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Flash Memory

Flash Memory

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Flash Memory

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  1. Flash Memory Computer Science & Engineering DepartmentArizona State University Tempe, AZ 85287 Dr. Yann-Hang Leeyhlee@asu.edu(480) 727-7507

  2. Flash Memory • A type of EEPROM (Electrically-Erasable programmable Read-Only Memory) • an older type of memory that used UV-light to erase • Non-volatile, solid state technology • Relatively limited lifespan • Information is stored in an array of memory cells made from floating-gate (FG) transistors • Packaged inside a memory card: • Extremely durable • Can withstand intense pressure • Immersion in water • Better kinetic shock resistance than hard disks

  3. USB Flash Memory • USB connector • USB mass storage controller device • Test points • Flash memory chip • Crystal oscillator • LED • Write-protect switch (Optional) • Space for second flash memory chip

  4. Floating Gate Operation • Not programmed • No electronics trapped on floating gate • WL=1 turns on transistor, pulling Bitline low • Data out = 1 • As floating gate has no effect • Programmed • Electronics is trapped on floating gate • Increase threshold voltage • Transistor remains off when WL=1 • Bitline=1 and Data out=0 Sept 2007

  5. Flash Memory Operations • Write: programmed or not • Read: • Erase: make cells non-programmed write operation read operation erase operation

  6. NOR and NAND Flash • NOR • BL = NOR (all WL’s of non-programmed cells) • The word line of selected row high • Un-programmed = 1 • Programmed = 0 • NAND • BL= NAND (all WL’s of non-programmed cells ) • All word lines high by default with exception of selected row • Un-programmed = 1 • Programmed = 0

  7. Comparison of NOR and NAND Flash • NAND Flash cells are 60% smaller than NOR Flash cells • Wear leveling: limit in the number of times NAND Flash blocks can reliably be programmed and erased. • NAND Flash array: grouped into a series of blocks, which are the smallest erasable entities • Random access time on NOR Flash < 0.075μs; on NAND Flash, for the first byte is significantly slower > 10μs • NAND Flash: faster PROGRAM and ERASE times

  8. Parallel and Serial Flash Chips • Parallel interface • Direct control of flash memory operations • Read, program, erase (sector or chip), • write inhibit, standby • Byte programming • software id phase • load address and data • Program (20s) • Serial interface (SPI, I2C, microwire) • Send commands and receive responses • Read, program (auto-increment), erase • Status, write protection, etc.

  9. Coldfire Flash Module • Interfaces • As a read-only memory to the ColdFire core • A backdoor mapping for all program, erase, and verify operations, as well as providing a read datapath for the DMA. • May be programmed via the EzPort, which is a serial Flash programming • Flash Configuration field (24 bytes) • Stored in flash memory base+0x400 • Protection and access restriction • On reset, read in the field to Flash controller • Flash program using BDM