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
Flash Storage Summits 2010
• Rapid Technology Process - 2009 : 3xnm- 2010 : 2xnm- 2011: 2Ynm
• Larger block size / Page size• Slower memory array timing• Increasing ECC requirement
NAND Flash Trend
Flash Storage Summits 2010
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
Flash Storage Summits 2010
• 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
Flash Storage Summits 2010
• 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
Flash Storage Summits 2010
• Stronger ECC engine can cover bit errors
• It can also extend the NAND P/E cycle
Increase ECC Requirement
Flash Storage Summits 2010
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
Flash Storage Summits 2010
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
Flash Storage Summits 2010
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
Flash Storage Summits 2010
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
Flash Storage Summits 2010
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
Flash Storage Summits 2010
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