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EE 587SoC Design & Test

Partha PandeSchool of EECS

Washington State Universitypande@eecs.wsu.edu

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System Design Issues

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Low Energy FPGA Architecture

• Architectural level optimization– Level 0 – Nearest Neighbor– Level 1 – Mesh – Level 2 - Hierarchical

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Different Architectures

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Paths in Interconnect

• Connection may be long, complex:

LE LE LE LE LE

LE LE LE LE LE

LE LE LE LE LE

Wiring channel

Wir

ing

chan

nel

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Interconnect Architecture

• Connections from wiring channels to LEs.• Connections between wires in the wiring channels.

LE LE

Wiring channel

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Switchbox

channel channel

chan

nel

chan

nel

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Mesh-based Interconnect Network

Switch BoxRouting of the data

Connect BoxConnects cell I/OsTo the global interconnect

InterconnectPoint

Courtesy Dehon and Wawrzyniek

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Circuit Level Optimization

• The connecting path from one CLB to another is an RC chain

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Low Swing Interconnect

Mode E (pj) D (ns) ED

Full Swing 72.3 1.9 137

Low Swing 31.4 2.3 72

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Low Power SRAM Design

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Memory Organization

Sense amplifiers/drivers

Column decoder

AK

AK-1

AL-1

Storage cell

Word line

Bit line

Input-Output (M bits)

A0

AK-1

2L-K

M.2K

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SRAM Cell

bit bit

VDD

Sense amplifier

PC

EQ

Output

BL BL

WL

Prechargecircuit

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Cell Array Power Management

• Smaller transistors• Low supply voltage• Lower voltage swing (0.1V – 0.3V for SRAM)

– Sense amplifier restores the full voltage swing for outside use.

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SRAM Cell Design

• 6 transistor SRAM cell reduces static current (leakage) but take more area

• Vth reduction in very low Vdd SRAMs suffer from large leakage current

Use multiple threshold devices:

Memory cell with high Vth (reduce leakage)

Peripheral circuits with low Vth (improve speed)

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Banked Organization

• Banking targets total switched capacitance to achieve reduced power and improved speed

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Divided Word Line

• Main idea: Divide each row of RAM cells into segments (blocks), use a decoder to access only one segment

• Only the memory cells in the activated block have their bit line pair driven

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Divided Word Line

• Pros: Improves speed (by decreasing word line delay) Lower power dissipation (by decreasing the number of bit

line pair activated)• However, local decoders add delay• Less cells/block reduces power, but increases area (more local

decoders)• Chang, 1997:

49.8% power reduction, 14.8% area penalty82.9% power reduction, 24.8% area penalty

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Reduced Bit Line Swing

• Limit voltage swing on bit lines to improve both speed and power:

1. Pulsed word line

2. Bit line isolation

• Need sense amplifiers for each column to sense/restore signal

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Pulsed Word Line

• Main idea: Isolate memory cells from the bit lines after sensing, to prevent the cells from changing the bit line voltage further

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Pulsed Word Line

Q

R S

Reset from dummy sense-amp Word enable

SA Sense Amplifiers

Memory Core

Accessed Row

Dum

my

Col

umn

Wor

d D

river

Wor

d D

ecod

er

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Pulsed Word Line

• Dummy bit lines reach full swing, but trigger pulse shut off when regular bit lines reach 10% swing

• Generation of word line pulses very critical

– Too long: power efficiency degraded

– Too short: Sense amplifiers operation may fail

• Generation of word line using delay lines is susceptible to process and temperature

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Bit Line Isolation

• Main idea: Isolate sense amplifiers from bit line after sensing, to prevent from having large voltage swings

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Row Decoders

Collection of 2M complex logic gatesOrganized in regular and dense fashion

(N)AND Decoder

NOR Decoder

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Hierarchical Decoders

• • •

• • •

A2A2

A2A3

WL 0

A2A3A2A3A2A3

A3 A3A0A0

A0A1A0A1A0A1A0A1

A1 A1

WL 1

Multi-stage implementation improves performance

NAND decoder usingNAND decoder using2-input pre-decoders2-input pre-decoders

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Data Retention in SRAM

(A)

1.30u

1.10u

900n

700n

500n

300n

100n

0.00 .600 1.20 1.80

Factor 7

0.13 m CMOSm

0.18 m CMOSm

VDD

I lea

kag

e

SRAM leakage increases with technology scaling

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Reducing Retention Current

• Turning off unused memory blocks• Increasing the thresholds by using body biasing• Inserting extra resistance in the leakage path• Lowering the supply voltage

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Suppressing Leakage in SRAM

SRAMcell

SRAMcell

SRAMcell

VDD,int

VDD

VDD VDDL

VSS,int

sleep

sleep

SRAMcell

SRAMcell

SRAMcell

VDD,int

sleep

low-threshold transistor

Reducing the supply voltageReducing the supply voltageInserting Extra ResistanceInserting Extra Resistance