A d v a n c e I n f o r m a t i o n
A 64-bit password is the only additional tool utilized in this method.
The password is stored in a one-time programmable (OTP) region of the flash
memory. Once the Password Mode Locking Bit is set, the password is perma-
nently set with no means to read, program, or erase it. The password is used to
clear the PPB Lock bit. The Password Unlock command must be written to the
flash, along with a password. The flash device internally compares the given
password with the pre-programmed password. If they match, the PPB Lock bit is
cleared, and the PPBs can be altered. If they do not match, the flash device does
nothing. There is a built-in 2 µs delay for each “password check.” This delay is
intended to thwart any efforts to run a program that tries all possible combina-
tions in order to crack the password.
Password and Password Mode Locking Bit
In order to select the Password sector protection scheme, the customer must first
program the password. One method of choosing a password would be to correlate
it to the unique Electronic Serial Number (ESN) of the particular flash device. An-
other method could generate a database where all the passwords are stored,
each of which correlates to a serial number on the device. Each ESN is different
for every flash device; therefore each password should be different for every flash
device. While programming in the password region, the customer may perform
Password Verify operations.
Once the desired password is programmed in, the customer must then set the
Password Mode Locking Bit. This operation achieves two objectives:
1) It permanently sets the device to operate using the Password Protection Mode.
It is not possible to reverse this function.
2) It also disables all further commands to the password region. All program, and
read operations are ignored.
Both of these objectives are important, and if not carefully considered, may lead
to unrecoverable errors. The user must be sure that the Password Protection
method is desired when setting the Password Mode Locking Bit. More importantly,
the user must be sure that the password is correct when the Password Mode
Locking Bit is set. Due to the fact that read operations are disabled, there is no
means to verify what the password is afterwards. If the password is lost after set-
ting the Password Mode Locking Bit, there will be no way to clear the PPB Lock bit.
The Password Mode Locking Bit, once set, prevents reading the 64-bit password
on the DQ bus and further password programming. The Password Mode Locking
Bit is not erasable. Once Password Mode Locking Bit is programmed, the Persis-
tent Sector Protection Locking Bit is disabled from programming, guaranteeing
that no changes to the protection scheme are allowed.
64-bit Password
The 64-bit Password is located in its own memory space and is accessible through
the use of the Password Program and Verify commands (see Password Verify
Command). The password function works in conjunction with the Password Mode
Locking Bit, which when set, prevents the Password Verify command from read-
ing the contents of the password on the pins of the device.
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