memory design - duke electrical and computer...

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ECE 261 Krish Chakrabarty 1

Memory Design

• Memory Types• Memory Organization• ROM design• RAM design• PLA design

Adapted from J. M. Rabaey, A. Chandrakasan and B. Nikolic, Digital Integrated Circuits, 2nd ed. Copyright 2003 Prentice Hall/Pearson.

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Semiconductor Memory Classification

Read-Write MemoryNon-VolatileRead-Write

MemoryRead-Only Memory

EPROM

E2PROM

FLASH

RandomAccess

Non-RandomAccess

SRAM

DRAM

Mask-Programmed

Programmable (PROM)

FIFO

Shift Register

CAM

LIFO

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Memory Timing: Definitions

Write cycleRead access Read access

Read cycle

Write access

Data written

Data valid

DATA

WRITE

READ

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Memory Architecture: Decoders

Word 0

Word 1

Word 2

Word N2 2

Word N2 1

Storagecell

M bits M bits

N

w o r d s

S0

S1

S2

SN2 2

A 0

A 1

AK2 1

K 5 log2N

SN2 1

Word 0

Word 1

Word 2

Word N2 2

Word N2 1

Storagecell

S0

Input-Output(M bits)

Intuitive architecture for N x M memoryToo many select signals:

N words == N select signalsK = log2N

Decoder reduces the number of select signals

Input-Output(M bits)

D e c o d e r

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Row

Dec

oder

Bit line2L 2 K

Word line

AKAK1 1

AL 2 1

A0

M.2K

AK2 1

Sense amplifiers / Drivers

Column decoder

Input-Output(M bits)

Storage cell

Array-Structured Memory ArchitectureProblem: ASPECT RATIO or HEIGHT >> WIDTH

Amplify swing torail-to-rail amplitude

Selects appropriateword

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Hierarchical Memory Architecture

Advantages:Advantages:1. Shorter wires within blocks1. Shorter wires within blocks2. Block address activates only 1 block => power savings2. Block address activates only 1 block => power savings

Globalamplifier/driver

Controlcircuitry

Global data busBlock selector

Block 0

Rowaddress

Columnaddress

Blockaddress

Block i Block P 2 1

I/O

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Read-Only Memory CellsWL

BL

WL

BL

1WL

BL

WL

BL

WL

BL

0

VDD

WL

BL

GND

Diode ROM MOS ROM 1 MOS ROM 2

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MOS OR ROM

WL[0]

VDD

BL[0]

WL[1]

WL[2]

WL[3]

Vbias

BL[1]

Pull-down loads

BL[2] BL[3]

VDD

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ROM Example• 4-word x 6-bit ROM

– Represented with dot diagram– Dots indicate 1’s in ROM

Word 0: 010101Word 1: 011001Word 2: 100101Word 3: 101010

ROM Array

2:4DEC

A0A1

Y0Y1Y2Y3Y4Y5

weakpseudo-nMOS

pullups

Looks like 6 4-input pseudo-nMOS NORs

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MOS NOR ROM

WL[0]

GND

BL [0]

WL [1]

WL [2]

WL [3]

VDD

BL [1]

Pull-up devices

BL [2] BL [3]

GND

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MOS NOR ROM Layout

Programmming using theActive Layer Only

Polysilicon

Metal1

Diffusion

Metal1 on Diffusion

Cell (9.5λ x 7λ)

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MOS NOR ROM Layout

Polysilicon

Metal1

Diffusion

Metal1 on Diffusion

Cell (11λ x 7λ)

Programmming usingthe Contact Layer Only

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MOS NAND ROM

All word lines high by default with exception of selected row

WL [0]

WL [1]

WL [2]

WL [3]

VDD

Pull-up devices

BL [3]BL [2]BL [1]BL [0]

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MOS NAND ROM Layout

No contact to VDD or GND necessary;

Loss in performance compared to NOR ROMdrastically reduced cell size

Polysilicon

Diffusion

Metal1 on Diffusion

Cell (8λ x 7λ)Programmming usingthe Metal-1 Layer Only

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NAND ROM LayoutCell (5λ x 6λ)

Polysilicon

Threshold-alteringimplant

Metal1 on Diffusion

Programmming usingImplants Only

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Decreasing Word Line Delay

Metal bypass

Polysilicon word lineK cells

Polysilicon word lineWLDriver

(b) Using a metal bypass

(a) Driving the word line from both sides

Metal word line

WL

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Precharged MOS NOR ROM

PMOS precharge device can be made as large as necessary,but clock driver becomes harder to design.

WL [0]

GND

BL [0]

WL [1]

WL [2]

WL [3]

VDD

BL [1]

Precharge devices

BL [2] BL [3]

GND

pref

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Read-Write Memories (RAM)STATIC (SRAM)

DYNAMIC (DRAM)

Data stored as long as supply is appliedLarge (6 transistors/cell)FastDifferential

Periodic refresh requiredSmall (1-3 transistors/cell)SlowerSingle Ended

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6-transistor CMOS SRAM Cell WL

BL

VDD

M5M6

M4

M1

M2

M3

BL

QQ

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6T-SRAM — Layout

VDD

GND

QQ

WL

BLBL

M1 M3

M4M2

M5 M6

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3-Transistor DRAM Cell

No constraints on device ratiosReads are non-destructiveValue stored at node X when writing a “1” = VWWL-VTn

WWL

BL1

M1 X

M3

M2

CS

BL2

RWL

VDD

VDD 2 VT

DVVDD 2 VTBL 2

BL 1

X

RWL

WWL

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3T-DRAM — Layout

BL2 BL1 GND

RWL

WWL

M3

M2

M1

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1-Transistor DRAM Cell

Write: CS is charged or discharged by asserting WL and BL.Read: Charge redistribution takes places between bit line and storage capacitance

Voltage swing is small; typically around 250 mV.

M1

CS

WL

BL

CBL

VDD 2 VT

WL

X

sensing

BL

GND

Write 1 Read 1

VDD

VDD /2 VDD /2

∆V BL VPRE– VBIT VPRE–CS

CS CBL+------------= =V

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DRAM Cell Observations1T DRAM requires a sense amplifier for each bit line, due to

charge redistribution read-out.DRAM memory cells are single ended in contrast to SRAM

cells.The read-out of the 1T DRAM cell is destructive; read and

refresh operations are necessary for correct operation.Unlike 3T cell, 1T cell requires presence of an extra

capacitance that must be explicitly included in the design.When writing a “1” into a DRAM cell, a threshold voltage is

lost. This charge loss can be circumvented by bootstrapping the word lines to a higher value than VDD

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1-T DRAM Cell

Uses Polysilicon-Diffusion CapacitanceExpensive in Area

M1 wordline

Diffusedbit line

Polysilicongate

Polysiliconplate

Capacitor

Cross-section Layout

Metal word line

Poly

SiO2

Field Oxiden+ n+

Inversion layerinduced byplate bias

Poly

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Periphery

DecodersSense AmplifiersInput/Output BuffersControl / Timing Circuitry

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Row DecodersCollection 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 A3A 0A0

A0A1A0A1A0A1A0A1

A1 A1

WL 1

Multi-stage implementation improves performance

NAND decoder usingNAND decoder using22--input preinput pre--decodersdecoders

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Dynamic DecodersPrecharge devices

VDD φ

GND

WL3

WL2

WL1

WL0

A0A0

GND

A1A1φ

WL3

A0A0 A1A1

WL 2

WL 1

WL 0

VDD

VDD

VDD

VDD

2-input NOR decoder 2-input NAND decoder

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4-to-1 tree based column decoder

Number of devices drastically reducedDelay increases quadratically with # of sections; prohibitive for large decoders

buffersprogressive sizingcombination of tree and pass transistor approaches

Solutions:

BL 0 BL 1 BL 2 BL 3

D

A 0

A 0

A1

A 1

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PLA versus ROMProgrammable Logic Arraystructured approach to random logic“two level logic implementation”

NOR-NOR (product of sums)NAND-NAND (sum of products)

IDENTICAL TO ROM!

Main differenceROM: fully populatedPLA: one element per minterm

Note: Importance of PLA’s has drastically reduced1. slow2. better software techniques (mutli-level logic

synthesis)But …

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Programmable Logic Array

GND GND GND GND

GND

GND

GND

VDD

VDD

X 0X 0 X 1 f0 f1X 1 X 2X 2

AND-plane OR-plane

Pseudo-NMOS PLA

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Dynamic PLA

GND

GNDVDD

VDD

X 0X 0 X 1 f0 f1X 1 X 2X 2

ANDf

ANDf

ORf

ORf

AND-plane OR-plane

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PLA LayoutVDD GNDφ

And-Plane Or-Plane

f0 f1x0 x0 x1 x1 x2 x2Pull-up devices Pull-up devices

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CAMs• Extension of ordinary memory (e.g. SRAM)

– Read and write memory as usual– Also match to see which words contain a key

CAM

adr data/key

matchread

write

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10T CAM Cell

• Add four match transistors to 6T SRAM– 56 x 43 λ unit cell

bit bit_b

word

match

cell

cell_b

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CAM Cell Operation

• Read and write like ordinary SRAM• For matching:

– Leave wordline low– Precharge matchlines– Place key on bitlines– Matchlines evaluate

• Miss line– Pseudo-nMOS NOR of match lines– Goes high if no words match

row decoder

weak

missmatch0

match1

match2

match3

clk

column circuitry

CAM cell

address

data

read/write