1

Encoding schemes for cross-talk power minimization in deep-submicron buses

Massimo PoncinoUniversità di Verona

Motivation• With technology scaling, the energy consumption of

on-chip buses becomes significant • Coupling capacitance (Cc) dominates the total

interconnect capacitance due to the aspect ratio of recent DSM technology (λλ = Cc/Cg > 2÷4 for 0.18µm )

• Current low-power techniques aim at reducing the coupling activity on parallel buses

Cross-Talk Phenomena• Opposite switching (cross-talk switching) represents

the worst case for:– Delay (reduced throughput)– Power (charge/discharge capacitance)– Signal integrity

Gnd plane

Cg Cg Cg

CcA1 V Cc A2

Energy Model

• Contributions:– Line to ground capacitance (Cg)– Lateral (or coupling) capacitance (Cc)

• Analytical model:Ebus=N (αc Cc + αs Cg) Vdd

2

αc: coupling activity αs: self activityN: number of bus linesVdd: supply voltage

Shielding Techniques

• Bus encoding techniques should target elimination of cross-talk activity

• Two main solutions to eliminate opposite switching between adjacent lines:– (Spatial) shielding:

• Insertion of neutral 0 or Vdd wires (shield wires) between data wires

– Bus encoding: • Use an encoding scheme that translates N -bit data words into

cross-talk free (N+a) -bit codewords

Shielding Techniques

Shield line (N-1 )

Data line (N)

Spatial shielding Bus encoding

M1 M2

ENC

DECN+a

bits

2

Temporal Shielding• Exploit redundancy to implement shielding over time• Redundancy is realized by adding a neutral all 1’s or

all 0’s word between two consecutive data words. – Opposite switching between adjacent wires is

completely avoided

• Possible problem:– Decrease in performance

• Solution:– Encoder packs two consecutive words at a time

• Shield sequence obtained has a higher self-activity than the original one– Solution: Build inter -symbols dynamically by and-ing

two consecutive 2N packed words

Temporal Shielding Algorithm

Cycle (i) w(i) x(i) comment0 0000 -1 0101 00000101 data2 1010 0000000 1 intersymbol3 0001 10100001 data4 1111 10100000 intersymbol5 1010 11111010 data6 1101 11011010 intersymbol7 1111 11011111 data

Example:Algorithm:

(wi ; w i+1) on odd cycles

Ii , i+1 on even cycles

where:

Ii ,i+1 = (wi+2 ;wi+3) • (w i ;w i+1)

xi

Experimental Results

Trace # patterns E[nJ] Spatial Sh. Saving [%] Temporal Sh.Saving [%]Bytecode 2644 7,32E-08 7,21E-08 1,5 6,12E-08 16,35HTML 22534 7,46E-07 7,17E-07 3,83 6,12E-07 17,9ImgTool 91396 2,04E-06 2,03E-06 0,66 1,74E-06 14,59JPEG 34985 1,40E-06 1,40E-06 0,02 1,10E-06 21,64M31 55067 1,65E-06 1,60E-06 2,93 1,36E-06 17,53MP3 25600 1,05E-06 1,03E-06 2,21 8,17E-07 22,18MPG 20880 7,80E-07 7,80E-07 0 6,27E-07 19,58Perl 3578 8,34E-08 8,04E-08 3,58 7,09E-08 14,94Random 7408 3,01E-07 2,99E-07 0,7 2,36E-07 21,59WAV 1194808 4,17E-05 4,17E-05 0,02 3,30E-05 20,78Average 1,55 18,71

Encoder Architecture

clk

RN

2-wordpacking

012N

Shieldbit

block

Shieldbit

block

Shieldbit

block

2N bit Data Bus

Shield Bit Block

clk

R

clk

R

clk

R

w(i)w(i+1)

MU

X

sel

I{w(i),w(i)}

Energy Savings vs. HW Encoder Consumption

- encoder- bus

Bus length (mm)

En

erg

y (J

)

0 0.5 1 1.5 2 2.5 3

3

2.5

2

1.5

1

0.5

0

Conclusions and Future Work

• The proposed encoding scheme is an alternative way to avoid cross-talk phenomena using a simpler codecarchitecture with an average energy savings around 18% for data buses

• It can be used with other encoding schemes (e.g.: Bus Invert or Coupling-Driven Bus Invert) to further reduce activity on buses

• Encoding schemes are more suitable for smaller technologies (<0.18µm) (higher λ), where cross-talkphenomena become more critical