NEW READ RETRY

Powerful and automatic solution to improve reading of NAND flash chips

Patryk Płudowski - RUSOLUT

What is Read Retry?

2

1. Command

2. Response

Write Page

Command

Read Retry

Standard

VendorSpecific

Read Page

Read ID

READ RETRY IS A SPECIFIC COMMAND SET TO TWEAK READING MODE

CONTROLLER NAND FLASH

3

Problem with chip reading – bit errors

DATA FROM RAW NAND DATA FROM CONTROLLER

• Read disturb (noise, connection problems)

• Program disturb

• Endurance of cells

• Data retention

Bit error sources

Error Correction Codes (ECC)

No errors Correctable Uncorrectable

Green RedLight green

Am

ou

nt

of

erro

rs Error Correction Capability

ECC CORRECTABILTY IS STRICTLY LIMITED BY ECC CAPABILITYUncorrectable errors can be repaired with Read Retry

4

ECC map

NAND flash internals

5

Pe

riph

erals

PAGE DECODER PAGE DECODER

Me

mo

ry array

Blo

ck

Blo

ck

PAGEPAGEPAGEPAGE

PAGE

PageCELL CELL CELL CELLCELL

PAGE IS THE SMALLEST R/W UNIT

Page decoder

6

Digital data

01101011

Analog data

Page decoderMemory

array

2.3v 2.9v 3.1v 1,7v NAND controller

2.3v >> 012.9v >> 103.1v >> 101,7v >> 11

ANALOG STATES FROM CELLS ARE CONVERTED INTO DIGITAL DATA

*Simplified view

Architecture of cells

7

1 Bit Per Cell2 Levels100,000 P/E Cycles

2 Bits Per Cell4 Levels10,000 P/E Cycles

3 Bits Per Cell8 Levels1,000 P/E Cycles

LOWER ENDURANCE

11

10

01

00

SLC MLC TLC

1

0

111

010

100

001

110

101

000

011V

olt

age

[V]

= St

ore

d d

ata

LARGER CAPACITY

LARGER ECC AREA

8

Thresholds in memory cells

THE DATA IN CELL IS STORED AS A VOLTAGE LEVEL

3.0V

2.2V

2.5V

2.8V

0V

Vo

ltag

e[V

] =

Sto

red

dat

a

11(2)

10(2)

00(2)

01(2)

Zones of detection

1.9V

2.7V

2.4V

2 Bits Per Cell 4 Levels 10,000 P/E Cycles

MLC memory cell

9

Bits inside memory cell

*Simplified view

2.0V

Cell 3

2.0V

Cell 2

Vo

ltag

e[V

] =

Sto

red

dat

a

THE DATA IN CELL IS STORED AS A VOLTAGE LEVELController is reading data according to specific zones

2.2V

2.5V

2.8V

0V

11(2)

10(2)

00(2)

01(2)

Cell 1 Cell 4

2.9V 3.0V

Zones of detection

MLC memory cellCycle 1 2 3 4 HEX

Initial 10 11 11 10 BE

10

Retention Error

Cell 1 Cell 4V

olt

age[

V]

= St

ore

d d

ata

2.7V2.75V

WHEN CHARGE LEAKS OUT FROM CELL WE GET BIT ERROR

*Simplified view

2.2V

2.5V

2.8V

0V

11(2)

10(2)

00(2)

01(2) Zones of detection

Cycle 1 2 3 4 HEX

Initial 10 11 11 10 BE

Default read 00 11 11 00 3C

2.0V

Cell 3

2.0V

Cell 2

MLC memory cell

11

Read-Retry Mechanism

2.5V

2.8V

2.7V

2.75V

Cell 4Cell 1

Default read

10(2)

00(2)

Vo

ltag

e[V

] =

Sto

red

dat

a

WHEN CHARGE LEAKS OUT FROM CELL WE GET BIT ERRORDegraded cell gives wrong data

*Simplified view

Cycle 1 2 3 4 HEX

Initial 10 11 11 10 BE

Default read 00 11 11 00 3C

MLC memory cell (focused view)

READ RETRY MECHANISM HELPS TO SHIFT READ VOLTAGE THRESHOLDS

12

Read-Retry Mechanism #1

Vo

ltag

e[V

] =

Sto

red

dat

a

Shift threshold – atempt 1

Default read

*Simplified view

2.5V

10(2)

2.72V00(2)

2.75V

Cell 4Cell 1

2.7V

Cycle 1 2 3 4 HEX

Initial 10 11 11 10 BE

Default read 00 11 11 00 3C

Atempt #1 00 11 11 10 3E

MLC memory cell (focused view)

13

Read-Retry Mechanism #2

Vo

ltag

e[V

] =

Sto

red

dat

a

Default read

READ RETRY MECHANISM HELPS TO SHIFT READ VOLTAGE THRESHOLDS

Shift threshold – atempt 2

*Simplified view

2.5V

10(2)

00(2)

2.75V

Cell 4Cell 1

2.7V2.68V

Cycle 1 2 3 4 HEX

Initial 10 11 11 10 BE

Default read 00 11 11 00 3C

Atempt #1 00 11 11 10 3E

Atempt #2 10 11 11 10 BE

MLC memory cell (focused view)

Sets of parameters

14

ALL ZONES NEED TO BE ADJUSTED SEPARETLY

Default threshold

Default threshold

Default threshold

2.2V

2.5V

2.8V

0V

Vo

ltag

e[V

] =

Sto

red

dat

a

11(2)

10(2)

00(2)

01(2)

2.1V

2.4V

2.7V

0V

11(2)

10(2)

01(2)

00(2)

2.08V

2.55V

2.81V

0V

11(2)

10(2)

00(2)

01(2)

SHIFTED READ THRESHOLDS

DEFAULT THRESHOLD THRESHOLD SET #1 THRESHOLD SET #2

DIFFERENT PARAMETERS PRODUCE DIFFERENT ERROR RATE

Read Retry effect

15

15

13

11

10

8

6

5

2

0

2

3

7

Amount of errors SET -5SET -4SET -3SET -2SET -1DefaultSET +1SET +2SET +3SET +4SET +5SET +6

?resS any?koy to r?`ooTQresS any kgy to rebootPresS any kgy to rebootPress any key to rebootPress any key to rg2ootPress any key tk Rg??otPress an? +ey tk Rg?ootPress an? +ey ?k Rg??otPress`a~? +ey ?? Rg??otPre{s`a~? +ey ?? Zg??otPve{s`a|? +%y ?? Zw??otPveos`?|? +?y ??jZw??ot

CORRECT

New firmware which allows us to read chips faster and change NAND protocol instantly

16

What we did to implement Read Retry?

We made new universal software platform just to start Read Retry analysis

17

More than 500 devices analyzedTO UNDERSTAND what controller does to improve reading

18

And finally after ~2000 human hours we made an automatic and smart Read Retry algorithm in VNR

19

Read Retry results compared to normal ECC ReRead

20

1-st pass 2-nd Pass 3-th pass 4-th pass 5th-pass 6-th pass 7-th pass

ReadRetry 73% 87% 100%

ReRead 42% 55% 64% 69% 73% 75% 77%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Co

rre

cte

d e

rro

rsEffectiveness of correction

READ RETRY HELPS TO CORRECT ERRORS FROM ANY SOURCEWhen ReRead corrects errors caused only by read disturb

Already supported chips

21

Almost all chips of:

Micron, Intel2C64444BA989A4E53CA52C84643CA52C846454A92C68044AA9B5646454A4

….

Sandisk, Toshiba453C86937A983A95937A983AA89276983C95937A453C96937645DE94937645DEA49276454CA8A276453E9B927E

….

Most popular of:

SamsungECDEA47A68ECD7947E64ECD798CE74ECDE98CE74

ECD798CAECDED9CA

….

HynixADD7149E34ADDE14A742ADD794DA74ADDE14AB42ADDE94EB74ADD7949160

….

MUCH MORE SOON

SUMMARY• READING NAND FLASH WITH READ RETRY HELPS TO ADJUST READING PROTOCOL AND SIGNIFICANTLY IMPROVE

QUALITY OF DATA

• ALL NEW CHIPS HAVE A BUILT-IN READ RETRY MECHANISM AND ITS USAGE IS THE ONLY PROPER WAY TO EXTRACT PHYSICAL IMAGE

• WORKING BETA VERSION IS RELEASED IN VNR 3.5.

• IN CURRENT VERSION OF SOFTWARE IT IS SPECIFICALLY DESIGNED FOR PARTICULAR MEMORY CHIP

IT IS A AVAILABLE BY REQUEST AT: SUPPORT@RUSOLUT.COM

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LET’S MOVE TO PRACTICAL PART