CompactFlash Memory Card Product Manual
SanDisk CompactFlash Memory Card Product Manual © 1998 SANDISK CORPORATION
12
1.7.6
Identify Drive Information
Word 51 of the Identify Drive Command
information has a default value of 0000H for the
SDP5A FlashDisk. The data field type
information for this word is “PIO data transfer
cycle timing mode 0.”
For the CompactFlash Memory Card, word 51 of
the Identify Drive Command information has a
default value of 0001H. The data field type
information for this word is “PIO data transfer
cycle timing mode 1.”
1.8
Functional Description
CompactFlash Memory Cards contain a high
level, intelligent subsystem as shown in the block
diagram,
Figure
1-1.
This
intelligent
(microprocessor)
subsystem
provides
many
capabilities not found in other types of memory
cards. These capabilities include:
1. Standard ATA register and command set
(same as found on most magnetic disk
drives).
2. Host independence from details of erasing
and programming flash memory.
3. Sophisticated
system for managing
defects (analogous to systems found in
magnetic disk drives).
4. Sophisticated system for error recovery
including a powerful error correction code
(ECC).
5. Power management for low power
operation.
1.8.1
Flash Technology Independence
The 512 byte sector size of CompactFlash Memory
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 CompactFlash
Memory Card. This command contains the address
and the number of sectors to write/read. 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 more and more
complex in the future. Since the CompactFlash
Memory Card Series 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
CompactFlash Memory Card today will be able to
access future SanDisk cards built with new flash
technology without having to update or change
host software.
1.8.2
Defect and Error Management
CompactFlash
Memory
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.
CompactFlash Memory Cards do a read after
write under margin conditions to verify that the
data is written correctly (except in the case of a
Write without Erase Command). In the rare case
that a bit is found to be defective, CompactFlash
Memory Cards replace this bad bit with a spare
bit within the sector header. If necessary,
CompactFlash Memory 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 CompactFlash Memory Card soft error rate
specification is much better than the magnetic
disk drive specification. In the extremely rare
case a read error does occur, CompactFlash
Memory 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
CompactFlash Memory Cards
unparalleled
reliability.
