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Flash Memory and SSD

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  1. Flash Memory and SSD Nov. 8, 2010 Sungjoo Yoo Embedded System Architecture Lab. ESA, POSTECH, 2010

  2. Agenda • NAND Flash memory • Internal operations and reliability • Flash memory organization and operations • Read, program, and erase • ECC (error correction code) • Wear leveling, etc. • Flash translation layer (FTL) • SSD architecture ESA, POSTECH, 2010

  3. Context: SSD in Notebook and PDA • SSD Benefits • High performance, low power, and reliability 10 Flash memory chips ESA, POSTECH, 2010

  4. An Example: Intel SSD • Especially good for random accesses w.r.t. HDD ESA, POSTECH, 2010

  5. [Source: J. Lee, 2007] NOR vs. NAND Summary • NOR Flash • Random, direct access interface • Fast random reads • Slow erase and write • Mainly for code storage • NAND Flash • Block I/O access • Higher density, lower cost • Better performance for erase and write • Mainly for (sequential) data storage ESA, POSTECH, 2010

  6. [Source: Samsung, 2000] Area Efficiency • Metal contacts in NOR cell are the limiting factor: 2.5X difference in area/cell ESA, POSTECH, 2010

  7. NAND Flash Memory Circuit [A 113mm² 32Gb 3b/cell NAND Flash Memory]

  8. [Source: Samsung, 2000] NAND Program & Erase ESA, POSTECH, 2010

  9. [Suh, 1995] Bias Conditions for Erase, Program, and Read

  10. [Source: Z. Wu, 2007] SLC vs. MLC • SLC (single level cell) vs. MLC MLC SLC probability 10 00 01 0 11 1 voltage Vth • Fast, less error • Low bit density • Slow, more error • High bit density + ECC (error correction code) E.g., RS, LDPC, BCH, … E.g., 4bit ECC for 512B ESA, POSTECH, 2010

  11. (2008) • Three major parasitic effects • Background pattern dependency (BDP) • Noise • Cell-to-Cell interference • Proposed solution : • Reduce the number of neighbour cells • Reduce the Vth shift A Zeroing Cell-to-Cell Interference Page Architecture With Temporary LSB Storing and Parallel MSB Program Scheme for MLC NAND Flash Memories

  12. Features of 32Gb 3b/cell (D3) NAND flash memory with sub-35nm CMOS process (This has reduced cost and increased density compared to conventional chips) A 113mm² 32Gb 3b/cell NAND Flash Memory

  13. - Achieving typical 2.3MB/s program performance with an ISPP scheme - lowering the pate programming current using self-boosting of program inhibit voltages A 3.3V 32Mb NAND Flash Memory with Incremental Step Pulse Programming Scheme ISPP conventional Vpgm Vpgm time time

  14. - Achieving typical 2.3MB/s program performance with an ISPP scheme - lowering the pate programming current using self-boosting of program inhibit voltages A 3.3V 32Mb NAND Flash Memory with Incremental Step Pulse Programming Scheme 0.6V Distribution of program cycles number in a chip, The ISPP device has the narrowest distribution Programmed cell with Vth distribution of device with ISPP and 2 devices with fixed Vpgm pulses

  15. NAND structure and Operation reading programming

  16. Circuits and algorithms which result in an approximately 30% increase in program throughput compared to the conventional scheme A 48nm 32Gb 8-Level NAND Flash Memory with 5.5MB/s Program Throughput

  17. [Shibata, 2008] Floating Gate Coupling Effects

  18. - Programming method suppressing the floating gate coupling effect and making the narrow Vth distribution for 16LC A 70nm 16Gb 16-Level-Cell NAND flash Memory

  19. - Programming method suppressing the floating gate coupling effect and making the narrow Vth distribution for 16LC A 70nm 16Gb 16-Level-Cell NAND flash Memory Vth distribution transition of 16LC Comparison of Vth distribution

  20. Features of 32Gb 3b/cell (D3) NAND flash memory with sub-35nm CMOS process (This has reduced cost and increased density compared to conventional chips) A 5.6MB/s 64Gb 4b/Cell NAND Flash Memory in 43nm CMOS

  21. Flash Reliability

  22. [Source: Samsung, 2000] NAND Flash Lifetime • # of erase operations is limited due to degradation  wear leveling & ECC are needed! ESA, POSTECH, 2010

  23. representing raw error data from multi-level-cell devices from four manufacturers,identifying the root-cause mechanisms,and estimating the resulting uncorrectable bit error rate(UBER) Bit Error Rate in NAND Flash Memories

  24. representing raw error data from multi-level-cell devices from four manufacturers,identifying the root-cause mechanisms,and estimating the resulting uncorrectable bit error rate(UBER) Bit Error Rate in NAND Flash Memories Detrapping SILC

  25. representing raw error data from multi-level-cell devices from four manufacturers,identifying the root-cause mechanisms,and estimating the resulting uncorrectable bit error rate(UBER) Bit Error Rate in NAND Flash Memories

  26. representing raw error data from multi-level-cell devices from four manufacturers,identifying the root-cause mechanisms,and estimating the resulting uncorrectable bit error rate(UBER) Bit Error Rate in NAND Flash Memories

  27. Column-Dependent Errors Redundant bitlines! Redundant bitlines? [Swenson, 2009]

  28. Agenda • NAND Flash memory • Internal operations and reliability • Flash memory organization and operations • Read, program, and erase • ECC (error correction code) • Wear leveling, etc. • Flash translation layer (FTL) • SSD architecture ESA, POSTECH, 2010

  29. [Source: J. Lee, 2007] Flash Operations • Operations • Read • Write or Program • Changes a desired state from 1 to 0 • Erase • Changes all the states from 0 to 1 • Unit • Page • Read/Write unit (in NAND) • Block • Erase unit write erase ESA, POSTECH, 2010

  30. [Source: Micron, 2006] NAND Flash Architecture: 2Gb Case tR tPROG ESA, POSTECH, 2010

  31. [Source: Micron, 2007e] Small vs. Large Block ESA, POSTECH, 2010

  32. [Source: Micron, 2007e] Performance Comparison Small block 12.65MB/s for read 2.33MB/s for program 33ns*2K = 66us tR = 15us tPROG = 200us Large block 16.13MB/s for read 5.20MB/s for program tR = 25us tPROG = 300us Runtime reduction! Note: The same erase time per block! ESA, POSTECH, 2010

  33. [Source: Micron, 2006] Read Operation 25us Data ESA, POSTECH, 2010

  34. [Source: Micron, 2006] Pin Description ESA, POSTECH, 2010

  35. [Source: Micron, 2006] Commands & I/O Multiplexing ESA, POSTECH, 2010

  36. [Source: Micron, 2006] Erase Operation ESA, POSTECH, 2010

  37. [Source: Micron, 2006] Program Operation ESA, POSTECH, 2010

  38. [Source: Micron, 2006] Program with Random Data Input • Often used for partial page program ESA, POSTECH, 2010

  39. [Source: Micron, 2006] Page Storage Methods ESA, POSTECH, 2010

  40. [Source: Micron, 2006] Read Operation (Revisited) ESA, POSTECH, 2010

  41. [Source: Micron, 2006] Page Read Cache Mode ESA, POSTECH, 2010

  42. [Source: Micron, 2007] Comparison between Normal Read and Page Mode Cache Read ESA, POSTECH, 2010

  43. [Source: Micron, 2006] Commands & I/O Multiplexing ESA, POSTECH, 2010

  44. [Source: Micron, 2007] Page Read Cache Mode Operation ESA, POSTECH, 2010

  45. [Source: Micron, 2007] Performance Comparison ESA, POSTECH, 2010

  46. [Source: Micron, 2006] Program Page Cache Mode ESA, POSTECH, 2010

  47. [Source: Micron, 2006] Overlapping Program Data Cycles and tPROG ESA, POSTECH, 2010

  48. [Source: Micron, 2006] Commands & I/O Multiplexing ESA, POSTECH, 2010

  49. [Source: Micron, 2007b] Program Page Cache Mode Operation ESA, POSTECH, 2010

  50. [Source: Micron, 2007b] Performance Comparison ESA, POSTECH, 2010