AM29BDD160G AMD [Advanced Micro Devices], AM29BDD160G Datasheet - Page 26

• Manufacturer Part Number: AM29BDD160G
• Description: 16 Megabit (1 M x 16-bit/512 K x 32-Bit), CMOS 2.5 Volt-only Burst Mode, Dual Boot, Simultaneous Read/Write Flash Memory
• Manufacturer: AMD [Advanced Micro Devices]

Table 11 contains all possible combinations of the
DYB, PPB, and PPB lock relating to the status of the
sector.
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 dy-
namically locked or unlocked. The DYB then controls
whether or not the sector is protected or unpro-
tected.
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 hav-
ing modified the contents of the protected sector. An
erase command to a protected sector enables status
polling for approximately 50 µs after which the de-
vice returns to read mode without having erased the
protected sector.
The programming of the DYB, PPB, and PPB lock for
a given sect or 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
prevents programming of the password protection
mode locking bit. This guarantees that an unautho-
rized user
Password Protection Mode
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:
24
DYB
Table 11. Sector Protection Schemes
0
0
0
1
1
0
1
1
PPB
0
0
1
0
1
1
0
1
Lock
PPB
0
1
0
0
0
1
1
1
changeable, DYB is changeable
changeable, DYB is changeable
Unprotected—PPB and DYB are
Protected—PPB and DYB are
Unprotected—PPB not
Protected—PPB not
Sector State
changeable
changeable
Am29BDD160G
A 64-bit password is the only additional tool utilized
in this method.
The password is stored in a one-time programma-
ble (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 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 combinations 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 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 num-
ber on the device. Each ESN is different for every
flash device; therefore each password should be dif-
ferent 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:
Both of these objectives are important, and if not
carefully considered, may lead to unrecoverable er-
rors. The user must be sure that the Password
Protection method is desired when setting the Pass-
word 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 setting the Password Mode
Locking Bit, there will be no way to clear the PPB
Lock bit.
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.
The only means to clear the PPB Lock bit is by writ-
ing a unique 64-bit Password to the device. The
Password Sector Protection method is otherwise
identical to the Persistent Sector Protection
method.
It permanently sets the device to operate using
the Password Protection Mode. It is not possible to
reverse this function.
It also disables all further commands to the pass-
word region. All program, and read operations are
ignored.