A d v a n c e I n f o r m a t i o n
the Password Protection Mode Lock Bit. This guarantees that a hacker could not
place the device in Password Protection Mode. The Password Protection Mode
Lock Bit resides in the “Lock Register”.
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 be-
tween 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 com-
pares 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 has been 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 de-
vice. 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 pass-
word 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 Protec-
tion 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 after-
wards. If the password is lost after programming the Password Protection Mode
50
S29GLxxxN MirrorBitTM Flash Family
27631A4 May 13, 2004
SPANSION [ SPANSION ]
