A D V A N C E I N F O R M A T I O N
Table 10. Sector Protection Schemes could not place the device in password protection
mode.
PPB
Lock
Password Protection Mode
DYB
PPB
Sector State
The Password Sector Protection Mode method allows
an even higher level of security than the Persistent
Sector Protection Mode. There are two main differ-
ences between the Persistent Sector Protection and
the Password Sector Protection Mode:
Unprotected—PPB and DYB are
changeable
0
0
0
1
Unprotected—PPB not
changeable, DYB is changeable
0
0
0
1
1
0
1
1
1
0
1
1
0
1
0
0
0
1
1
1
■ When the device is first powered on, or comes out
of a reset cycle, the PPB Lock bit set to the locked
state, rather than cleared to the unlocked state.
Protected—PPB and DYB are
changeable
■ The only means to clear the PPB Lock bit is by writ-
ing a unique 64-bit Password to the device.
Protected—PPB not
changeable, DYB is changeable
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.
Table 10 contains all possible combinations of the
DYB, PPB, and PPB lock relating to the status of the
sector.
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 permanently
set with no means to read, program, or erase it. The
password is used to clear the PPB Lock bit. The Pass-
word Unlock command must be written to the flash,
along with a password. The flash device internally
compares the given password with the pre-pro-
grammed 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 combinations in order to crack
the password.
In summary, if the PPB is set, and the PPB lock is set,
the sector is protected and the protection can not be
removed until the next power cycle clears the PPB
lock. If the PPB is cleared, the sector can be dynami-
cally locked or unlocked. The DYB then controls
whether or not the sector is protected or unprotected.
If the user attempts to program or erase a protected
sector, the device ignores the command and returns to
read mode. A program command to a protected sector
enables status polling for approximately 1 µs before
the device returns to read mode without having modi-
fied the contents of the protected sector. An erase
command to a protected sector enables status polling
for approximately 50 µs after which the device returns
to read mode without having erased the protected sec-
tor.
Password and Password Mode Locking
Bit
In order to select the Password sector protection
scheme, the customer must first program the pass-
word. One method of choosing a password would be
to correlate it to the unique Electronic Serial Number
(ESN) of the particular flash device. Another 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 pass-
word region, the customer may perform Password
Verify operations.
The programming of the DYB, PPB, and PPB lock for
a given sector can be verified by writing a
DYB/PPB/PPB lock verify command to the device.
Persistent Sector Protection Mode
Locking Bit
Like the password mode locking bit, a Persistent Sec-
tor Protection mode locking bit exists to guarantee that
the device remain in software sector protection. Once
set, the Persistent Sector Protection locking bit pre-
vents programming of the password protection mode
locking bit. This guarantees that an unauthorized user
Once the desired password is programmed in, the
customer must then set the Password Mode Locking
Bit. This operation achieves two objectives:
April 8, 2003
Am29BDD160G
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SPANSION [ SPANSION ]
