Field-programmable gate array

A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturing – hence "field-programmable". The FPGA configuration is generally specified using a hardware description language (HDL), similar to that used for an application-specific integrated circuit (ASIC). (Circuit diagrams were previously used to specify the configuration, as they were for ASICs, but this is increasingly rare.)

FPGAs contain an array of programmable logic blocks, and a hierarchy of reconfigurable interconnects that allow the blocks to be "wired together", like many logic gates that can be inter-wired in different configurations. Logic blocks can be configured to perform complex combinational functions, or merely simple logic gates like AND and XOR. In most FPGAs, logic blocks also include memory elements, which may be simple flip-flops or more complete blocks of memory.^[1]

Technical design

Contemporary field-programmable gate arrays (FPGAs) have large resources of logic gates and RAM blocks to implement complex digital computations. As FPGA designs employ very fast I/Os and bidirectional data buses it becomes a challenge to verify correct timing of valid data within setup time and hold time. Floor planning enables resources allocation within FPGAs to meet these time constraints. FPGAs can be used to implement any logical function that an ASIC could perform. The ability to update the functionality after shipping, partial re-configuration of a portion of the design^[2] and the low non-recurring engineering costs relative to an ASIC design (notwithstanding the generally higher unit cost), offer advantages for many applications.^[1]

Some FPGAs have analog features in addition to digital functions. The most common analog feature is programmable slew rate and drive strength on each output pin, allowing the engineer to set slow rates on lightly loaded pins that would otherwise ring or couple unacceptably, and to set stronger, faster rates on heavily loaded pins on high-speed channels that would otherwise run too slowly.^[3][4] Another relatively common analog feature is differential comparators on input pins designed to be connected to differential signaling channels. A few "mixed signal FPGAs" have integrated peripheral analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) with analog signal conditioning blocks allowing them to operate as a system-on-a-chip.^[5] Such devices blur the line between an FPGA, which carries digital ones and zeros on its internal programmable interconnect fabric, and field-programmable analog array (FPAA), which carries analog values on its internal programmable interconnect fabric.

History

The FPGA industry sprouted from programmable read-only memory (PROM) and programmable logic devices (PLDs). PROMs and PLDs both had the option of being programmed in batches in a factory or in the field (field-programmable). However, programmable logic was hard-wired between logic gates.^[6]

In the late 1980s, the Naval Surface Warfare Center funded an experiment proposed by Steve Casselman to develop a computer that would implement 600,000 reprogrammable gates. Casselman was successful and a patent related to the system was issued in 1992.^[6]

Some of the industry's foundational concepts and technologies for programmable logic arrays, gates, and logic blocks are founded in patents awarded to David W. Page and LuVerne R. Peterson in 1985.^[7][8]

Altera was founded in 1983 and delivered the industry’s first reprogrammable logic device in 1984 – the EP300 – which featured a quartz window in the package that allowed users to shine an ultra-violet lamp on the die to erase the EPROM cells that held the device configuration.^[9]

Xilinx co-founders Ross Freeman and Bernard Vonderschmitt invented the first commercially viable field-programmable gate array in 1985 – the XC2064.^[10] The XC2064 had programmable gates and programmable interconnects between gates, the beginnings of a new technology and market.^[11] The XC2064 had 64 configurable logic blocks (CLBs), with two three-input lookup tables (LUTs).^[12] More than 20 years later, Freeman was entered into the National Inventors Hall of Fame for his invention.^[13][14]

Altera and Xilinx continued unchallenged and quickly grew from 1985 to the mid-1990s, when competitors sprouted up, eroding significant market share. By 1993, Actel (now Microsemi) was serving about 18 percent of the market.^[11] By 2010, Altera (31 percent), Actel (10 percent) and Xilinx (36 percent) together represented approximately 77 percent of the FPGA market.^[15]

The 1990s were an explosive period of time for FPGAs, both in sophistication and the volume of production. In the early 1990s, FPGAs were primarily used in telecommunications and networking. By the end of the decade, FPGAs found their way into consumer, automotive, and industrial applications.^[16]

Modern developments

A recent trend has been to take the coarse-grained architectural approach a step further by combining the logic blocks and interconnects of traditional FPGAs with embedded microprocessors and related peripherals to form a complete "system on a programmable chip". This work mirrors the architecture by Ron Perlof and Hana Potash of Burroughs Advanced Systems Group which combined a reconfigurable CPU architecture on a single chip called the SB24. That work was done in 1982. Examples of such hybrid technologies can be found in the Xilinx Zynq-7000 All Programmable SoC, which includes a 1.0 GHz dual-core ARM Cortex-A9 MPCore processor embedded within the FPGA's logic fabric or in the Altera Arria V FPGA, which includes an 800 MHz dual-core ARM Cortex-A9 MPCore. The Atmel FPSLIC is another such device, which uses an AVR processor in combination with Atmel's programmable logic architecture. The Microsemi SmartFusion devices incorporate an ARM Cortex-M3 hard processor core (with up to 512 kB of flash and 64 kB of RAM) and analog peripherals such as a multi-channel ADC and DACs to their flash-based FPGA fabric.

In 2010, Xilinx Inc introduced the first All Programmable System on a Chip branded Zynq™-7000 that fused features of an ARM high-end microcontroller (hard-core implementations of a 32-bit processor, memory, and I/O) with an 28 nm FPGA fabric to make it easier for embedded designers to use. The extensible processing platform enables system architects and embedded software developers to apply a combination of serial and parallel processing to their embedded system designs, for which the general trend has been to progressively increasing complexity. The high level of integration helps to reduce power consumption and dissipation, and the reduced parts count versus using an FPGA with a separate CPU chip leads to a lower parts cost, a smaller system, and higher reliability since most failures in modern electronics occur on PCBs in the connections between chips instead of within the chips themselves.^[17][18][19]

An alternate approach to using hard-macro processors is to make use of soft processor cores that are implemented within the FPGA logic. Nios II, MicroBlaze and Mico32 are examples of popular softcore processors.

As previously mentioned, many modern FPGAs have the ability to be reprogrammed at "run time", and this is leading to the idea of reconfigurable computing or reconfigurable systems – CPUs that reconfigure themselves to suit the task at hand.

Additionally, new, non-FPGA architectures are beginning to emerge. Software-configurable microprocessors such as the Stretch S5000 adopt a hybrid approach by providing an array of processor cores and FPGA-like programmable cores on the same chip.

Companies like Microsoft have started to use FPGA to accelerate high-performance, computationally intensive systems (like the data centers that operate their Bing search engine), due to the performance per Watt advantage FPGAs deliver.^[20]

Gates

• 1982: 8192 gates, Burroughs Advances Systems Group, integrated into the S-Type 24-bit processor for reprogrammable I/O.^[7][8]
• 1987: 9,000 gates, Xilinx^[11]
• 1992: 600,000, Naval Surface Warfare Department^[6]
• Early 2000s: Millions^[16]

Market size

• 1985: First commercial FPGA : Xilinx XC2064^[11]
• 1987: $14 million^[11]
• ≈1993: >$385 million^[11]
• 2005: $1.9 billion^[21]
• 2010 estimates: $2.75 billion^[21]
• 2013: $5.4 billion ^[22]
• 2020 estimate: $9.8 billion ^[22]

Design starts

• 2005: 80,000^[23]
• 2008: 90,000^[24]

FPGA comparisons

Historically, FPGAs have been slower, less energy efficient and generally achieved less functionality than their fixed ASIC counterparts. An older study had shown that designs implemented on FPGAs need on average 40 times as much area, draw 12 times as much dynamic power, and run at one third the speed of corresponding ASIC implementations. More recently, FPGAs such as the Xilinx Virtex-7 or the Altera Stratix 5 have come to rival corresponding ASIC and ASSP solutions by providing significantly reduced power, increased speed, lower materials cost, minimal implementation real-estate, and increased possibilities for re-configuration 'on-the-fly'. Where previously a design may have included 6 to 10 ASICs, the same design can now be achieved using only one FPGA.^[25]

Advantages of FPGAs include the ability to re-program in the field to fix bugs, and may include a shorter time to market and lower non-recurring engineering costs. Vendors can also take a middle road by developing their hardware on ordinary FPGAs, but manufacture their final version as an ASIC so that it can no longer be modified after the design has been committed.

Xilinx claims that several market and technology dynamics are changing the ASIC/FPGA paradigm:^[26]

• Integrated circuit development costs are rising aggressively
• ASIC complexity has lengthened development time
• R&D resources and headcount are decreasing
• Revenue losses for slow time-to-market are increasing
• Financial constraints in a poor economy are driving low-cost technologies

These trends make FPGAs a better alternative than ASICs for a larger number of higher-volume applications than they have been historically used for, to which the company attributes the growing number of FPGA design starts (see History).^[26]

Some FPGAs have the capability of partial re-configuration that lets one portion of the device be re-programmed while other portions continue running.

Complex programmable logic devices (CPLD)

The primary differences between CPLDs (complex programmable logic devices) and FPGAs are architectural. A CPLD has a somewhat restrictive structure consisting of one or more programmable sum-of-products logic arrays feeding a relatively small number of clocked registers. The result of this is less flexibility, with the advantage of more predictable timing delays and a higher logic-to-interconnect ratio. The FPGA architectures, on the other hand, are dominated by interconnect. This makes them far more flexible (in terms of the range of designs that are practical for implementation within them) but also far more complex to design for.

In practice, the distinction between FPGAs and CPLDs is often one of size as FPGAs are usually much larger in terms of resources than CPLDs. Typically only FPGAs contain more complex embedded functions such as adders, multipliers, memory, and serdes. Another common distinction is that CPLDs contain embedded flash to store their configuration while FPGAs usually, but not always, require external nonvolatile memory.

Security considerations

With respect to security, FPGAs have both advantages and disadvantages as compared to ASICs or secure microprocessors. FPGAs' flexibility makes malicious modifications during fabrication a lower risk.^[27] Previously, for many FPGAs, the design bitstream was exposed while the FPGA loads it from external memory (typically on every power-on). All major FPGA vendors now offer a spectrum of security solutions to designers such as bitstream encryption and authentication. For example, Altera and Xilinx offer AES (up to 256 bit) encryption for bitstreams stored in an external flash memory.

FPGAs that store their configuration internally in nonvolatile flash memory, such as Microsemi's ProAsic 3 or Lattice's XP2 programmable devices, do not expose the bitstream and do not need encryption. In addition, flash memory for a lookup table provides single event upset protection for space applications.. Customers wanting a higher guarantee of tamper resistance can use write-once, Antifuse FPGAs from vendors such as Microsemi.

With its Stratix 10 FPGAs and SoCs, Altera introduced a Secure Device Manager and physically uncloneable functions to provide high levels of protection against physical attacks.^[28]

Applications

Technically speaking, an FPGA can be used to solve any problem which is computable. This is trivially proven by the fact FPGA can be used to implement a soft microprocessor, such as the Xilinx MicroBlaze or Altera Nios II. Their advantage lies in that they are sometimes significantly faster for some applications due to their parallel nature and optimality in terms of the number of gates used for a certain process.

Specific applications of FPGAs include digital signal processing, software-defined radio, ASIC prototyping, medical imaging, computer vision, speech recognition, cryptography, bioinformatics, computer hardware emulation, radio astronomy, metal detection 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 point where some are now marketed as full systems on chips (SoC). Particularly with the introduction of dedicated multipliers into FPGA architectures in the late 1990s, applications which had traditionally been the sole reserve of DSPs began to incorporate FPGAs instead.^[29][30]

Another trend on the usage of FPGAs is hardware acceleration, where one can use the FPGA to accelerate certain parts of an algorithm and share part of the computation between the FPGA and a generic processor.

Traditionally, FPGAs have been reserved for specific vertical applications where the volume of production is small. For these low-volume applications, the premium that companies pay in hardware costs per unit for a programmable chip is more affordable than the development resources spent on creating an ASIC for a low-volume application. Today, new cost and performance dynamics have broadened the range of viable applications.

Common FPGA Applications:

Architecture

Logic blocks

The most common FPGA architecture^[1] consists of an array of logic blocks (called Configurable Logic Block, CLB, or Logic Array Block, LAB, depending on vendor), I/O pads, and routing channels. Generally, all the routing channels have the same width (number of wires). Multiple I/O pads may fit into the height of one row or the width of one column in the array.

An application circuit must be mapped into an FPGA with adequate resources. While the number of CLBs/LABs and I/Os required is easily determined from the design, the number of routing tracks needed may vary considerably even among designs with the same amount of logic. For example, a crossbar switch requires much more routing than a systolic array with the same gate count. Since unused routing tracks increase the cost (and decrease the performance) of the part without providing any benefit, FPGA manufacturers try to provide just enough tracks so that most designs that will fit in terms of Lookup tables (LUTs) and I/Os can be routed. This is determined by estimates such as those derived from Rent's rule or by experiments with existing designs.

In general, a logic block (CLB or LAB) consists of a few logical cells (called ALM, LE, Slice etc.). A typical cell consists of a 4-input LUT, a Full adder (FA) and a D-type flip-flop, as shown below. The LUTs are in this figure split into two 3-input LUTs. In normal mode those are combined into a 4-input LUT through the left mux. In arithmetic mode, their outputs are fed to the FA. The selection of mode is programmed into the middle multiplexer. The output can be either synchronous or asynchronous, depending on the programming of the mux to the right, in the figure example. In practice, entire or parts of the FA are put as functions into the LUTs in order to save space.^[31][32][33]

Hard blocks

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 I/O logic and embedded memories.

Higher-end FPGAs can contain high speed multi-gigabit transceivers and hard IP cores such as processor cores, Ethernet MACs, PCI/PCI Express controllers, and external memory controllers. These cores exist alongside the programmable fabric, but they are built out of transistors instead of LUTs so they have ASIC level performance and power consumption while not consuming a significant amount of fabric resources, leaving more of the fabric free for the application-specific logic. The multi-gigabit transceivers also contain high performance analog input and output circuitry along with high-speed serializers and deserializers, components which cannot be built out of LUTs. Higher-level PHY layer functionality such as line coding may or may not be implemented alongside the serializers and deserializers in hard logic, depending on the FPGA.

Clocking

Most of the circuitry built inside of an FPGA is synchronous circuitry that requires a clock signal. FPGAs contain dedicated global and regional routing networks for clock and reset so they can be delivered with minimal skew. Also, FPGAs generally contain analog PLL and/or DLL components to synthesize new clock frequencies as well as attenuate jitter. Complex designs can use multiple clocks with different frequency and phase relationships, each forming separate clock domains. These clock signals can be generated locally by an oscillator or they can be recovered from a high speed serial data stream. Care must be taken when building clock domain crossing circuitry to avoid metastability. FPGAs generally contain block RAMs that are capable of working as dual port RAMs with different clocks, aiding in the construction of building FIFOs and dual port buffers that connect differing clock domains.

3D architectures

To shrink the size and power consumption of FPGAs, vendors such as Tabula and Xilinx have introduced new 3D or stacked architectures.^[34][35] Following the introduction of its 28 nm 7-series FPGAs, Xilinx revealed that several of the highest-density parts in those FPGA product lines will be constructed using multiple dies in one package, employing technology developed for 3D construction and stacked-die assemblies.

Xilinx’s approach stacks several (three or four) active FPGA die side-by-side on a silicon interposer – a single piece of silicon that carries passive interconnect.^[35][36] The multi-die construction also allows different parts of the FPGA to be created with different process technologies, as the process requirements are different between the FPGA fabric itself and the very high speed 28 Gbit/s serial transceivers. An FPGA built in this way is called a heterogeneous FPGA.^[37]

Altera’s heterogeneous approach involves using a single monolithic FPGA die and connecting other die/technologies to the FPGA using Intel’s embedded multi-die interconnect bridge (EMIB) technology.^[38]

FPGA design and programming

To define the behavior of the FPGA, the user provides a hardware description language (HDL) or a schematic design. The HDL form is more suited to work with large structures because it's possible to just specify them numerically rather than having to draw every piece by hand. However, schematic entry can allow for easier visualisation of a design.

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. This file is transferred to the FPGA/CPLD via a serial interface (JTAG) or to an external memory device like an EEPROM.

The most common HDLs are VHDL and Verilog, although 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 through the introduction of alternative languages. National Instruments' LabVIEW graphical programming language (sometimes referred to as "G") has an FPGA add-in module available to target and program FPGA hardware.

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 (typically released under free and open source licenses such as the GPL, BSD or similar license), and other sources.

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 simulate 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.

More recently, OpenCL is being used by programmers to take advantage of the performance and power efficiencies that FPGAs provide. OpenCL allows programmers to develop code in the C programming language and target FPGA functions as OpenCL kernels using OpenCL constructs.^[39]

Basic process technology types

• SRAM – based on static memory technology. In-system programmable and re-programmable. Requires external boot devices. CMOS. Currently in use. It is worth noting that flash or EEPROM devices may often load contents into internal SRAM that controls routing and logic.
• Fuse – One-time programmable. Bipolar. Obsolete.
• Antifuse – One-time programmable. CMOS.
• PROM – Programmable Read-Only Memory technology. One-time programmable because of plastic packaging. Obsolete.
• EPROM – Erasable Programmable Read-Only Memory technology. One-time programmable but with window, can be erased with ultraviolet (UV) light. CMOS. Obsolete.
• EEPROM – Electrically Erasable Programmable Read-Only Memory technology. Can be erased, even in plastic packages. Some but not all EEPROM devices can be in-system programmed. CMOS.
• Flash – Flash-erase EPROM technology. Can be erased, even in plastic packages. Some but not all flash devices can be in-system programmed. Usually, a flash cell is smaller than an equivalent EEPROM cell and is therefore less expensive to manufacture. CMOS.

Major manufacturers

Xilinx and Altera are the current FPGA market leaders and long-time industry rivals.^[40] Together, they control over 80 percent of the market.^[41] Both Xilinx and Altera provide proprietary Windows and Linux design software (ISE/Vivado and Quartus) which enable engineers to design, analyse, simulate and synthesize (compile) their designs.^[42][43]

Other manufacturers include:

• Lattice Semiconductor (SRAM based with integrated configuration flash, instant-on, low power, live reconfiguration)
• Microsemi (previously Actel, antifuse, flash-based, mixed-signal)
• SiliconBlue Technologies (extremely low power SRAM-based FPGAs with optional integrated nonvolatile configuration memory; acquired by Lattice in 2011)
• Achronix (SRAM based, 1.5 GHz fabric speed),^[44]
• QuickLogic (handheld focused CSSP, no general purpose FPGAs)
• e2v (previously QP semiconductor)^[45]
• Atmel (second source of some Altera-compatible devices; also FPSLIC mentioned above)^[45]

In March 2010, Tabula announced their FPGA technology that uses time-multiplexed logic and interconnect that claims potential cost savings for high-density applications.^[46] On March 24, 2015, Tabula officially shut down.^[47]

On June 1, 2015, Intel announced it would acquire Altera for approximately $16.7 billion and completed the acquisition on December 30, 2015.^[48]

See also

References

Further reading

• Sadrozinski, Hartmut F.-W.; Wu, Jinyuan (2010). Applications of Field-Programmable Gate Arrays in Scientific Research. Taylor & Francis. ISBN 978-1-4398-4133-4. 
• Wirth, Niklaus (1995). Digital Circuit Design An Introduction Textbook. Springer. ISBN 3-540-58577-X. 

External links

• FPGA Development course on Youtube
• University of North Carolina at Charlotte's Reconfigurable Computing Laboratory
• Vaughn Betz's FPGA Architecture Page
• Tutorials and Examples on FPGAs
• What is an FPGA? on YouTube

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