am42bds6408h Meet Spansion Inc., am42bds6408h Datasheet - Page 20

• Manufacturer Part Number: am42bds6408h
• Description: Cmos 1.8 Volt-only Simultaneous Read/write, Burst Mode Flash Memory, And 8 Mbit 512 K X 16-bit Sram
• Manufacturer: Meet Spansion Inc.

PPB 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 to disable any further
changes to the PPBs during system operation.
The WP# write protect pin adds a final level of hard-
ware protection to the four highest and four lowest 4
Kbyte sectors (SA0 - SA3, SA138 - SA141 for a dual
boot). When this pin is low it is not possible to change
the contents of these four sectors. These sectors gen-
erally hold system boot code. So, the WP# pin can
prevent any changes to the boot code that could over-
ride the choices made while setting up sector protec-
tion 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.
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
Table 4
PPB, and PPB lock relating to the status of the sector.
18
DYB
IL
0
0
0
1
1
0
1
1
.
Table 4. Sector Protection Schemes
contains all possible combinations of the DYB,
PPB
0
0
1
0
1
1
0
1
Lock
PPB
0
1
0
0
0
1
1
1
Unprotected—PPB and DYB are
changeable
Unprotected—PPB not
changeable, DYB is changeable
Protected—PPB and DYB are
changeable
Protected—PPB not
changeable, DYB is changeable
A D V A N C E
Sector State
Am42BDS6408H
I N F O R M A T I O N
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
g i ve n s e c t o r c a n b e v e r i f i e d b y w r i t i n g 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:
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 of the flash memory. Once the Password
Mode Locking Bit is set, the password is permanently
set with no means to read, program, or erase it. The
password is used to clear the PPB Lock bit. The Pass-
word Unlock command must be written to the flash,
along with a password. The flash device internally
compares the given password with the pre-pro-
When the device is first powered on, or comes out
of a reset cycle, the PPB Lock bit is set to the
locked state, rather than cleared to the unlocked
state.
The only means to clear the PPB Lock bit is by writ-
ing a unique 64-bit Password to the device.
October 23, 2003