IRJET- A Survey on Reconstruct Structural Design of FPGA

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 07 Issue: 01 | Jan 2020 www.irjet.net p-ISSN: 2395-0072
© 2020, IRJET | Impact Factor value: 7.34 | ISO 9001:2008 Certified Journal | Page 1639
A Survey on Reconstruct Structural Design of FPGA
Rohit Raj1, Navneet Singh2
1,2Assistant Professor, Gurdasidevi Institute of Management & Technology, Budhlada- India
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Abstract - The digital circuit for eternity has been to a great
extent subjective by new types of Field Programmable devices
which are having complicated architectures e.g. FPGAs and
CPLDs, in the current years. This paper is for evaluating those
architectures in conditions of area and speed concert.
Key Words: FPGA, Look-Up Table (LUTs), Xilinx, CAD
Tools, Configurable Logic Block (CLB)
1. INTRODUCTION
Today's the biggest Coordinated Circuits into the business
sectors are amazingly complex field- programmable gate
array items (FPGA). For instance, the Xilinx, Altera has
created the programmable gadgets around 10 million
transistors in 0.5-micron advances and measures about 1.8
cm l.5 cms. These outcomes are because of the advances in
amazingly explore and improvement both in scholasticsand
businesses. The aggregate investigation of at present
accessible and quantities of the most industrial gadgets of
field-programmable gadgets are outlined in an ongoing
article in IEEE Plan and Trial of PCs. In this paper I am
attempting to depict, assessing the improvement in
engineering of FPGA that impact the absolute chip zone and
execution of speed. With the assistance of research
contemplates I will attempt to keep the steady highlights
utilized for business items.
The general methodology for this talk, I will followappeared
in figure 1. The originator presentthe explorationprocedure
is essentially founded on examination to consider the FPGA
engineering. The specialists to probe the structures must
create CAD tools to plan into the proposed IC. At that point
they do search for the parameters of the structures (for
example logic block convolution, interconnect adaptability,
and so on.) at that point going to make the computer aided
design apparatuses, for map the benchmark circuits intothe
theoretical ICs to at last assess the show of the
constructional design. As Figure 1 shows the iterative
endeavors of in vogue to complete the trials by consistently
altering the engineering and the utilization of computer-
aided design apparatuses.
2. RECENT DEVELOPMENTS
The results I am going to discuss in the next paper will be
about the current implementations in recent/present FPGA
products and might be affected in future architectures of
FPGA(s).
3. LOGIC-BLOCK COMPLEXITY
The complexity of Logic Block is themajorconcernedfor any
FPGA architecture as per the reports in recent research
publications. This is most of the time related about how a
single logic block should be handling to implement a
logic/digital circuitry in once. By the practical perceptions,
the basic block i.e. Lookup table (LUT), also canbetreated as
a memory, assumed a logic block as the number of inputs K.
Then as per the Rose et al. in study, the effect of N on both
the speed performance and the area required in
implementation of a ICs.
Fig 1: FPGA research Approach.
Fig 2: Logic Block functionality effects: on Speed
Fig 3: Logic Block Functionality effects: on Power.
The above Figures 2 and 3 is about trying to summarize
results for speed and performancearea.Figure2depicts that

the total relative area needed for differentvaluesofLUTs (K)
in an FPGA while assuming identical logic blocks. Normally
the FPGA had a minimum area for 4 LUTs, thus the other
data points of Figure 2 are normalized to those results. For
example, the figure shows that for K = 2 implementation
requires 1.5 times more area on other hand for K = 7, twice
as much. Thus as K increases, the total number of LUTs
decreases at the same time, area per LUT increases.
The different values of K also affectspeedperformanceasper
shown in Figure3. Where the verticalaxisrepresentscritical-
path delays averaged over a set of circuits. The performance
with very small LUTs poor, and larger logic blocks result in
better performance. As per figure 3 shows that up to a point
Improvements decreases beyond Ks of 5 or 6. As per the
results I summarize the results of this logic-blockcomplexity
study indicate that LUTs should have about 4 inputs to
optimize area.For speed performance, about 6 inputsisbest.
4. FPGA INTERCONNECT STRUCTURE
The interconnect structure is another fundamental
parameter besides the Logic Blocks, that determines an
FPGA's architecture. The studyofinterconnectsinFPGAhave
the structure illustrated in Figure 4. It also can be seen the
number and orientation of routing switches and wire in the
FPGA's interconnect structure. The wires exist in both
horizontal and vertical routing channels between rows and
columns of logic blocks to complete the Interconnect
structure. In this architecture the Routing switches appears
in two places. The two blocks i.e. the C blocks to connect the
logic- block pins to the routing wires, while the S block
connects one wire segment to another.Althoughinthisstudy
it has been assumed that allwire segments span onlya single
logic block and that joining two wires together at S blocks in
order to form longer connections. This paper’s major
concerned in the calculation of the order of programmable
routing switches (a measureof theareaneededfortheFPGA)
to place in the C and S block. There are two factors on which
designers/researchers are always investigating.
First is the number of wiresegmentsthateachlogic-blockpin
can connect to in a C block i.e. Fc, and other is for how many
other wire segments a wire segment entering an S block can
connect to i.e. Fs.
Fig 4: FPGA Routing Structure
Fig 5: Routing Flexibility
The figure 5 shows the results of experiment where each
curve in the figure depicts the specific value of Fs from 2
(lowest curve) to 6 (highest). While the horizontal axis
represents Fc as a percentage. This graph shows the
percentage of required connections (routability) in
designed/benchmark circuits that the CAD tools could
successfully complete for the given values of Fc and Fs.
Thus the figure 5 shows clearly that if Fc is at least 50% of
available tracks routability is good while forlowvaluesofFc,
routing circuits is difficult. Also, forall values of Fs, aslongas
Fc is greater than 50%, routability is high only exception is
with Fs=2. Thus, Fc should be high,andFscanbegreaterthan
or equal to 3. An example of acommercial productwiththese
characteristics is the Xilinx XC4000 series.
5. HARDWIRED LOGIC BLOCKS
In FPGAs the total signal propagation time depends on
Routing Delays which is 40 to 60% of total. Thus, to design
routing delays is the major factor while design/choose an
FPGA architectures. The hardwired logic block plays a vital
role to avoid routing delay. The figure 6 shows the scope of
the research by showing an example of a hardwired logic
block and its application. The figure 6a shows cascading of
three 4-input LUTs hardwired together to form a single
hardwired logic block. Thus, the advantage of hardwired
connections for circuits mapped into it through fewer
programmable switches.Anotherexampleofthesameshows
in Figures 6b and 6c. Figure 6b is an example circuit where
we might map eight normal 4- input LUTs. In this case, the
four programmable connections are responsible for the
longest path through the circuit, and thus the circuit would
traverse in series through four switches.
In contrast, Figure 6c shows the same circuit mapped into
hardwired logic blocks by using only one programmable
switch in order to make working of the circuit to be made
considerably faster. But it has the main drawback is that it
will add complexity to the CAD tools would need while
mapping circuits into the Blocks.

Fig 6: Example of hardwired logic block (a). A typical
circuit mapped into three 4-input LUTs (b) has four
connections in its critical path; the same circuit mapped
into hardwired logic block (c) has only one connection in
the critical path.
To measure/show the effects on speed performance, the
figure 7 shows the comparison of speed performance of
benchmark circuits to the results achievableusingnormal 4-
input LUTs. The horizontal axis corresponds to relative area
while vertical axis corresponds to relative speed
performance. The set of curves for hardwired logic blocks
based on 2- to 7-input LUTs shown in figure, in which each
curve showing results averaged over many hardwired
arrangements. From the figure it is easy to say that
hardwired logic blocks can provide a significant benefit in
terms of speed performance.
Fig 7: Impact of hardwired logic blocks on speed and area
6. DATA PATH IN FPGAS
Designers/researchers adopted a different approach to
optimize a chip for a specific class of circuits for general-
purpose use. An example of this philosophy is the
architectureofFPGAproposedbyCherepachaandLewiswho
proposed an architecture that takes advantage of properties
in data path circuit by manipulating a set of bits-as opposed
to a single bit-by performing arithmetic, multiplexing, and
other operations to be performed. Thus wecanalsooptimize
data path circuits in the routing structures and the logic
blocks in architecture. The figure8showshowdatapathscan
share routing bits by manipulate two bits of data in the same
way passing them through (LUTs programmed as) a
multiplexer and then an adder. The traditional approach
shows in figure 8a, in which eight programmable switches
(SRAM controlled pass transistors in this example) route the
two bits through the Interconnect. On the other hand, Figure
9b shows the circuit manipulates both the upper and lower
data bits in exactly the sameway such that the datapathscan
share the SRAM cells associated with eachbit.Andthiscanbe
achieved by only four SUM bits instead of eight, thus saving
chip area.
Fig 8: Sharing of routing bits in data path FPGAs.
Implementation without sharing {a] uses eight SRAM
cells; with sharing (b], four SRAM cells.
7. LOGIC EMULATION BY FPGA
The FPGA architecture can be designed for application
specific for logic emulation as per study provided by Jones
and Lewis. Previously Researchers/designers have to use
either very expensive hardware accelerators or slow
software approaches for logic emulation. Thus there would
be a requirement for an obvious approach to implement the
circuit by programming into an FPGA in such a way that this
should be functionally correct, and mightbehavingdifferent
timing than the real circuit if built using a different
technology. For this purpose the study showed how to
multiplex a single LUT over time to serveastheequivalent of
an entire array of LUTs. For this approach a set of SRAM
memories required as an extra hardware, which have very
high density-to hold the different results generated over
time by the multiplexed LUT.
Such type of architecture of the time multiplexed FPGA
depicts in figure 9. In this architecture foremulatinga circuit
is by representing each node in the circuit (i.e., each output
of a LUT) as a location in the SRAM block called the node
memory. It executes a “program” stored in SRAM block
called logic instruction memory and the single 4-input LUT
computes the value over time as the circuitoperatesfor each
node. We can also show this concept as LUT is like a CPU,
and the program stored by logic instructionmemorythatthe
CPU executes so each“instruction” executed bytheCPU(LUT
in this case) specifies several parameters such as the truth
table function for the LUT, the addresses of four nodes to
read as inputs to the 4-input LUT, and an addressin which to

store the generated output. Also, the 4-input LUT can
optionally feed a flip-flop; each instruction includes
configuration information for the flip- flop (suchastheclock
node).
Fig 9: Architecture of gate array (FPGA)
While going through the Figure 9 we also found it has an
additional blocks such as a cache and a third memory block
in order to improve the efficiency of thearchitecture,suchas
a fast cache memory does in a computer system. This
architecture also having an I/O unit that allows us to build
larger emulators by interconnecting multiple FPGAs.
This type of architecture one can found in most of recent
typical commercial FPGA series, e.g. the Xilinx XC4000,
Virtex, Zynq series etc. Also it offers a muchcheapersolution
in terms of gates per unit area, and the speed performance
achieved is much greater than that of software simulations,
the only other inexpensive solution.
8. CONCLUSION
The main objective of this paper is to provide insight into
FPGA architectural design. As the FPGA technology will
always be remain an exciting and dynamic technology for at
least the next several years as academics and industry
continue to develop increasingly sophisticated devices
through innovative research studies.
REFERENCES
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[3] Rose, Jonathan, et al. "Architecture of field-
programmable gate arrays: The effect of logic block
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[4] Rose, Jonathan, and Stephen Brown. "Flexibility of
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[8] Aggarwal, Aditya A., and David M. Lewis. "Routing
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[9] Cherepacha, Don, and David Lewis. "DP-FPGA: An
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