Mobile Storage Applications: Controller Development Challenges

Flash Storage Summits 2010Flash Storage Summits 2010

Mobile Storage Applications: Controller Development

Challenges

Robert HsiehSilicon Motion, Inc.

Flash Storage Summits 2010

• SD - UHSI = 104MB/s - UHS II = 300MB/s

• eMMC- v4.4 = 104MB/s

• UFS - Target 300MB/s

What Throughput Host Supports


• Rapid Technology Process - 2009 : 3xnm- 2010 : 2xnm- 2011: 2Ynm

• Larger block size / Page size• Slower memory array timing• Increasing ECC requirement

NAND Flash Trend


The Challenges of Performance

• Slower memory array timing - tProg (T-program) and tR (T-

read) parameters is greater• More Page size / per block

- Decrease Random Performance


• Enable NAND High speed Interface- Toggle mode / ONFI Sync.- 200MB/s ~ 400MB/s in future

• Improve Random IOPS- Efficient memory management

• Scalable Design- Dual Channel / Interleave

How to Handle Performance


• The PV state drifts down and hence causes data retention issue

• Read/write disturbance and adjacent cell interference

• Vth distribution is broadening as P/E cycles increases

The Challenges of Reliability


• Stronger ECC engine can cover bit errors

• It can also extend the NAND P/E cycle

Increase ECC Requirement


Vth Tracking• Under different

ambient conditions (temperature, humidity, access count, … etc), the Vth may drift in different levels or directions

• Insert some PV7 in each page at pre-determined locations to track the Vth drift


Data-Shaping

• The uncompressed data blocks have very unbalanced distributions of 0 and 1.

• In new generation MLC and TLC flash, the unbalanced distribution damages the data stored in flash cells

• Data-shaping is necessary to be used to scramble the original data.

• After applying data-shaping to a sector, this sector cannot be compressed any more


Static Refresh and Early Retirement (1)

• In everyday use, more than 60% are repeated read operations, and the accumulated charge loss would eventually result in data loss

• The refresh of the reference cells, ‘StaticDataRefresh’, is achieved by erasing and re-programming the data into the same cell or into another cell when the correctable error number is lager than a predefined level


Static Refresh and Early Retirement (2)

• This implementation practically restores the data to its original, error-free state, and hence, lengthening the life of the data

• As a block ages over time, it cannot reliably store charge anymore, EarlyRetirement enters the scene


Power Consumption

• Meet Mobile application requirement- Standby current- Sleep current

• Keep low-powered mode for SoC memory

• Programmable clock frequency for slower clock mode

• Limit the number of NAND dies in active


Conclusion

• Mobile storage emphasize Random Performance, Reliability, and Power consumption

• Silicon Motion present the potential challenges that controller development may face

• A Dedicated controller is more suitable for mobile storage application


Thank You !

Mobile Storage Applications: Controller Development Challenges

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