MD1665-D64-J SanDisk, MD1665-D64-J Datasheet - Page 11

• Manufacturer Part Number: MD1665-D64-J
• Description: UDOC FLASH DISK 64MB
• Manufacturer: SanDisk

Specifications of MD1665-D64-J

• Memory Type: FLASH - Nand
• Memory Size: 64MB
• Package / Case: Module
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Speed: -

4.3
Flash memory can be erased a limited number of times. This number is called the erase cycle limit,
or write endurance limit, and is defined by the flash array vendor. The erase cycle limit applies to
each individual erase block in the flash device. In uDiskOnChip, the erase cycle limit of the flash is
100,000 erase cycles. This means that after approximately 100,000 erase cycles, the erase block
begins to generate storage errors at a rate significantly higher than the error rate that is typical to the
flash.
In a typical application, and especially if a file system is used, specific pages are constantly updated
(e.g., the page/s that contain the FAT, registry, etc.). Without any special handling, these pages
would wear out more rapidly than other pages, reducing the lifetime of the entire flash.
To overcome this inherent deficiency, TrueFFS uses M-Systems’ patented wear-leveling algorithm.
This wear-leveling algorithm ensures that consecutive writes of a specific sector are not written
physically to the same page in the flash. This spreads flash media usage evenly across all pages,
thereby maximizing flash lifetime.
M-Systems’ wear-leveling mechanism provides more than 5 million write/erase cycles for reliable
data storage over an extended period
Dynamic Wear-Leveling
TrueFFS uses statistical allocation to perform dynamic wear-leveling on newly written data. This
minimizes the number of erase cycles per block. As a block erase is the most time-consuming
operation, dynamic wear-leveling has a major impact on overall performance. This impact cannot
be noticed during the first write to flash (since there is no need to erase blocks beforehand), but
becomes more and more noticeable as the flash media becomes full.
Static Wear-Leveling
Areas on the flash media may contain static files, characterized by blocks of data that remain
unchanged for very long periods of time, or even for the whole device lifetime. If wear-leveling
were only applied on newly written pages, static areas would never be cycled. This limited
application of wear-leveling would lower life expectancy significantly in cases where flash memory
contains large static areas. To overcome this problem, TrueFFS forces data transfer in static areas as
well as in dynamic areas, thereby applying wear-leveling to the entire media.
4.4
TrueFFS uses algorithms based on erase after write instead of erase before write to ensure data
integrity during normal operation and in the event of a power failure. Used areas are reclaimed for
erasing and writing the flash management information into them only after an operation is complete.
This procedure serves as a check on data integrity.
The erase-after-write algorithm is also used to update and store mapping information on the flash
memory. This keeps the mapping information coherent even during power failures. The only
mapping information held in RAM is a table pointing to the location of the actual mapping
information. This table is reconstructed during power-up or after reset from the information stored
in the flash memory.
11
Wear-Leveling
Power Failure Management
uDiskOnChip Data Sheet, Rev. 2.2
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94-SR-003-01-8L