FPGA and ASIC Technology Comparison

FPGA and ASIC Technology Comparison

Part 1

Fundamentals of FPGA Design

1day

Designing forPerformance

2days

Advanced FPGAImplementation

2days

Intro to VHDL or Intro to Verilog

3days


FPGA vs. ASIC Design FlowASIC to FPGA

Coding Conversion

Virtex-5 Coding Techniques Spartan-3 Coding Techniques

Curriculum Path

forASIC Design

Minimum

: 6 months design

experience


Welcome

If you are an experienced ASIC designer transitioning to FPGAs, this course will help you reduce your learning curve by leveraging your ASIC experienceCareful attention to how FPGAs are different than ASICs will help you create a fast and reliable FPGA design

Objectives

After completing this training you will be able to:Describe the differences between ASIC and FPGA architecturesExplain the features of Xilinx FPGA architectureBenefit from the Xilinx dedicated resources

Contrasting Architectures

ASIC architecture compared to the Xilinx FPGA architecture– Gates versus LUTs– Delays– Performance

Fundamental part selection considerations– Cost– Size– Performance– Volume– Analog circuitry– Time to market– Reprogrammability

Standard Cell

Advantages– Lowest price for high-volume

production (greater than 200K per year)– Fastest clock frequency (performance)– Unlimited size– Integrated analog functions

• Custom ASICs– Low power

Disadvantages– Highest non-recurring engineering

costs– Longest design cycle– Limited vendor IP with high cost– High cost for engineering change

orders

Embedded Array

Advantages– Low price for medium-volume to

high-volume production– Performance only slightly slower than a

standard cell– 50+ million gates– Custom macro support– More flexibility than an FPGA– Low power

Disadvantages– High non-recurring engineering costs– Design cycle longer than an FPGA – Vendor IP has high cost– Generally digital only

Xilinx FPGAsField-Programmable Gate Arrays

Advantages– Lowest cost for low-volume to medium-

volume production– No non-recurring engineering costs– Standard product – Fastest time to market– Xilinx has extensive library of IP

• Inexpensive compared to ASIC vendors– Ability to make bug fixes quickly

and inexpensivelyDisadvantages– Slower performance– Size limited to ~25 million system gates– Digital only

Field-Programmable Gate Arrays

Xilinx FPGAs are made using SRAMToday’s FPGAs use 65-nm copper CMOS process Potential to accommodate 25M system gates – Includes RAM and logic gates

Performance up to 550 MHzIntegrated synthesis, simulation, and place & route tools– PC and UNIX– Inexpensive: $2500 or less for the ISE Design Suite

• Use of third-party tools will increase costs• Free ISE WebPACK is available

Configuration Introduction

When does configuration happen?– On power up– On demand

Why do FPGAs need to be configured?− FPGA configuration memory is volatile− Configuration data is stored in a PROM or other external data

source

What do you need to know about FPGA configuration?− What happens during configuration− How to set up various configuration modes and daisy chains

Configuration

Cost of ownership is reduced with the ability to reconfigure the hardware—extending the life of the product

• Reduces the costly physical deployment of repair technicians

• Extends the life of the product Upgrades Bug fixes Adding additional functionality Faster time to market Partial reconfiguration

FPGA

FPGA Configuration Methods

Xilinx Cables:JTAGSlave SerialSlave SelectMAP

Microprocessor:JTAGSlave SerialSlave SelectMAP

Xilinx PROMs: Slave/Master Serial Slave/Master SelectMAP

Commodity Flash:Slave SelectMAPSPI*BPI*

*SPI and BPI support is available in the newer Virtex™-5 and Spartan™-3E families

Five Primary Elements

Routing

Xilinx FPGAs

Dedicated blocks

Input and output blocks

Configurable logic blocks

* Clocking Resources

Logic Cells

Logic cells include– Combinatorial logic, arithmetic

logic, and a register

Combinatorial logic is implemented using Look-Up Tables (LUTs)Register functions can include latches, JK, SR, D, and T-type flip-flopsArithmetic logic is a dedicated carry chain for implementing fast arithmetic operations

CarryChain

LUTCarry in

Carry out

D Q

S/R

Combinatorial Logic

LUT

LUTs function as a ROM

Combinatorial Logic

Z

They generate the output value… for a given set of inputs

ABCD

FE

0 0 0 0 0 0 00 0 0 0 0 1 00 0 0 0 1 0 00 0 0 0 1 1 10 0 0 1 0 0 1

0 0 0 1 0 1 1 . . .

0 0 1 1 0 0 00 0 1 1 0 1 00 0 1 1 1 0 00 0 1 1 1 1 1

A B C D E F Z

0 0 0 1 0 1

Constant delay through a LUTLimited by the number of inputs and outputs, not by complexity

Wide Input Functions

For wider input functions, LUTs can be combined using a multiplexer

These muxes are dedicated, so they are fast

LUT

LUT

LUTMUX

LUT-Based Memory

Can store 64 bits of memory as either a RAM or a ROMFundamentally, the LUT is a ROMCan become RAM with activation of configuration write strobeCombine multiple LUTs for larger memories—larger in both in depth and width

128 x 8 is not uncommon6-input LUT contains two 5-input LUTs, which adds more flexibility

LUT

Carry Logic

The carry logic chain is dedicated logic that computes high-speed arithmetic logic functionsThe carry chain generally consists of a multiplexer and an XOR gate– The LUT computes the multiplexer selector – The multiplexer determines the carry-out– The XOR gate computes the addition

Memory Blocks

Support single- and dual-port synchronous operations In dual-port mode, these RAM blocks support fully independent ports for both reading and writingEach RAM block can be configured for 36 kb– Can be used as 2 independent 18-kb

RAMsDedicated cascade logic allows 2 RAM blocks to be configured as 72 kbBlocks of memory are generally spread out across the dieDedicated FIFO logic enables each RAM to be configured as a FIFO

Port A: 8 bits

Port B: 32 bits

Configuration Depth Data Bits Parity Bits

32k x 1 32 kb 1 0

16k x 2 16 kb 2 0

8k x 4 8 kb 4 0

4k x 9 4 kb 8 1

2k x 18 2 kb 16 2

1k x 36 1 kb 32 4

Block RAM Configurations

Configurations available on each portIndependent configurations on ports A and B, read and write– Supports data-width

conversion, including parity bits

IN 8 bit

OUT 32 bit

IOB Element

Input path– Two DDR registers

Output path– Two DDR registers– Two 3-state enable DDR registers

Each path can be combinatorial or registeredSeparate clocks and clock enables for I and OSet and reset signals are shared

IOB Element

Default I/O standard varies by family– Fast and slow slew rate– Programmable drive strength– Other I/O standards

Built in SERDES functionality– ISERDES divides input data by up to 10– OSERDES multiplies output data by up to 10

DSP Slice

25x18 Multiply ALU Mode

Pattern DetectionIndependent

C input

Dedicated ACascading

Routing

A combination of programmable and dedicated routing linesDedicated routing– Global clocks with predefined clock tree– Regional clocks and IO clocks– Global low-skew routing resources for other

high-fanout signals– Carry chain routing– Dedicated routing among other dedicated

resources

General interconnect– Routing of local signals between CLBs and

IOBs

Clock Management

Dedicated clock trees are pre-optimized clock networks that balance the skew and minimize delay

Virtex-5 FPGA has 32 separate clock networksSpartan-3 FPGA has 8 separate clock networksEach can be configured for a built-in clock enable (BUFGCE) or switching clock sources (BUFGMUX)Local clock routing includes regional (BUFR) and SERDES (BUFIO)

Clock Management

PLLDigital Clock Manager (DCM) consists of…

– Digital Delay Locked Loop (DLL)

– Digital Frequency Synthesis (DFS)

– Digital Phase Shifter (DPS)

CMT

I/O Translators

Programmable input and output thresholdsSupported standards include– LVCMOS (several classes), LVPECL, HSTL

(several classes), SSTL (several classes), PCI, PCI-X, LVDS (several classes), GTL, GTL+, andHyperTransport™ (LDT) technology

- Supported standards vary, check your data sheet

Different I/O standards require a separate input and output reference voltage for each bank supporting a separate I/O standardGenerally, each bank can support several standards, as long as they share the same vref (input) or vcco (output)

Dedicated and Special ResourcesClock management (CMT)– DCM and PLL– Dedicated clock trees (not

shown)

Test logic– Built-in JTAG

I/O translators– Supporting many different thresholds

Other resources– Dual-Data Rate (DDR) registers in IOB

– SERDES resources

Dedicated Cores– Block RAM– DSP Slices– Gigabit transceivers, MGTs (all devices)– Tri-mode Ethernet MAC (all devices)– PCI Express® core (all devices)

Additional FXT Cores– PowerPC® 440 processors (not shown)– Faster GTX transceiver (not shown)

Other Resources

Embedded processor cores– 32-bit PowerPC 440 processor core (hard)– MicroBlaze processor core (soft)

Digitally controlled termination resistance (DCI)

FPGA flexibility– Reconfigurable logic– Time to market– Lowest “cost of change”

Xilinx combinatorial resources use flexible LUTsXilinx slices also contain registers, carry logic, clocking resources, and dedicated muxes to improve the performance for all applicationsXilinx FPGAs have dedicated resources for DSP, RAM, PCI, EMAC, and I/O that make these critical paths equivalent to a custom ASIC

Summary

Where Can I Learn More?

Xilinx online documents – www.support.xilinx.com

• Software manuals• Data sheets• Application notes• User guides

Xilinx Education Services courses– www.xilinx.com/training

• Xilinx tools and architecture courses• Hardware description language courses• Free Videos


Part 2

Fundamentals of FPGA Design

1day

Designing forPerformance

2days

Advanced FPGAImplementation

2days

Intro to VHDL or Intro to Verilog

3days


FPGA vs. ASIC Design FlowASIC to FPGA

Coding Conversion

Virtex-5 Coding Techniques Spartan-3 Coding Techniques

Curriculum Path

forASIC Design


Welcome

If you are an experienced ASIC designer transitioning to FPGAs, this course will help you reduce your learning curve by leveraging your ASIC experienceCareful attention to how FPGAs are different than ASICs will help you create a fast and reliable FPGA design

Objectives

After completing this training you will be able to:Describe how a simple logic implementation can differ between ASIC and FPGAsRecognize gate counts as an estimation of design sizeExplain some of the FPGA design practices you must follow to get peak performance in your FPGA

Gate Comparison

In retargeting HDL code for an ASIC design to an FPGA, gate conversion is rarely one to one

A 0.13-µ standard cell can have up to 100K gates per mm2

– A Virtex®-5 FPGA has about 20K usable gates per mm2

Why the difference?

Xilinx has programmable logic in addition to the functional logic– Routing– Multiplexers– Configuration memory registers

This means built-in design flexibility!

Gate Translation

Separate out logic, flip-flops, RAM, cores, and I/O– Partition cores into logic and RAM

Assume– 6 to 24 gates per LUT (depending on the number of inputs used)– RAM bits are equivalent– Up to 100 ASIC gates per I/O; translate to IOBs– 7 gates per register

So what design strategy do you think you need to use?– To get the most out of the FPGA try to use as many features as possible,

especially the FPGA’s dedicated hardware

Example

ASIC 250K logic gates

Four 32-kb blocks of RAM

243 pads, includingpower and ground

FPGA 20,800 to 41,600 LUTs

Equivalent

Equivalent number of pins

Depending on the number of LUTs needed, this design could use a

Virtex-5 LX30, LX50, or LX85 FPGA

Any ASIC-to-FPGA gate counting method is only a rough estimate.

Taking ASIC code directly to an FPGA will not utilize the dedicated resources of the FPGA.

Gate Counts

Gate counts are influenced byCoding styleMetal layersProcess geometryLibrary qualityPlacement and routing algorithmsCore contents (RAM versus gates)I/O requirementsSpecial features

CONCLUSION

8-input AND gate

AND Gate Example

For vec(7.0)

assign and_out = & vec;

VerilogVerilog

For vec(7 downto 0)

and_out <= vec(0) AND vec(1) AND vec(2) AND vec(3) AND vec(4) AND vec(5) AND vec(6) AND vec(7);

VHD

LVH

DL

ASIC Implementation

8-input AND gateTwo four-input NAND gates feeding a two-input NOR gate

Approximate delay in a standard-cell ASIC with 0.13-µ process = 0.47 ns

Approximate gate count = 14

Beware of ASIC libraries with very wide gate types!

Xilinx Implementation

Approximate max delay in a Virtex-5 FPGA = 0.435 ns

Approximate gate count = 18 gates

8-input AND gate implemented in three 4-input LUTs and two logic levels

Approximate max delay in a Spartan®-3 FPGA = 0.678 ns

Approximate gate count = 18 gates

Question

How many 4-input LUTs would be required to implement a 32-input OR gate?

How many Logic Levels would they generate?

Answer

How many 4-input LUTs would be required to implement a 32-input OR gate? 11

How many Logic Levels would they generate? 3

If net delays ~ .3 ns and LUT delays ~.2 ns then total delay would be 2(.3) + 3(.2) ~ 1.2 ns

…in a Spartan®-3 FPGA

How do you think this would be implemented in Virtex-5 with a 6-input LUT? (Answer: 7 LUTs and 2 Logic Levels)

LUT

LUT

LUT

LUT

LUT

LUT

LUT

LUT

LUT

LUT

LUT

Tri-State Busses

Some ASIC designs have large tri-state busses– There are no tri-state buffers associated with each slice in the newest

FPGAs– These will have to be re-synthesized and be mapped to LUTs and the F7

and F8 dedicated muxes– You may need to code these with a CASE statement and a high-Z output– The F7 can implement an 8-to-1 mux – The F8 can implement a 16-to-1 mux

Registered AND gate

process (clk)beginif rising_edge(clk) then

vec_q <= vec;and_out <= vec_q(0) AND vec_q(1) AND vec_q(2) AND vec_q(3) AND vec_q(4) AND vec_q(5) AND vec_q(6) AND vec_q(7);

end if;end process;

VHD

LVH

DL

always @ (posedge clk)begin

vec_q <= vec;and_out <= & vec_q;

end

Verilog

Performance Comparison

A comparison of the achieved performance for the registered 8-input AND gate– Virtex-5 FPGA

• ~550 MHz• ~88 gates

– 0.13-µ standard cell ASIC• ~850 MHz• ~77 gates

Typical high-performance frequencies (no optimization for the FPGA)– Virtex-5 FPGA

• ~275 MHz for four-levels of LUT (combinatorial) logic– 0.13-µ standard cell ASIC

• ~550 MHz for equivalent logic

Don’t forget to optimize your HDL code!

ASIC versus FPGA

Combinatorial logic implemented in an ASIC is typically faster than in an FPGA implementation– The fine-grain architecture of an ASIC allows wider input functions

to be implemented with significantly less delay– ASICs have a dedicated routing structure rather than a

programmable routing structureCritical paths typically include I/O, RAM, PCI™ technology, EMAC, and DSP resources– Xilinx has dedicated FPGA resources to implement these

functions, making these paths equivalent to an ASIC implementation• Remember: Xilinx Virtex-5 devices are cutting-edge ASICs

Don’t forget to include Xilinx-dedicated resources in your design!

Pipelining

fMAX = n MHz

D Q Two Logic Levels D Q

fMAX 2n MHz

One Level

One Level

D Q D Q D Q

Sequential Design

How do you get high performance from an FPGA?

Pipelining– For large combinatorial paths, additional registers may need to

be inferred to break up combinatorial paths to increase performance

– This technique increases the size of the design– This is not as likely to be needed for Virtex-5 FPGA designs

because the Virtex-5 FPGA has a 6-input LUT– Evaluate the number of logic levels your design has by

generating a timing report from the ISE® Design Suite or your synthesis tool

– Usually the registers are added at a hierarchical boundary

Don’t forget to evaluate the number of logic levels for your timing-critical paths!

Timing Constraints

How do you get high performance from an FPGA?

Timing constraints– Timing constraints communicate the performance goals to the

implementation tools– Global timing constraints constrain virtually all the paths in your design

based on your system frequency, input, and output times (PERIOD, OFFSET IN, OFFSET OUT)

– Path-specific timing constraints need to be added to constrain multi-cycle paths and false paths

Adding timing constraints is essential if you want good system speed!

Coding Style

How do you get high performance out of an FPGA?

Coding style has a large impact on the performance– Because FPGA combinatorial and routing resources are inherently slower,

the HDL coding style needs to be improved – Write your code to limit the number of logic levels inferred– Learn about proper HDL coding styles by listening to the REL modules

Don’t waste time! Evaluate your HDL!

Synchronous Design

How do you get reliability out of an FPGA?

Always build a synchronous design– Asynchronous circuits are less reliable– Lot variations exist for all FPGAs, which means that your design has to be

able to work for faster devices

Timing constraints– Cannot fix asynchronous design problems—only you can

Synchronous Design Methodology

One clock (or at least as few as possible)Use one edge (all flip-flops use rising or falling edge)Use D-type flip-flopsRegister the outputs of each behavioral blockIn place of multiple clocks, use clock enablesSynchronize asynchronous signals to the “single” clock (synchronization circuits)Do NOT create– Gated, derived, or divided clocks– Local asynchronous set/reset– Avoid global asynchronous set/reset

Get it right the first time!

Summary

Don’t worry too much about gate counting methodologies. They are only rough estimates, anywayOptimize your HDL coding styleInstantiate Xilinx-dedicated hardware resources into your design to improve your system speed and maximize what you get from your FPGAPipeline your timing-critical pathsTiming constraints are a primary means for improving system speedGet your design to work properly the first time by designing synchronously

Where Can I Learn More?

Xilinx Answers Browser – www.support.xilinx.com Answers Browser window

• Enter keywords like “pipelining” or “period constraint”Xilinx Education Services courses– www.xilinx.com/training

• Xilinx tools and architecture courses Fundamentals of FPGA Design

» Learn about synchronous design, global timing constraints, the Architecture Wizard, and the CORE Generator™ tool

Designing for Performance» Learn about avoiding metastability, path-specific timing

constraints, and the Timing Analyzer • Free Video-based Training

» Learn about proper HDL coding techniques

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FPGA and ASIC Technology Comparison