An Improved “Soft” eFPGA Design and Implementation Strategy

Victor Aken’Ova, Guy Lemieux, Resve Saleh

SoC Research Lab, University of British Columbia

Vancouver, BC Canada

2

Overview• Introduction and Motivation

– Embedded FPGA (eFPGA)• Soft Embedded FPGAs

– Configurable Architecture• Improving Soft eFPGAs

– Tactical Standard Cells– Structured eFPGA layout

• Results• Summary and Conclusions

3

Introduction

• SoC designs are getting more complex and costly

• Programmability can be built into SoCs to

amortize costs by reducing chip re-spins

No Flexibility

Software Flexibility

Hardware FlexibilityeFPGAs

4

Applications for eFPGA Fabrics

13

2

eFPGA for product differentiation

An eFPGA for CPU acceleration

An eFPGA for revisions

CPU

5

Motivation

• shortcomings of existing eFPGA design approaches– Hard eFPGA

• Highly efficient full-custom layouts but inflexible

– Soft eFPGA • Very flexible but inefficient standard cell layouts

• alternative approach: flexible + efficient

6

“Hard” eFPGA Approach with a library of 3 Cores

??

Restrictive! overcapacity increases area and delay overheads

1

3

2

RTL

user circuit

?

7

The “Soft” eFPGA Approach

eFPGA RTL Generator

ASIC flow autogenerated eFPGA

Generic Standard Cells7x area and 2x delay versus full-custom

much less logic and routing overcapacity

8

Some Solutions to Problems of Existing Approaches

• retain eFPGA generator idea for flexibility

• use tactical cells to reduce area + delay

• use structured approach for efficiency

But…

9

Our Improved Design Approach“Soft++”

StructuredASIC FLOW

autogenerated eFPGA

Tactical +Generic Cellscombine best of soft and hard approaches

GOALGOAL

eFPGA RTL Generator

10

Island-style eFPGA Architecture

• used island-style architecture because

– Mainstream: existing FPGA CAD tools can

can be leveraged

– can exploit its regular structure to improve

design efficiency

• Created parameterized eFPGA in VHDL

11

Island-style eFPGA Architecture

(a) Island-style eFPGA (b) eFPGA Tile Layout

L

L: Left Edge TILE

B

B: Bottom Edge TILE

C

C: Corner TILE

12

Unstructured vs. Structured eFPGA Design Approach

Soft eFPGASoft eFPGA

Fixed Fixed LogicLogic

Fixed Fixed LogicLogic

(a) unstructured eFPGA layout (b) structured eFPGA layout

tile1tile2

tile3 tile4

13

Measured Impact of Structure on eFPGA Quality

• Significant improvements in logic capacity

– result of a more efficient CAD methodology

• wire-only critical path delay less by 21%

• Cut CAD design time by as much as 6X

14

Architecture-specific Tactical Cells – The Concept

• improve quality by creating few tactical

standard cells to replace generic cells

• detailed analysis of design profile should

reveal areas that yield significant gains

15

Standard cell Area Breakdown for Island-Style Architecture

flip-flops and multiplexers dominate eFPGA area

LUT mux 39%

input mux 13%

LUT30%

switch 16%

other12%

flip-flops 46%

muxes 42%

16

Architecture-specific Tactical Cells – Flip-Flop vs. SRAM

An SRAM circuit has fewer transistorsfewer transistors = less area

(b) typical SRAM cell(a) typical D flip-flop

~2:1 area ratio!VDD

write readbit bitb

GND

D

clock clock

Q

17

Custom Layout of Standard Cell – Flip-Flop vs. SRAM

Standard Cell Flip-flop Tactical SRAM Cell

2.5Xvdd

gnd

1X vdd

gnd

18

Architecture-specific Tactical Cells – CMOS vs. Pass Gate

pass tree logic uses fewer transistorsfewer transistors and is fasteris faster

D

OS0

S0

C

B

A

S0

S1

S1S0

~4:1 area ratio!

after extra output inverterdecompose into NAND, INV

O

S0

D

C

B

A

VDDS1 S1S0

19

Layout Technique for Pass-Tree Multiplexers

n-well cut-outs allow denser pass transistor tree layouts

n-well cutout

n-well n-well

gnd gnd

vdd vdd

underutilized region extra NMOS(denser cell)

20

Architecture-specific Tactical Cells – Cell Area

EquivalentStandard cell Area (um2)

CustomTactical cell Area (um2)

improvement Factor

Cell

1-SRAM

16:1 MUX

32:1 MUX

4-LUT

5-LUT

61 24 2.5899 146 6.1

2228 293 7.6

1875 530 3.53.910614180

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Area Impact of Tactical Standard Cells – eFPGA Area

eFPGA

(c) full-custom(b) soft ++(a) soft

eFPGA

-58%

eFPGA

-85%

soft ++ ~2.4X smaller than soft = 58% area savings

22

Graphs of Area and Delay SavingsA

rea

Del

ay

Benchmarks

Benchmarks

2.4X Better1.6 – 2.8X full-custom area

1.4X Better

1.1X of full-custom delay

23

Fabricated Chip Designs with eFPGAs (180nm process)

(a) gradual architecture (b) island-style architecture

24

Summary

• eFPGA area improved 58% (on average)

– 2 to 2.8X larger than full-custom equivalent (worst case)

• eFPGA delay improved 40% (average)

– within 10% of delay of full-custom versions

• exploited the regularity of island-style architecture to increase logic capacity

25

End of Talk

26

Question and Answer Slide

Logic Capacity

Are

aSoft

Soft++

soft++ fills some of performance gap left by hard

hard

custom