48 S29GLxxxN MirrorBitTM Flash Family 27631A4 May 13, 2004
Advance Information
state”. Setting the PPB Lock Bit to the “freeze state” disables all program and
erase commands to the Non-Volatile PPB bits. In effect, the PPB Lock Bit locks the
PPB bits into their current state. The only way to clear the PPB Lock Bit to the
“unfreeze state” is to go through a power cycle, or hardware reset. The Software
Reset command will not clear the PPB Lock Bit to the “unfreeze state”. System
boot code can determine if any changes to the PPB bits are needed e.g. to allow
new system code to be downloaded. If no changes are needed then the boot code
can set the PPB Lock Bit to disable any further changes to the PPB bits during
system operation.
The WP# write protect pin adds a final level of hardware protection. When this
pin is low it is not possible to change the contents of the WP# protected sectors.
These sectors generally hold system boot code. So, the WP# pin can prevent 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 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 Set command sequence
is all that is necessary. The DYB Set and DYB Clear commands for the dynamic
sectors switch the DYB bits 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 to the “unfreeze
state” by either putting the device through a power-cycle, or hardware reset. The
PPB bits can then be changed to reflect the desired settings. Setting the PPB Lock
Bit once again to the “freeze state” will lock the PPB bits, and the device operates
normally again.
Note: to achieve the best protection, it's recommended to execute the PPB Lock
Bit Set command early in the boot code, and protect the boot code by holding
WP# = V
IL
.
Persistent Protection Bit (PPB)
A single Persistent (non-volatile) Protection Bit is assigned to each sector. If a PPB
is programmed to the protected state through the “PPB Program” command, that
sector will be protected from program or erase operations will be read-only. If a
PPB requires erasure, all of the sector PPB bits must first be erased in parallel
through the “All PPB Erase” command. The “All PPB Erase” command will prepro-
grammed all PPB bits prior to PPB erasing. All PPB bits erase in parallel, unlike
programming where individual PPB bits are programmable. The PPB bits have the
same endurance as the flash memory.
Programming the PPB bit requires the typical word programming time without uti-
lizing the Write Buffer. During a PPB bit programming and A11 PPB bit erasing
sequence execution, the DQ6 Toggle Bit I will toggle until the programming of the
PPB bit or erasing of all PPB bits has completed to indicate programming and
erasing status. Erasing all of the PPB bits at once requires typical sector erase
time. During the erasing of all PPB bits, the DQ3 Sector Erase Timer bit will output
a 1 to indicate the erasure of all PPB bits are in progress. When the erasure of all
PPB bits has completed, the DQ3 Sector Erase Timer bit will output a 0 to indicate
that all PPB bits have been erased. Reading the PPB Status bit requires the initial
access time of the device.
