S71PL129JC0 datasheet(36/153 Pages) SPANSION

36 / 153 page

S71PL129JC0/S71PL129JB0/S71PL129JA0

S71PL129Jxx_00_A8 October 28, 2005

Advance

Info rmation

the PPB and the DYB related to that sector. For the sectors that have the PPBs

cleared, the DYBs control whether or not the sector is protected or unprotected.

By issuing the DYB Write command sequences, the DYBs are set or cleared, thus

placing each sector in the protected or unprotected state. These are the so-called

Dynamic Locked or Unlocked states. These states are called dynamic states be-

cause it is very easy to switch back and forth between the protected and

unprotected conditions. This allows software to easily protect sectors against in-

advertent 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 the PPBs 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 lim-

ited to 100 erase cycles.

The PPB Lock bit adds an additional level of protection. Once all PPBs are pro-

grammed 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 ef-

fect, 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 operation.

The WP#/ACC write protect pin adds a final level of hardware protection to sec-

tors SA1-133, SA1-134, SA2-0 and SA2-1. 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 prevent any changes to the boot code that could

override the choices made while setting up sector protection during system

initialization.

For customers who are concerned about malicious viruses there is another level

of security - the persistently locked state. To persistently protect a given sector

or sector group, the PPBs associated with that sector need to be set to “1”. Once

all PPBs are programmed to the desired settings, the PPB Lock should be set to

“1”. Setting the PPB Lock automatically disables all program and erase commands

to the Non-Volatile PPBs. In effect, the PPB Lock “freezes” the PPBs into their cur-

rent state. The only way to clear the PPB Lock is to go through a power cycle.

It is possible to have sectors that have been persistently locked, and sectors that

are left in the dynamic state. The sectors in the dynamic state are all unprotected.

If there is a need to protect some of them, a simple DYB Write command se-

quence is all that is necessary. The DYB write command for the dynamic sectors

switch the DYBs to signify protected and unprotected, 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 re-

flect the desired settings. Setting the PPB lock bit once again 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 protect the boot code by holding WP#/ACC = VIL.

Table 17 contains all possible combinations of the DYB, PPB, and PPB lock relating

to the status of the sector.

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