AM29PDL128G datasheet(27/71 Pages) AMD

27 / 71 page

Am29PDL128G

October 28, 2004

P R E L IM IN A R Y

Note: To achieve the best protection, it is recom-

mended to execute the PPB lock bit set command

early in the boot code, and protect the boot code by

holding WP# = V

IL.

Table 9.

Sector Protection Schemes

Table 9 contains all possible combinations of the DYB-

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

giv en s e c t or c an be v e r i fied b y w r it ing 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 a hacker 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 the first eight bytes of the

SecSi Sector. 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 com-

mand must be written to the flash, along with a pass-

word. The flash device internally compares the given

password with the pre-programmed password. If the

passwords match, the PPB Lock bit is cleared, and the

PPBs can be altered. If the passwords do not match,

the flash device does nothing. There is a built-in 2 µs

delay for each “password check.” This delay is in-

tended to thwart any efforts to run a program that tries

all possible combinations to crack the password.

Because the password occupies the first eight bytes of

the SecSi Sector, the password must be programmed

before either the password protection mode is se-

lected or the SecSi Sector protection bit is pro-

grammed (to use both the SecSi Sector and Password

Protection at the same time). See “Utilizing Password

and SecSi Sector Concurrently” on page 30 for more

information.

Password and Password Mode Locking

Bit

To select the Password sector protection scheme, the

customer must first program the password. AMD rec-

ommends 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 differ-

ent 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:

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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