Design of Embedded
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Field Programmable Gate
Arrays and Applications
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After going through this lesson the student will be able to
Define what is a field programmable gate array (FPGA)
Distinguish between an FPGA and a stored-memory processor
List and explain the principle of operation of the various functional units within an FPGA
Compare the architecture and performance specifications of various commercially
Describe the steps in using an FPGA in an embedded system
An FPGA is a device that contains a matrix of reconfigurable gate array logic circuitry.
When a FPGA is configured, the internal circuitry is connected in a way that creates a hardware
implementation of the software application. Unlike processors, FPGAs use dedicated hardware
for processing logic and do not have an operating system. FPGAs are truly parallel in nature so
different processing operations do not have to compete for the same resources. As a result, the
performance of one part of the application is not affected when additional processing is added.
Also, multiple control loops can run on a single FPGA device at different rates. FPGA-based
control systems can enforce critical interlock logic and can be designed to prevent I/O forcing by
an operator. However, unlike hard-wired printed circuit board (PCB) designs which have fixed
hardware resources, FPGA-based systems can literally rewire their internal circuitry to allow
reconfiguration after the control system is deployed to the field. FPGA devices deliver the
performance and reliability of dedicated hardware circuitry.
A single FPGA can replace thousands of discrete components by incorporating millions of logic
gates in a single integrated circuit (IC) chip. The internal resources of an FPGA chip consist of a
matrix of configurable logic blocks (CLBs) surrounded by a periphery of I/O blocks shown in
Fig. 20.1. Signals are routed within the FPGA matrix by programmable interconnect switches
and wire routes.
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Fig. 20.1 Internal Structure of FPGA
In an FPGA logic blocks are implemented using multiple level low fan-in gates, which gives it a
more compact design compared to an implementation with two-level AND-OR logic. FPGA
provides its user a way to configure:
1. The intersection between the logic blocks and
2. The function of each logic block.
Logic block of an FPGA can be configured in such a way that it can provide functionality as
simple as that of transistor or as complex as that of a microprocessor. It can used to implement
different combinations of combinational and sequential logic functions. Logic blocks of an
FPGA can be implemented by any of the following:
combinational gates like basic NAND gates or XOR gates
n-input Lookup tables
Wide fan-in And-OR structure.
Routing in FPGAs consists of wire segments of varying lengths which can be interconnected via
electrically programmable switches. Density of logic block used in an FPGA depends on length
and number of wire segments used for routing. Number of segments used for interconnection
typically is a tradeoff between density of logic blocks used and amount of area used up for
routing. Simplified version of FPGA internal architecture with routing is shown in Fig. 20.2.
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Fig. 20.2 Simplified Internal Structure of FPGA
Why do we need FPGAs?
By the early 1980’s large scale integrated circuits (LSI) formed the back bone of most of
the logic circuits in major systems. Microprocessors, bus/IO controllers, system timers etc were
implemented using integrated circuit fabrication technology. Random “glue logic” or
interconnects were still required to help connect the large integrated circuits in order to:
1. Generate global control signals (for resets etc.)
2. Data signals from one subsystem to another sub system.
Systems typically consisted of few large scale integrated components and large number of SSI
(small scale integrated circuit) and MSI (medium scale integrated circuit) components.Intial
attempt to solve this problem led to development of Custom ICs which were to replace the large
amount of interconnect. This reduced system complexity and manufacturing cost, and improved
performance. However, custom ICs have their own disadvantages. They are relatively very
expensive to develop, and delay introduced for product to market (time to market) because of
increased design time. There are two kinds of costs involved in development of custom ICs
1. Cost of development and design
2. Cost of manufacture
(A tradeoff usually exists between the two costs)
Therefore the custom IC approach was only viable for products with very high volume, and
which were not time to market sensitive.FPGAs were introduced as an alternative to custom ICs
for implementing entire system on one chip and to provide flexibility of reporogramability to the
user. Introduction of FPGAs resulted in improvement of density relative to discrete SSI/MSI
components (within around 10x of custom ICs). Another advantage of FPGAs over Custom ICs
is that with the help of computer aided design (CAD) tools circuits could be implemented in a
short amount of time (no physical layout process, no mask making, no IC manufacturing)
Evaluation of FPGA
In the world of digital electronic systems, there are three basic kinds of devices: memory,
microprocessors, and logic. Memory devices store random information such as the contents of a
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spreadsheet or database. Microprocessors execute software instructions to perform a wide variety
of tasks such as running a word processing program or video game. Logic devices provide
specific functions, including device-to-device interfacing, data communication, signal
processing, data display, timing and control operations, and almost every other function a system
The first type of user-programmable chip that could implement logic circuits was the
Programmable Read-Only Memory (PROM), in which address lines can be used as logic circuit
inputs and data lines as outputs. Logic functions, however, rarely require more than a few
product terms, and a PROM contains a full decoder for its address inputs. PROMS are thus an
inefficient architecture for realizing logic circuits, and so are rarely used in practice for that
purpose. The device that came as a replacement for the PROM’s are programmable logic devices
or in short PLA. Logically, a PLA is a circuit that allows implementing Boolean functions in
sum-of-product form. The typical implementation consists of input buffers for all inputs, the
programmable AND-matrix followed by the programmable OR-matrix, and output buffers. The
input buffers provide both the original and the inverted values of each PLA input. The input lines
run horizontally into the AND matrix, while the so-called product-term lines run vertically.
Therefore, the size of the AND matrix is twice the number of inputs times the number of
When PLAs were introduced in the early 1970s, by Philips, their main drawbacks were that they
were expensive to manufacture and offered somewhat poor speed-performance. Both
disadvantages were due to the two levels of configurable logic, because programmable logic
planes were difficult to manufacture and introduced significant propagation delays. To overcome
these weaknesses, Programmable Array Logic (PAL) devices were developed. PALs provide
only a single level of programmability, consisting of a programmable “wired” AND plane that
feeds fixed OR-gates. PALs usually contain flip-flops connected to the OR-gate outputs so that
sequential circuits can be realized. These are often referred to as Simple Programmable Logic
Devices (SPLDs). Fig. 20.3 shows a simplified structure of PLA and PAL.
Fig. 20.3 Simplified Structure of PLA and PAL
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With the advancement of technology, it has become possible to produce devices with
higher capacities than SPLD’s.As chip densities increased, it was natural for the PLD
manufacturers to evolve their products into larger (logically, but not necessarily physically) parts
called Complex Programmable Logic Devices (CPLDs). For most practical purposes, CPLDs
can be thought of as multiple PLDs (plus some programmable interconnect) in a single chip. The
larger size of a CPLD allows to implement either more logic equations or a more complicated
Fig. 20.4 Internal structure of a CPLD
Fig. 20.4 contains a block diagram of a hypothetical CPLD. Each of the four logic blocks shown
there is the equivalent of one PLD. However, in an actual CPLD there may be more (or less) than
four logic blocks. These logic blocks are themselves comprised of macrocells and interconnect
wiring, just like an ordinary PLD.
Unlike the programmable interconnect within a PLD, the switch matrix within a CPLD
may or may not be fully connected. In other words, some of the theoretically possible
connections between logic block outputs and inputs may not actually be supported within a given
CPLD. The effect of this is most often to make 100% utilization of the macrocells very difficult
to achieve. Some hardware designs simply won't fit within a given CPLD, even though there are
sufficient logic gates and flip-flops available. Because CPLDs can hold larger designs than
PLDs, their potential uses are more varied. They are still sometimes used for simple applications
like address decoding, but more often contain high-performance control-logic or complex finite
state machines. At the high-end (in terms of numbers of gates), there is also a lot of overlap in
potential applications with FPGAs. Traditionally, CPLDs have been chosen over FPGAs
whenever high-performance logic is required. Because of its less flexible internal architecture,
the delay through a CPLD (measured in nanoseconds) is more predictable and usually shorter.
The development of the FPGA was distinct from the SPLD/CPLD evolution just described.This
is apparent from the architecture of FPGA shown in Fig 20.1. FPGAs offer the highest amount of
logic density, the most features, and the highest performance. The largest FPGA now shipping,
part of the Xilinx Virtex™ line of devices, provides eight million "system gates" (the relative
density of logic). These advanced devices also offer features such as built-in hardwired
processors (such as the IBM Power PC), substantial amounts of memory, clock management
systems, and support for many of the latest, very fast device-to-device signaling technologies.
FPGAs are used in a wide variety of applications ranging from data processing and storage, to
instrumentation, telecommunications, and digital signal processing. The value of programmable
logic has always been its ability to shorten development cycles for electronic equipment
manufacturers and help them get their product to market faster. As PLD (Programmable Logic
Device) suppliers continue to integrate more functions inside their devices, reduce costs, and
increase the availability of time-saving IP cores, programmable logic is certain to expand its
popularity with digital designers.
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FPGA Structural Classification
Basic structure of an FPGA includes logic elements, programmable interconnects and memory.
Arrangement of these blocks is specific to particular manufacturer. On the basis of internal
arrangement of blocks FPGAs can be divided into three classes:
This architecture consists of logic elements (called CLBs) arranged in rows and columns
of a matrix and interconnect laid out between them shown in Fig 20.2. This symmetrical matrix
is surrounded by I/O blocks which connect it to outside world. Each CLB consists of n-input
Lookup table and a pair of programmable flip flops. I/O blocks also control functions such as tristate control, output transition speed. Interconnects provide routing path. Direct interconnects
between adjacent logic elements have smaller delay compared to general purpose interconnect
Row based architecture
Row based architecture shown in Fig 20.5 consists of alternating rows of logic modules
and programmable interconnect tracks. Input output blocks is located in the periphery of the
rows. One row may be connected to adjacent rows via vertical interconnect. Logic modules can
be implemented in various combinations. Combinatorial modules contain only combinational
elements which Sequential modules contain both combinational elements along with flip flops.
This sequential module can implement complex combinatorial-sequential functions. Routing
tracks are divided into smaller segments connected by anti-fuse elements between them.
This architecture is designed in hierarchical manner with top level containing only logic
blocks and interconnects. Each logic block contains number of logic modules. And each logic
module has combinatorial as well as sequential functional elements. Each of these functional
elements is controlled by the programmed memory. Communication between logic blocks is
achieved by programmable interconnect arrays. Input output blocks surround this scheme of
logic blocks and interconnects. This type of architecture is shown in Fig 20.6.
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Fig. 20.5 Row based Architecture
Fig. 20.6 Hierarchical PLD
FPGA Classification on user programmable switch technologies
FPGAs are based on an array of logic modules and a supply of uncommitted wires to
route signals. In gate arrays these wires are connected by a mask design during manufacture. In
FPGAs, however, these wires are connected by the user and therefore must use an electronic
device to connect them. Three types of devices have been commonly used to do this, pass
transistors controlled by an SRAM cell, a flash or EEPROM cell to pass the signal, or a direct
connect using antifuses. Each of these interconnect devices have their own advantages and
disadvantages. This has a major affect on the design, architecture, and performance of the FPGA.
Classification of FPGAs on user programmable switch technology is given in Fig. 20.7 shown
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Actel ACT1 & 2
Fig. 20.7 FPGA Classification on user programmable technology
The major advantage of SRAM based device is that they are infinitely re-programmable
and can be soldered into the system and have their function changed quickly by merely changing
the contents of a PROM. They therefore have simple development mechanics. They can also be
changed in the field by uploading new application code, a feature attractive to designers. It does
however come with a price as the interconnect element has high impedance and capacitance as
well as consuming much more area than other technologies. Hence wires are very expensive and
slow. The FPGA architect is therefore forced to make large inefficient logic modules (typically a
look up table or LUT).The other disadvantages are: They needs to be reprogrammed each time
when power is applied, needs an external memory to store program and require large area. Fig.
20.8 shows two applications of SRAM cells: for controlling the gate nodes of pass-transistor
switches and to control the select lines of multiplexers that drive logic block inputs. The figures
gives an example of the connection of one logic block (represented by the AND-gate in the upper
left corner) to another through two pass-transistor switches, and then a multiplexer, all controlled
by SRAM cells . Whether an FPGA uses pass-transistors or multiplexers or both depends on the
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Fig. 20.8 SRAM-controlled Programmable Switches.
The antifuse based cell is the highest density interconnect by being a true cross point.
Thus the designer has a much larger number of interconnects so logic modules can be smaller
and more efficient. Place and route software also has a much easier time. These devices however
are only one-time programmable and therefore have to be thrown out every time a change is
made in the design. The Antifuse has an inherently low capacitance and resistance such that the
fastest parts are all Antifuse based. The disadvantage of the antifuse is the requirement to
integrate the fabrication of the antifuses into the IC process, which means the process will always
lag the SRAM process in scaling. Antifuses are suitable for FPGAs because they can be built
using modified CMOS technology. As an example, Actel’s antifuse structure is depicted in Fig.
20.9. The figure shows that an antifuse is positioned between two interconnect wires and
physically consists of three sandwiched layers: the top and bottom layers are conductors, and the
middle layer is an insulator. When unprogrammed, the insulator isolates the top and bottom
layers, but when programmed the insulator changes to become a low-resistance link. It uses
Poly-Si and n+ diffusion as conductors and ONO as an insulator, but other antifuses rely on
metal for conductors, with amorphous silicon as the middle layer.
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Fig. 20.9 Actel Antifuse Structure.
The EEPROM/FLASH cell in FPGAs can be used in two ways, as a control device as in
an SRAM cell or as a directly programmable switch. When used as a switch they can be very
efficient as interconnect and can be reprogrammable at the same time. They are also non-volatile
so they do not require an extra PROM for loading. They, however, do have their detractions. The
EEPROM process is complicated and therefore also lags SRAM technology.
Logic Block and Routing Techniques
Crosspoint FPGA: consist of two types of logic blocks. One is transistor pair tiles in which
transistor pairs run in parallel lines as shown in figure below:
Fig. 20.10 Transistor pair tiles in cross-point FPGA.
second type of logic blocks are RAM logic which can be used to implement random access
Plessey FPGA: Basic building block here is 2-input NAND gate which is connected to each
other to implement desired function.
Fig. 20.11 Plessey Logic Block
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Both Crosspoint and Plessey are fine grain logic blocks. Fine grain logic blocks have an
advantage in high percentage usage of logic blocks but they require large number of wire
segments and programmable switches which occupy lot of area.
Actel Logic Block: If inputs of a multiplexer are connected to a constant or to a signal, it can be
used to implement different logic functions. For example a 2-input multiplexer with inputs a and
b, select, will implement function ac + bc´. If b=0 then it will implement ac, and if a=0 it will
Fig. 20.12 Actel Logic Block
Typically an Actel logic block consists of multiple number of multiplexers and logic gates.
Xilinx Logic block
In Xilinx logic block Look up table is used to implement any number of different
functionality. The input lines go into the input and enable of lookup table. The output of the
lookup table gives the result of the logic function that it implements. Lookup table is
implemented using SRAM.
Fig. 20.13 Xilinx - LUT based
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A k-input logic function is implemented using 2^k * 1 size SRAM. Number of different possible
functions for k input LUT is 2^2^k. Advantage of such an architecture is that it supports
implementation of so many logic functions, however the disadvantage is unusually large number
of memory cells required to implement such a logic block in case number of inputs is large. Fig.
20.13 shows 5-input LUT based implementation of logic block LUT based design provides for
better logic block utilization. A k-input LUT based logic block can be implemented in number of
different ways with tradeoff between performance and logic density.
Set by configuration
4-input “look up table”
An n-lut can be shown as a direct implementation of a function truth-table. Each of the latch
holds the value of the function corresponding to one input combination. For Example: 2-lut
shown in figure below implements 2 input AND and OR functions.
INPUTS AND OR
Altera Logic Block
Altera's logic block has evolved from earlier PLDs. It consists of wide fan in (up to 100
input) AND gates feeding into an OR gate with 3-8 inputs. The advantage of large fan in AND
gate based implementation is that few logic blocks can implement the entire functionality
thereby reducing the amount of area required by interconnects. On the other hand disadvantage is
the low density usage of logic blocks in a design that requires fewer input logic. Another
disadvantage is the use of pull up devices (AND gates) that consume static power. To improve
power manufacturers provide low power consuming logic blocks at the expense of delay. Such
logic blocks have gates with high threshold as a result they consume less power. Such logic
blocks can be used in non-critical paths.
Altera, Xilinx are coarse grain architecture.
Example: Altera’s FLEX 8000 series consists of a three-level hierarchy. However, the lowest
level of the hierarchy consists of a set of lookup tables, rather than an SPLD like block, and so
the FLEX 8000 is categorized here as an FPGA. It should be noted, however, that FLEX 8000 is
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a combination of FPGA and CPLD technologies. FLEX 8000 is SRAM-based and features a
four-input LUT as its basic logic block. Logic capacity ranges from about 4000 gates to more
than 15,000 for the 8000 series. The overall architecture of FLEX 8000 is illustrated in Fig.
(8 Logic Elements &
Fig. 20.14 Architecture of Altera FLEX 8000 FPGAs.
The basic logic block, called a Logic Element (LE) contains a four-input LUT, a flip-flop, and
special-purpose carry circuitry for arithmetic circuits. The LE also includes cascade circuitry that
allows for efficient implementation of wide AND functions. Details of the LE are illustrated in
Fig. 20.15 Altera FLEX 8000 Logic Element (LE).
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In the FLEX 8000, LEs are grouped into sets of 8, called Logic Array Blocks (LABs, a term
borrowed from Altera’s CPLDs). As shown in Fig. 20.16, each LAB contains local interconnect
and each local wire can connect any LE to any other LE within the same LAB. Local
interconnect also connects to the FLEX 8000’s global interconnect, called FastTrack. All
FastTrack wires horizontal wires are identical, and so interconnect delays in the FLEX 8000 are
more predictable than FPGAs that employ many smaller length segments because there are fewer
programmable switches in the longer path
From Fast Track
cntrl Cascade, carry
To Fast Track
To Fast Track
To Fast Track
to adjacent LAB
Fig. 20.16 Altera FLEX 8000 Logic Array Block (LAB).
FPGA Design Flow
One of the most important advantages of FPGA based design is that users can design it
using CAD tools provided by design automation companies. Generic design flow of an FPGA
includes following steps:
At this stage designer has to decide what portion of his functionality has to be implemented on
FPGA and how to integrate that functionality with rest of the system.
I/O integration with rest of the system
Input Output streams of the FPGA are integrated with rest of the Printed Circuit Board, which
allows the design of the PCB early in design process. FPGA vendors provide extra automation
software solutions for I/O design process.
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Designer describes design functionality either by using schematic editors or by using one of the
various Hardware Description Languages (HDLs) like Verilog or VHDL.
Once design has been defined CAD tools are used to implement the design on a given FPGA.
Synthesis includes generic optimization, slack optimizations, power optimizations followed by
placement and routing. Implementation includes Partition, Place and route. The output of design
implementation phase is bit-stream file.
Bit stream file is fed to a simulator which simulates the design functionality and reports errors in
desired behavior of the design. Timing tools are used to determine maximum clock frequency of
the design. Now the design is loading onto the target FPGA device and testing is done in real
Hardware design and development
The process of creating digital logic is not unlike the embedded software development
process. A description of the hardware's structure and behavior is written in a high-level
hardware description language (usually VHDL or Verilog) and that code is then compiled and
downloaded prior to execution. Of course, schematic capture is also an option for design entry,
but it has become less popular as designs have become more complex and the language-based
tools have improved. The overall process of hardware development for programmable logic is
shown in Fig. 20.17 and described in the paragraphs that follow.
Perhaps the most striking difference between hardware and software design is the way a
developer must think about the problem. Software developers tend to think sequentially, even
when they are developing a multithreaded application. The lines of source code that they write
are always executed in that order, at least within a given thread. If there is an operating system it
is used to create the appearance of parallelism, but there is still just one execution engine. During
design entry, hardware designers must think-and program-in parallel. All of the input signals are
processed in parallel, as they travel through a set of execution engines-each one a series of
macrocells and interconnections-toward their destination output signals. Therefore, the
statements of a hardware description language create structures, all of which are "executed" at
the very same time.
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Place and Route
Fig. 20.17 Programmable logic design process
Typically, the design entry step is followed or interspersed with periods of functional simulation.
That's where a simulator is used to execute the design and confirm that the correct outputs are
produced for a given set of test inputs. Although problems with the size or timing of the
hardware may still crop up later, the designer can at least be sure that his logic is functionally
correct before going on to the next stage of development.
Compilation only begins after a functionally correct representation of the hardware exists. This
hardware compilation consists of two distinct steps. First, an intermediate representation of the
hardware design is produced. This step is called synthesis and the result is a representation called
a netlist. The netlist is device independent, so its contents do not depend on the particulars of the
FPGA or CPLD; it is usually stored in a standard format called the Electronic Design
Interchange Format (EDIF).
The second step in the translation process is called place & route. This step involves mapping the
logical structures described in the netlist onto actual macrocells, interconnections, and input and
output pins. This process is similar to the equivalent step in the development of a printed circuit
board, and it may likewise allow for either automatic or manual layout optimizations. The result
of the place & route process is a bitstream. This name is used generically, despite the fact that
each CPLD or FPGA (or family) has its own, usually proprietary, bitstream format. Suffice it to
say that the bitstream is the binary data that must be loaded into the FPGA or CPLD to cause that
chip to execute a particular hardware design.
Increasingly there are also debuggers available that at least allow for single-stepping the
hardware design as it executes in the programmable logic device. But those only complement a
simulation environment that is able to use some of the information generated during the place &
route step to provide gate-level simulation. Obviously, this type of integration of device-specific
information into a generic simulator requires a good working relationship between the chip and
simulation tool vendors.
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Things to Ponder
Q.1 Define the following acronyms as they apply to digital logic circuits:
Q2.How granularity of logic block influences the performance of an FPGA?
Q3. Why would anyone use programmable logic devices (PLD, PAL, PLA, CPLD, FPGA,
etc.) in place of traditional "hard-wired" logic such as NAND, NOR, AND, and OR gates? Are
there any applications where hard-wired logic would do a better job than a programmable
Q4.Some programmable logic devices (and PROM memory devices as well) use tiny fuses
which are intentionally "blown" in specific patterns to represent the desired program.
Programming a device by blowing tiny fuses inside of it carries certain advantages and
disadvantages - describe what some of these are.
Q5. Use one 4 x 8 x 4 PLA to implement the function.
F1 ( w, x, y, z ) = wx ' y ' z + wx ' yz '+ wxy '
F2 ( w, x, y, z )= wx ' y + x ' y ' z
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