SanDisk SDSDH-1024 Product Manual - Page 7

Functional Description, Independent Flash Technology, Defect and Error Management

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Revision 2.2 Chapter 1 - Introduction SanDisk SD Card Product Manual 1.4 Functional Description SanDisk SD cards contain a high-level, intelligent subsystem as shown in Figure 1-1. This intelligent (microprocessor) subsystem provides many capabilities not found in other types of memory cards. These capabilities include: • Host independence from details of erasing and programming flash memory • Sophisticated system for managing defects (analogous to systems found in magnetic disk drives) • Sophisticated system for error recovery including a powerful error correction code (ECC) • Power management for low-power operation 1.5 Independent Flash Technology The 512-byte sector size of the SanDisk SD Card is the same as that in an IDE magnetic disk drive. To write or read a sector (or multiple sectors), the host computer software simply issues a read or write command to the SD Card. This command contains the address. The host software then waits for the command to complete. The host software does not get involved in the details of how the flash memory is erased, programmed or read. This is extremely important as flash devices are expected to get increasingly complex in the future. Because the SD Card uses an intelligent on-board controller, the host system software will not require changing as new flash memory evolves. In other words, systems that support the SD Card today will be able to access future SD cards built with new flash technology without having to update or change host software. 1.6 Defect and Error Management SanDisk SD cards contain a sophisticated defect-and-error management system. This system is analogous to the systems found in magnetic disk drives and in many cases offers enhancements. For instance, disk drives do not typically perform a read after write to confirm the data is written correctly because of the performance penalty that would be incurred. SD cards do a read after write under margin conditions to verify that the data is written correctly. In the rare case that a bit is found to be defective, SD cards replace this bad bit with a spare bit within the sector header. If necessary, SD cards will even replace the entire sector with a spare sector. This is completely transparent to the host and does not consume any user data space. The SD Card's soft error rate specification is much better than the magnetic disk drive specification. In the extremely rare case a read error does occur, SD cards have innovative algorithms to recover the data. This is similar to using retries on a disk drive but is much more sophisticated. The last line of defense is to employ a powerful ECC to correct the data. If ECC is used to recover data, defective bits are replaced with spare bits to ensure they do not cause any future problems. These defect and error management systems coupled with the solid-state construction give SD cards unparalleled reliability. © 2004 SanDisk Corporation 1-3 12/08/04

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Chapter 1 – Introduction
Revision 2.2
SanDisk SD Card Product Manual
© 2004 SanDisk Corporation
1-3
12/08/04
1.4
Functional Description
SanDisk SD cards contain a high-level, intelligent subsystem as shown in Figure 1-1.
This
intelligent (microprocessor) subsystem provides many capabilities not found in other types
of memory cards. These capabilities include:
•
Host independence from details of erasing and programming flash memory
•
Sophisticated system for managing defects (analogous to systems found in magnetic
disk drives)
•
Sophisticated system for error recovery including a powerful error correction code
(ECC)
•
Power management for low-power operation
1.5
Independent Flash Technology
The 512-byte sector size of the SanDisk SD Card is the same as that in an IDE magnetic
disk drive. To write or read a sector (or multiple sectors), the host computer software
simply issues a read or write command to the SD Card. This command contains the
address. The host software then waits for the command to complete. The host software
does not get involved in the details of how the flash memory is erased, programmed or
read. This is extremely important as flash devices are expected to get increasingly complex
in the future. Because the SD Card uses an intelligent on-board controller, the host system
software will not require changing as new flash memory evolves. In other words, systems
that support the SD Card today will be able to access future SD cards built with new flash
technology without having to update or change host software.
1.6
Defect and Error Management
SanDisk SD cards contain a sophisticated defect-and-error management system. This
system is analogous to the systems found in magnetic disk drives and in many cases offers
enhancements. For instance, disk drives do not typically perform a read after write to
confirm the data is written correctly because of the performance penalty that would be
incurred.
SD cards do a read after write under margin conditions to verify that the data is
written correctly. In the rare case that a bit is found to be defective, SD cards replace this
bad bit with a spare bit within the sector header. If necessary, SD cards will even replace
the entire sector with a spare sector. This is completely transparent to the host and does not
consume any user data space.
The SD Card’s soft error rate specification is much better than the magnetic disk drive
specification. In the extremely rare case a read error does occur, SD cards have innovative
algorithms to recover the data. This is similar to using retries on a disk drive but is much
more sophisticated. The last line of defense is to employ a powerful ECC to correct the
data. If ECC is used to recover data, defective bits are replaced with spare bits to ensure
they do not cause any future problems. These defect and error management systems
coupled with the solid-state construction give SD cards unparalleled reliability.