AM29BDD160G datasheet(28/79 Pages) AMD

28 / 79 page

Am29BDD160G

June 7, 2006

Table 11.Sector Protection Schemes

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 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.

The programming of the DYB, PPB, and PPB lock for

a g i ven s ect or c a n b e ver i f i ed b y wr i t i ng 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

could not place the device in password protection

mode.

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:

■ 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.

A 64-bit password is the only additional tool utilized in

this method.

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.

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.

Once the desired password is programmed in, the

customer must then set the Password Mode Locking

Bit. This operation achieves two objectives:

DYB

PPB

Lock

Sector State

000

Unprotected—PPB and DYB are

changeable

001

Unprotected—PPB not

changeable, DYB is changeable

010

Protected—PPB and DYB are

changeable

100

110

011

Protected—PPB not

changeable, DYB is changeable

101

111

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