Transitioning from e-mmc to UFS: Controller Design. Kevin Liu ASolid Technology Co., Ltd.

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1 Transitioning from e-mmc to UFS: Controller Design Kevin Liu ASolid Technology Co., Ltd. Flash Storage Summits 2

2 Agenda emmc vs. UFS Flash Trend & Challenges Key Requirements for Embedded Controller Design Flash Storage Summits 2

3 Common Memory Interface Today NOR/XIP UFS DRAM System ~5MB/s NAND emmc System V ~ 3MB/s V2 ~ 6MB/s DRAM UFS emmc Controller SD card emmc NAND UFS Controller UFS NAND Flash Storage Summits 2

4 NAND Process Shrinking Faster than Moore s Law ECC (Bits) Process (nm) Endurence (K cycles) 3 2 TLC MLC SLC Flash Storage Summits 2

5 NAND Organization Trend 5 Larger page sizes improves sequential throughput. More pages per block affects random write performance Block size (K Bytes) Pages per block Page Size (K Bytes) Flash Storage Summits 2

6 Embedded vs. Consumer Applications Requirements for embedded applications are different than typical consumer applications in serveral ways Higher density More die Faster random operations Page-based block management DRAM cache? Faster/More I/O channels Multiple ECC engines Simultaneous operations Better endurance and reliability Stronger ECC FSP More consistent use over time Balanced block management to reduce write amplification Flash Storage Summits 2

7 8LC (TLC) Normal Read Thresholds MSB Thresholds After Program LSB Threshold CSB Thresholds Flash Storage Summits 2

8 EW Cycling vs Retention Lobes shift EW Cycling Lobes become wider Lobes shift Retention Dependence on cycle count Using the same set of thresholds before and after EW cycling and/or retention may not be a good idea Flash Storage Summits 2

9 Reliability Challenges BER equalized V th Equally spaced V th Flash Storage Summits 2

10 Flash Signal Processor (FSP) Adaptive read threshold positioning Adaptive program lobe positioning Efficient management of threshold history Adjustable programming speed/accuracy Randomizer to reduce interference Parameters of different flash Flash regions Storage Summits 2 W/L W/L W/L 2 W/L n High error rate 3 Vitim Cell 4 2 Low error rate B/L B/L B/L 2 B/L m

11 Solutions for Handling Flash Reliably NAND Technologies SLC/MLC/TLC Applications Consumer Embedded Technology & Trend Design Management FSP (Flash Signal Processing) ECC Advanced Firmware Algorithm Reliability Endurance Performance Delivery Flash Storage Summits 2

12 Recycling/Garbage Collection The process of removing invalid blocks in flash to create empty blocks to write to It s better to steal flash bandwidth from the host (background garbage collection) Blocks should be chosen wisely Should take into account both free space and cycle counts for Wear leveling TRIM command helps in space domain Real Time Clock may help in time domain Flash Storage Summits 2

13 Effective Wear-leveling Global wear- leveling Balance block usage over space domain Static wear-leveling Balance block usage over time domain may increase write amplification factor and hence kill flash life lower down performance if it s not managed Note : WriteAmplifcationFac tor appropriately Flash Storage Summits 2 FlashWrite HostWrite

14 Endurance vs. Random IOPS Page Based Mapping Block Based Mapping Flash Storage Summits 2

15 Power Cycling Complicates Memory Management Protect User's Data Guaranteeing data integrity is a MUST TLC much harder than MLC Required in embedded applications Previously written data Data in flight Advanced Error Recovery Techniques are needed Full Map resident in Flash Cache full map in SRAM/DRAM Cache partial map in DRAM Upload to host Flash Storage Summits 2

16 Power Management To control max power Ration power among active flash die and controller Smart state based power management PHY power management In general, a trade-off between power consumption wake-up latency Flash Storage Summits 2

17 Manufacturing Flexibility emmc/eufs are differentiated by optimizing for different flash devices and applications Should be achieved through manufacturing time customizations Firmware and MP tool must be flexible enough to fulfill different requirements Flash Storage Summits 2

18 Putting It All Together FSP Reliability Advanced Firmware Algorithm Performance Key embedded attributes that makes things great Endurance emmc/ UFS Controller NAND ECC Flexibility Power Consumption Flash Storage Summits 2