Roy LeeAdvisor: Lei He

royjylee@ucla.edu http://eda.ee.ucla.edu

October 26, 20111

SEU Mitigation for FPGA-based Systems

Outline

• Introduction

• In-Place Decomposition (IPD)

• In-Place LUT Polarity Inversion (IPV)

• Experimental Results

• Conclusions & Future Works

Robustness in FPGAs• FPGAs are extensively used not only for prototyping but

also in a wide range of applications such as internet networking and communication equipment, and robustness is among the most important design objectives

• An effective approach for reducing the impact of Single Event Upset can lead to higher mean-time-to-failure(MTTF), increased quality of service, and reduced maintenance cost

Single Event Upset (SEU)• Single Event Upsets (SEUs) : one of the main causes of reliability

reduction caused by charge particle strikes due to cosmic radiation which create soft errors

• Major effect on circuits : change the logic state of a static memory element

• Trend : SEU vulnerability is increasing with technology shrinking

SEU in FPGAs• Most of the commercial FPGAs employ SRAM as their

configuration memory elements for higher logic density and programming flexibility

• Three of the major memory elements in FPGAs : user flip-flop, block RAM, and configuration RAM

User Flip-Flop Block RAM Configuration RAM

05000000

100000001500000020000000250000003000000035000000400000004500000050000000

xc5vlx220t

Qua

ntity

Single Event Upset in FPGA• The circuit effect of SEUs in a FPGA is permanent until

the FPGA is re-programmed Interconnect :

0

1

0

1

0

1

0

1

I1 I2

Configuration Bits

LUT

0

0

0

I1

I2

Configuration Bits

MUX

Logic :

SEU on Configuration RAM is much more critical!

Demand for In-Place Reliability Optimizations

• Triple Modular Redundancy (TMR) is the most popular fault tolerant technique, but it requires more than 3X overhead on power, area, and cost

• For non-mission critical applications, such as communication systems, robustness improvement with little or no overhead is highly demanded

• In-place optimization techniques provide reliability improvement while preserving circuit placement and routing, and therefore the overhead is minimal

In-Place Resyntheses Flow• Mitigation after placement and routing without change of

placement and routing (and no change on design closure)

Design Entry

Logic Synthesis

Map

Bitstream

Placement and Routing

In-Place Decomposition (IPD)

In-Place LUT Polarity Inversion (IPV)

In-Place Resyntheses

Outline

• Introduction

• In-Place Decomposition (IPD)

• In-Place LUT Polarity Inversion (IPV)

• Experimental Results

• Conclusions & Future Works

Fault Metrics

|)}()(|{|21 xCxCxSER bnb

• Soft Error Rate(SER) of a configuration SRAM bit:

• Mean-Time-To-Failure (MTTF)• System level measurement of reliability• For single fault model, MTTF 1/average(SERb)

In-Place LUT Decomposition• Leveraging the dual-output feature of LUT architecture

and the built-in carry chains

Dual-output 6LUT

Xilinx Virtex-5 6-input LUT architecture

Original LUT Decomposed LUT

Decomposition

LUT Decomposition• Decomposition : F = C( F1, F2, ……, Fn )

(C is the converging logic function)

Example 1 : In-Place Duplication

• The average SER of the LUT is : (S0+……+ S31)/32

5-input AND function

Input Output SER00000 0 S0

00001 0 S1

00010 0 S2

11110 0 S30

11111 1 S31

……

……

5-input

……

Input Output SER

00000 0 S0

00001 0 S1

00010 0 S2

11110 0 S30

11111 1 S31

• The average SER of the LUT is reduced to (2*S31)/32

Covered……

Orig

DupF

…...

…...

0 -> 1

0

……

……

Input Output SER

00000 0 S0

00001 0 S1

00010 0 S2

11110 0 S30

11111 1 S31

……

……

…… Covered

Example 1 : In-Place Duplication

Input Output SER

000 0 AS0

001 0 AS1

110 0 AS6

111 1 AS7

• The number of SRAM bits used is reduced from 32 to 12, and the SERs of unused bits are 0

• The average SER is also reduced due to logic masking of the converging logic

……

3LUT

2LUT

F

……

……

Input Output SER

00 0 BS0

01 0 BS1

10 0 BS2

11 1 BS3

Example 2 : In-Place Decomposition

Outline

• Introduction

• In-Place Decomposition (IPD)

• In-Place LUT Polarity Inversion (IPV)

• Experimental Results

• Conclusions & Future Works

Fault Masking for MUX

• Fault is masked when logic(i) = logic(j)

……

b1 b…

MUX m

pin i

pin j

bm…

logic(i)

logic(j)

SEU on a routing MUX

Example of Fault Masking

b1 bk…

MUX m

1 1 1 0 1 0 0 1 1 1 (+)

0 1 0 1 0 1 1 0 0 1 (+)

bm…

1 0 1 0 1 0 1 0 1 0

……

1 0 1 0 1 0 1 0 1 0

v(i)

v(j)

observ(m)

soft error

0 0 0 1 0 1 1 0 0 0 (–)

0 1 0 1 0 1 1 0 0 1 (+)

1 0 1 0 1 0 1 0 1 0

……

0 0 0 0 0 0 0 0 0 0

b1 bk…

MUX m

bm…

SER(bk)=( v(i) v(j) ) · observ(m)

observ(m) is the fault observability at MUX m : the probability of the fault that can be propagated to the primary outputs

LUT Polarity Inversion

(000) = 0(001) = 1(010) = 0(011) = 1(100) = 0(101) = 1(110) = 1(111) = 0

a,+

c,+

b,+

(000) = 1(001) = 0(010) = 1(011) = 0(100) = 1(101) = 0(110) = 0(111) = 1

(000) = 1(001) = 0(010) = 0(011) = 1(100) = 1(101) = 0(110) = 1(111) = 0

o,+

a,+

c,+

b,+ o,-

a,-

c,-

b,+ o,+

Polarity can be determined independently for each input and the output of an LUT

LUT inversion

Fanout adjustment

Inversion to Reduce SER

……

b1 b…

MUX m

pin i, +

pin j, +

bm…

1 (90%) 0 (10%)1 (20%) 0 (80%)

……

b1 b…

MUX m

pin i, +

pin j, -

bm…

1 (90%) 0 (10%)0 (20%) 1 (80%)

SER: 1-0.9*0.2-0.1*0.8=0.74

SER: 1-0.9*0.8-0.1*0.2=0.26

Outline

• Introduction

• In-Place Decomposition (IPD)

• In-Place LUT Polarity Inversion (IPV)

• Experimental Results

• Conclusions & Future Works

Improvement by IPD

LUT-Level Chip-level

ABC IPD ABC IPD

alu4 0.34% 0.11% 0.45% 3.23%

apex2 0.29% 0.04% 0.33% 2.67%

apex4 1.16% 0.25% 1.41% 10.63%

des 1.42% 1.01% 2.43% 13.95%

ex1010 1.24% 0.29% 1.53% 11.60%

exp5p 0.73% 0.24% 0.97% 7.06%

misex3 0.55% 0.10% 0.65% 5.08%

pdc 0.91% 0.11% 1.02% 8.51%

seq 0.63% 0.11% 0.74% 5.78%

spla 1.14% 0.16% 1.30% 10.67%

SER Ratio 1.00 0.22 1.00 0.94

MTTF Imp. 1.00 4.52 1.00 1.07

IPV increase LUT-level MTTF by 4.52x, and chip-level MTTF by 1.07x (due to dominance of interconnects)

SER reduction for MCNC benchmarks mapped to 6-input LUTs

Improvement by IPV

IPV on average increases chip-level MTTF by 3.07XLess than 50% LUTs need to be inverted

Interconnect Chip-level

ABC IPV ABC IPV

alu4 3.06% 1.54% 3.40% 0.88%

apex2 2.61% 0.70% 2.90% 1.04%

apex4 10.44% 2.13% 11.60% 6.37%

des 12.78% 11.71% 14.20% 13.03%

ex1010 11.16% 1.50% 12.40% 4.40%

exp5p 6.57% 3.46% 7.30% 4.10%

misex3 4.95% 1.66% 5.50% 2.20%

pdc 8.19% 0.65% 9.10% 1.24%

seq 5.67% 1.67% 6.30% 1.28%

spla 10.26% 0.73% 11.40% 1.47%

SER Ratio 1.00 0.25 1.00 0.33

MTTF Imp. 1.00 3.99 1.00 3.07

SER for MCNC benchmarks mapped to 6-input LUTs

Improvement by Combined Algorithms

Combined Algorithms LUT Level Chip Level

IPF + IPD 66.53% 19.63%

IPF + IPV 14.76% 65.30%

IPD + IPV 76.06% 67.48%

IPF + IPD + IPV 66.53% 70.53%

IPF+IPD+IPV reduces chip-level SER by 70.53% 3.39x chip-level MTTF increase

Averaged SER reduction for MCNC benchmarks mapped to 6-input LUTs

Zhe Feng, Naifeng Jing and Lei He, “IPF: In-Place X-Filling to Migrate Soft Errors in SRAM-Based FPGAS,” FPL 2011

Conclusions & Future Works• Proposed two robust resynthesis techniques, In-Place

Decomposition(IPD) for logic and In-Place LUT Polarity Inversion(IPV) for interconnect, to improve circuit robustness without global overhead

• We show on average 3.39X MTTF improvement on the MCNC benchmark circuits when combining IPD, IPV, and IPF

• In the future, we will develop more in-place resynthesis techniques and investigate the interaction among different techniques

Thank you!