D at a
S hee t
8.14
Password Sector Protection
The Password Sector Protection method allows an even higher level of security than the Persistent Sector
Protection method. There are two main differences between the Persistent Sector Protection and the
Password Sector Protection methods:
When the device is first powered on, or comes out of a reset cycle, the PPB Lock Bit is set to the locked
state, or the freeze state, rather than cleared to the unlocked state, or the unfreeze state.
The only means to clear and unfreeze the PPB Lock Bit is by writing a unique 64-bit Password to the
device.
The Password Sector Protection method is otherwise identical to the Persistent Sector Protection method.
A 64-bit password is the only additional tool utilized in this method.
The password is stored in a one-time programmable (OTP) region outside of the flash memory. Once the
Password Protection Mode Lock Bit is set, the password is permanently set with no means to read, program,
or erase it. The password is used to clear and unfreeze 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 to the
unfreezed state,
and the
PPB bits 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
after the valid 64-bit password is entered for the PPB Lock Bit to be cleared to the
“unfreezed state”. This delay is intended to thwart any efforts to run a program that tries all possible
combinations in order to crack the password.
8.15
Password and Password Protection Mode Lock Bit
In order to select the Password Sector Protection method, the customer must first program the password.
The factory recommends that the password be somehow correlated to the unique Electronic Serial Number
(ESN) of the particular flash 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 Read operations. Once the desired password is programmed in, the customer must then
set the Password Protection Mode Lock 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 Sector Protection method is desired when programming the Password
Protection Mode Lock Bit. More importantly, the user must be sure that the password is correct when the
Password Protection Mode Lock Bit is programmed. Due to the fact that read operations are disabled, there is
no means to read what the password is afterwards. If the password is lost after programming the Password
Protection Mode Lock Bit, there is no way to clear and unfreeze the PPB Lock Bit. The Password Protection
Mode Lock Bit, once programmed, prevents reading the 64-bit password on the DQ bus and further password
programming. The Password Protection Mode Lock Bit is not erasable. Once Password Protection Mode
Lock Bit is programmed, the Persistent Protection Mode Lock Bit is disabled from programming, guaranteeing
that no changes to the protection scheme are allowed.
8.16
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 Password Read commands. The password function works in conjunction with the Password
Protection Mode Lock Bit, which when programmed, prevents the Password Read command from reading
the contents of the password on the pins of the device.
34
S29GL-N MirrorBit
®
Flash Family
S29GL-N_01_09 November 16, 2007
SPANSION [ SPANSION ]
