AM75PDL191CHHA Datasheet, PDF (42/129 Pages) SPANSION – 128 Megabit (8 M x 16-Bit) CMOS 3.0 Volt-only, Simultaneous Read/Write Flash Memory

ADVANCE INFORMATION
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, 268, and 269 in
PDL 127 or in SA1-133, SA1-134, SA2-0, SA2-1 in
PDL 129. 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 pro-
tection 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 dis-
abled by either putting the device through a power-cy-
cle, 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 de-
vice 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 = V_IL.
Table 11. Sector Protection Schemes
DYB
0
0
0
1
1
0
1
1
PPB
0
0
1
0
1
1
0
1
PPB
Lock
Sector State
0
Unprotected—PPB and DYB are
changeable
1
Unprotected—PPB not
changeable, DYB is changeable
0
0
Protected—PPB and DYB are
changeable
0
1
1
Protected—PPB not
changeable, DYB is changeable
1
Table 11 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 sector can be verified by writing 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-
40
Am75PDL191CHHa/Am75PDL193CHHa
January 14, 2004