Embed Size (px)
Citation preview
Towards minimizing read time for NAND Flash
Towards minimizing read time for NAND Flash
GlobecomDecember 5th, 2012
Borja Peleato, Rajiv Agarwal, John Cioffi (Stanford University) Minghai Qin, Paul H. Siegel (UCSD)
2Towards Minimizing Read Time for NAND FLash
Outline
• Introduction– NAND Flash write and read– Memory structure
• Problem: choosing read thresholds• Progressive read algorithm
– Estimating Min-BER threshold– Generating soft information
• Results
3Towards Minimizing Read Time for NAND FLash
N+ Source N+ DrainP-Type SiliconSubstrate
Floating gate
Control gate
Dielectric
Cell write procedure
4Towards Minimizing Read Time for NAND FLash
N+ Source N+ Drain
P-Type SiliconSubstrate
Floating gate
Control gate
Dielectric
Vt = 2V
Cell read procedure
+_Vt = 1V
Exact voltage unknown, read returns 1 if Vt < Vcell and 0 otherwise
5Towards Minimizing Read Time for NAND FLash
Memory structure
Page (~105 cells): write/read unit
Block (~128 pages): erase unit
• Main sources of noise:– Over-programming (and write noise)– Leakage– Inter-Cell-Interference (ICI)
Voltage
Nu
mb
er o
f ce
lls
Noise increases with cell scaling and wear
6Towards Minimizing Read Time for NAND FLash
Voltage
Nu
mb
er o
f ce
lls
• How do we choose read voltages when noise estimates are not available?
Problem
7Towards Minimizing Read Time for NAND FLash
If then hence
Solve for read threshold with min BER
1
21
1
11121 1,
t
Qt
Qtt
• Estimate Gaussian noise from noisy cdf samples
voltage
Pro
babi
lity
Progressive Read Algorithm
ynt
Qt
Qty
2
2
1
1
2
1
2
1)(
),(),( 222
211
t ())ˆ,ˆ(),ˆ,ˆ(),(),...,,( 22114411 ytyt
t̂
t̂
8Towards Minimizing Read Time for NAND FLash
Min BER threshold
• Minimum estimation error when reads are close to the means and spread out
voltage
Pro
babi
lity
2*ˆ
*ˆˆˆˆ
*)(*)ˆ(
),(,,,21121
t
yyt
etBERtBER
nnOeeee
t
9Towards Minimizing Read Time for NAND FLash
Soft information
• If hard decoding fails, re-use reads for soft decoding– For lower failure rate, reads clustered in uncertainty
region…
voltage
Pro
babi
lity
– For best combined noise estimation and decoding, two reads in each region
if means and variances are known
10Towards Minimizing Read Time for NAND FLash
S1 S2 S3
| Aµ - µ | / µ 1.2 ∙ 10-2 0.13 2.1 ∙ 10-2
| Aσ – σ | / σ 7.5 ∙ 10-2 0.59 0.13
| At* - t* | / t* 2.2 ∙ 10-2 4.2 ∙ 10-2 2.4 ∙10-2
| BER(At*) - BER(t*) | BER(t*) 1.6 ∙ 10-2 6.1 ∙10-2 1.9 ∙10-2
LDPC fail rate 1 0.14 7.4 ∙10-3
Genie LDPC fail rate 1 0 3 ∙ 10-4
S1 S2 S3
| Aµ - µ | / µ 1.2 ∙ 10-2 0.13
| Aσ – σ | / σ 7.5 ∙ 10-2 0.59
| At* - t* | / t* 2.2 ∙ 10-2 4.2 ∙ 10-2
| BER(At*) - BER(t*) | BER(t*) 1.6 ∙ 10-2 6.1 ∙10-2
LDPC fail rate 1 0.14
Genie LDPC fail rate 1 0
S1 S2 S3
| Aµ - µ | / µ 1.2 ∙ 10-2
| Aσ – σ | / σ 7.5 ∙ 10-2
| At* - t* | / t* 2.2 ∙ 10-2
| BER(At*) - BER(t*) | BER(t*) 1.6 ∙ 10-2
LDPC fail rate 1
Genie LDPC fail rate 1
S1 S2 S3
| Aµ - µ | / µ
| Aσ – σ | / σ
| At* - t* | / t*
| BER(At*) - BER(t*) | BER(t*)
LDPC fail rate
Genie LDPC fail rate
Result: ½ read operations
4 reads instead of 8 2x speed
11Towards Minimizing Read Time for NAND FLash
Thank you!
