Transcript Flash memories

Flash memories
Amit Weissenstern
JTAG course 2009
Hebrew University Jerusalem Israel
Usage of Flash memories
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Your computer's BIOS chip
Flash memory cards (MMC, CF, SD, etc…)
USB storage devices (flash drives)
PCMCIA memory cards
Memory cards for video game consoles
Many more…
Background
• Toshiba presented the NOR flash (1984)
• Toshiba announced NAND flash (1987)
• Intel introduced the first commercial NOR type
flash chip (1988)
• Global NAND flash memory annual revenue is
$15.2 billion (2008)
Types of memories
Memories
Volatile
SRAM
Non Volatile
PROM
Flash
ROM
DRAM
EEPROM
EPROM
Flash - Advantages
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Non-volatile
Electrically erased and reprogrammed
Solid state
Can be accessed (read) at high speed.
Single/Multi Level
Flash - Disadvantages
• Limited life of about 100,000 to 1,000,000
erase and write operations.
• Erasing is a block-level function.
• Longer time periods are required for erasing
and writing.
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Access Time (μs):
1000000
15000
300
10000
DRAM
Flash
35
100
Hard Disk
1
1
Bit level structure
Clear flash = All bits are set to one.
Writing to flash is a two-step process:
• The memory must first be erased as a block
(all ones).
• A write operation only writes a zero into a
memory cell.
Flash file system
Use of specifically designed flash file
systems/controller to perform wear- leveling
and error correction.
• Spread writes over the media.
• Defragmentation.
• Update storage method.
NAND vs. NOR
NOR
• Enables direct access to each memory cell.
• Fast random access, but slower write speed.
• Uses silicon less efficiently, resulting in higher cost per bit.
NAND
• Enables sequential access to memory cells.
• Faster write and erase speed.
• Uses silicon more efficiently, resulting in lower cost per bit.
NAND has far outpaced NOR, due to faster write and erase
performance and lower cost.
Building blocks
Write – NOR flash
Read – NOR flash
Erase – NOR flash
Flash memory and Testability
Testability for Flash memory:
• Structural Tests
• Functional tests
Flash memory & JTAG technology:
• Use of flash in ‘On-Board Programming’
- Thank you -