S71GS256 datasheet(58/195 Pages) SPANSION

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S29GLxxxN MirrorBitTM Flash Family

S29GLxxxN_MCP_A1 December 15, 2004

Advan ce

In form ati o n

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

Lock Bit, there will be no way to clear and unfreeze the PPB Lock Bit. The Pass-

word Protection Mode Lock Bit, once programmed, prevents reading the 64-bit

password on the DQ bus and further password programming. The Password Pro-

tection Mode Lock Bit is not erasable. Once Password Protection Mode Lock Bit is

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