AM29PDL640G datasheet(19/61 Pages) SPANSION

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Am29PDL640G

February 26, 2003

P R E L I M INARY

switch back and forth between the protected and un-

protected conditions. This allows software to easily

protect sectors against inadvertent changes yet does

not prevent the easy removal of protection when

changes are needed. The DYBs maybe set or cleared

as often as needed.

The PPBs allow for a more static, and difficult to

change, level of protection. The PPBs retain their state

across power cycles because they are non-volatile. In-

dividual PPBs are set with a command but must all be

cleared as a group through a complex sequence of

program and erasing commands. The PPBs are also

limited to 100 erase cycles.

The PPB Lock bit adds an additional level of protec-

tion. Once all PPBs are programmed to the desired

settings, the PPB Lock may be set to “1”. Setting the

PPB Lock disables all program and erase commands

to the non-volatile PPBs. In effect, the PPB Lock Bit

locks the PPBs into their current state. The only way to

clear the PPB Lock is to go through a power cycle.

System boot code can determine if any changes to the

PPB are needed; for example, to allow new system

code to be downloaded. If no changes are needed

then the boot code can set the PPB Lock to disable

any further changes to the PPBs during system opera-

tion.

The WP#/ACC write protect pin adds a final level of

hardware protection to sectors 0, 1, 140, and 141.

When this pin is low it is not possible to change the

contents of these sectors. These sectors generally

hold system boot code. The WP#/ACC pin can pre-

vent any changes to the boot code that could override

the choices made while setting up sector protection

during system initialization.

It is possible to have sectors that have been persis-

tently locked, and sectors that are left in the dynamic

state. The sectors in the dynamic state are all unpro-

tected. If there is a need to protect some of them, a

simple DYB Write command sequence is all that is

necessary. The DYB write command for the dynamic

sectors switch the DYBs to signify protected and un-

protected, respectively. If there is a need to change

the status of the persistently locked sectors, a few

more steps are required. First, the PPB Lock bit must

be disabled by either putting the device through a

power-cycle, or hardware reset. The PPBs can then

be changed to reflect the desired settings. Setting the

PPB lock bit once again will lock the PPBs, and the

device operates normally again.

The best protection is achieved by executing the PPB

lock bit set command early in the boot code, and pro-

tect the boot code by holding WP#/ACC = VIL.

Table 9.

Sector Protection Schemes

Table 9 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 given secto r can be v e r i fied by wr iti 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 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:

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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