D a t a
S h e e t
Password Sector Protection
The Password Sector Protection method allows an even higher level of security than the Per-
sistent 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 Pass-
word to the device.
The Password Sector Protection method is otherwise identical to the Persistent Sector Protec-
tion 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 mem-
ory. 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.
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 Pro-
tection 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 oper-
ations are ignored.
Both of these objectives are important, and if not carefully considered, may lead to unrecov-
erable 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 pro-
grammed. 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 Protec-
tion 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 eras-
able. 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.
42
S29GL-N MirrorBit™ Flash Family
S29GL-N_00_B3 October 13, 2006
SPANSION [ SPANSION ]
