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1.8.3
Endurance
CompactFlash Memory Cards have an endurance
specification for each sector of 300,000 writes
(reading a logical sector is unlimited). This is far
beyond what is needed in nearly all applications
of CompactFlash Memory Cards. Even very heavy
use of CompactFlash Card in digital cameras,
cellular phones, PDAs, personal communicators,
pagers, voice recorders, ruggedized handheld
computers, palmtop and notebook computers will
use only a fraction of the total endurance over the
typical
computer's five
instance, it would take over 34 years to wear out
an area on the CompactFlash Card on which a
file of any size (from 512 bytes to capacity) was
rewritten 3 times per hour, 8 hours a day, 365 days
per year.
With typical applications (PIM software, word
processing, spreadsheets, etc.), the endurance
limit is not of any practical concern to the vast
majority of users.
1.8.4
Wear Leveling
CompactFlash Memory Card Series products do
not require or perform a Wear Level operation.
The command is supported as a NOP operation to
maintain backward compatibility with existing
software utilities.
1.8.5
Using the Erase Sector and Write without
Erase Commands
The Erase Sector and Write without Erase
commands provide the capability to substantially
increase
the
write
CompactFlash Memory Card. Once a sector has
been erased using the Erase Sector command, a
write to that sector will be much faster. This is
because a normal write operation includes a
separate sector erase prior to write.
An example of where these commands may be
useful is in a digital camera. The camera user may
have plenty of time to erase pictures but may wish
to take several pictures in rapid succession. To
accomplish this, the host system (i.e., camera)
SanDisk CompactFlash Memory Card Product Manual © 1998 SANDISK CORPORATION
CompactFlash Memory Card Product Manual
year
lifetime.
For
performance
of
the
would use the Erase Sectors command to pre-erase
the sectors that will store the pictures. When the
pictures are taken, the camera can store them in
the previously erased sectors much faster than in
non-erased sectors.
1.8.5.1
Interaction with Systems not Aware of
the Erase Sector and Write without
Erase Commands
Many
systems
that
CompactFlash Memory Cards may not be aware of
the Erase Sector and Write without Erase
Commands. These systems would not issue these
commands but such a system might attempt a
normal write or a normal read to a pre-erased
sector.
A normal write to a pre-erased sector will function
correctly, but will be at the normal write speed
that is slower than a Write without Erase
command.
If a normal read is attempted to a "pre-erased"
sector, CompactFlash Memory Cards will detect it
is pre-erased and will return zero data and will
not report an error even though the data ECC is
not valid.
If an "un-aware" host system over-writes a pre-
erased sector with a normal write and then the
CompactFlash Memory Card is moved to the
system that created the erased sectors, a situation
exists where a Write without Erase might be
attempted to a "normal" sector. If this occurs, the
CompactFlash Card will perform a normal write
which means it will first erase the sector and then
do a full write with all margin modes enabled.
This write will of course be slower than if the
sector were in fact pre-erased.
1.8.5.2
Limitations and Issues
The advantage of the Write without Erase and
Erase Sector commands is that they shift the bulk
of the erase and write time to the Erase Sector
command. The Erase Sector command performs
most of the normal tasks needed. To increase the
speed of the Write without Erase command, the
final margin verify done in a normal write
can
read
and
write
13
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