more complex combinational functions such as decoders or simple math functions. In most FPGAs, these programmable logic components (or logic blocks, in FPGA parlance) also include memory elements, which may be simple flip-flops or more complete blocks of memories.

A hierarchy of programmable interconnects allows the logic blocks of an FPGA to be interconnected as needed by the system designer, somewhat like a one-chip programmable breadboard. These logic blocks and interconnects can be programmed after the manufacturing process by the customer/designer (hence the term "field programmable", i.e. programmable in the field) so that the FPGA can perform whatever logical function is needed.

FPGAs are generally slower than their application-specific integrated circuit (ASIC) counterparts, can't handle as complex a design, and draw more power. However, they have several advantages such as a shorter time to market, ability to re-program in the field to fix bugs, and lower non-recurring engineering costs. Vendors can sell cheaper, less flexible versions of their FPGAs which cannot be modified after the design is committed. The development of these designs is made on regular FPGAs and then migrated into a fixed version that more resembles an ASIC. Complex programmable logic devices, or CPLDs, are another alternative.

Applications

Applications of FPGAs include DSP, software-defined radio, aerospace and defense systems, ASIC prototyping, medical imaging, computer vision, speech recognition, cryptography, bioinformatics, computer hardware emulation and a growing range of other areas. FPGAs originally began as competitors to CPLDs and competed in a similar space, that of glue logic for PCBs. As their size, capabilities, and speed increased, they began to take over larger and larger functions to the state where some are now marketed as full systems on chips (SOC).

FPGAs especially find applications in any area or algorithm that can make use of the massive parallelism offered by their architecture. One such area is code breaking, in particular brute-force attack, of cryptographic algorithms.

FPGAs are also widely used for systems validation including pre-silicon validation, post-silicon validation, and firmware development. This allows chip companies to validate their design before the chip is produced in the factory, reducing the time to market.

Architecture

An application circuit must be mapinto an FPGA with adequate resources.

Logic block

Typical Logic Block

The typical FPGA logic block consists of a 4-input lookup table (LUT), and a flip-flop.

There is only one output, which can be either the registered or the unregistered LUT output. The logic block has four inputs for the LUT and a clock input. Since clock signals (and often other high-fanout signals) are normally routed via special-purpose dedicated routing networks in commercial FPGAs, they and other signals are separately managed.

Signal Routing

Each input is accessible from one side of the logic block, while the output pin can connect to routing wires in both the channel to the right and the channel below the logic block.

Each logic block output pin can connect to any of the wiring segments in the channels adjacent to it.

Similarly, an I/O pad can connect to any one of the wiring segments in the channel adjacent to it. For example, an I/O pad at the top of the chip can connect to any of the W wires (where W is the channel width) in the horizontal channel immediately below it.

Generally, the FPGA routing is unsegmented. That is, each wiring segment spans only one logic block before it terminates in a switch box. By turning on some of the programmable switches within a switch box, longer paths can be constructed. For higher speed interconnect, some FPGA architectures use longer routing lines that span multiple logic blocks.

Whenever a vertical and a horizontal channel intersect there is a switch box. In this architecture, when a wire enters a switch box, there are three programmable switches that allow it to connect to three other wires in adjacent channel segments. The pattern, or topology, of switches used in this architecture is the planar or domain-based switch box topology. In this switch box topology, a wire in track number one connects only to wires in track number one in adjacent channel segments, wires in track number 2 connect only to other wires in track number 2 and so on. The figure below illustrates the connections in a switch box.

FPGA design and programming

To define the behavior of the FPGA the user provides a hardware description language (HDL) or a schematic design. Common HDLs are VHDL and Verilog. Then, using an electronic design automation tool, a technology-mapped netlist is generated. The netlist can then be fitted to the actual FPGA architecture using a process called place-and-route, usually performed by the FPGA company's proprietary place-and-route software. The user will validate the map, place and route results via timing analysis, simulation, and other verification methodologies. Once the design and validation process is complete, the binary file generated (also using the FPGA company's proprietary software) is used to (re)configure the FPGA.

In a typical design flow, an FPGA application developer will simulate the design at multiple stages throughout the design process. Initially the RTL description in VHDL or Verilog is simulated by creating test benches to stimulate the system and observe results. Then, after the synthesis engine has mapped the design to a netlist, the netlist is translated to a gate level description where simulation is repeated to confirm the synthesis proceeded without errors. Finally the design is laid out in the FPGA at which point propagation delays can be added and the simulation run again with these values back-annotated onto the netlist.

In an attempt to reduce the complexity of designing in HDLs, which have been compared to the equivalent of assembly languages, there are moves to raise the abstraction level of the design. Companies such as Cadence, Synopsys and Celoxica are promoting SystemC as a way to combine high level languages with concurrency models to allow faster design cycles for FPGAs than is possible using traditional HDLs. Approaches based on standard C or C++ (with libraries or other extensions allowing parallel programming) are found in the Catapult C tools from Mentor Graphics, and in the Impulse C tools from Impulse Accelerated Technologies. Annapolis Micro Systems, Inc.'s CoreFire Design Suite and National Instruments LabVIEW FPGA provide a graphical dataflow approach to high-level design entry. Languages such as SystemVerilog, SystemVHDL, and Handel-C (from Celoxica) seek to accomplish the same goal, but are aimed at making existing hardware engineers more productive versus making FPGAs more accessible to existing software engineers.

Cores

Modern FPGA families expand upon the above capabilities to include higher level functionality fixed into the silicon. Having these common functions embedded into the silicon reduces the area required and gives those functions increased speed compared to building them from primitives. Examples of these include multipliers, generic DSP blocks, embedded processors, high speed IO logic and embedded memories.

To simplify the design of complex systems in FPGAs, there exist libraries of predefined complex functions and circuits that have been tested and optimized to speed up the design process. These predefined circuits are commonly called IP cores, and are available from FPGA vendors and third-party IP suppliers (rarely free, and typically released under proprietary licenses). Other predefined circuits are available from developer communities such as OpenCores.org (typically "free", and released under the GPL, BSD or similar license), and other sources.

Embedded Memory

Sram

Embedded processors

Embedded processors targetting FPGAs are provided as softcores IPs, described in an HDL, such as VHDL or Verilog. Most of the softcores are provided through FPGA vendors' proprietary design software and target their specific devices.