AT25DF641A-MH-T ATMEL [ATMEL Corporation], AT25DF641A-MH-T Datasheet - Page 28

• Manufacturer Part Number: AT25DF641A-MH-T
• Description: 64-Mbit 2.7V Minimum Serial Peripheral Interface Serial Flash Memory
• Manufacturer: ATMEL [ATMEL Corporation]

10. Security Commands
10.1
28
Sector Lockdown
Atmel AT25DF641A [Preliminary]
Certain applications require that portions of the Flash memory array be permanently protected
against malicious attempts at altering program code, data modules, security information, or
encryption/decryption algorithms, keys, and routines. To address these applications, the device
incorporates a sector lockdown mechanism that allows any combination of individual 64-Kbyte
sectors to be permanently locked so that they become read only. Once a sector is locked down,
it can never be erased or programmed again, and it can never be unlocked from the locked
down state.
Each 64-Kbyte physical sector has a corresponding single-bit Sector Lockdown Register that is
used to control the lockdown status of that sector. These registers are nonvolatile and will retain
their state even after a device power-cycle or reset operation. The following table outlines the
two states of the Sector Lockdown Registers.
Table 10-1.
Issuing the Sector Lockdown command to a particular sector address will set the corresponding
Sector Lockdown Register to the logical “1” state. Each Sector Lockdown Register can only be
set once; therefore, once set to the logical “1” state, a Sector Lockdown Register cannot be reset
back to the logical “0” state.
Before the Sector Lockdown command can be issued, the Write Enable command must have
been previously issued to set the WEL bit in the Status Register to a logical “1”. In addition, the
Sector Lockdown Enabled (SLE) bit in the Status Register must have also been previously set to
the logical “1” state by using the Write Status Register Byte 2 command (see
ister Byte 2” on page
asserted and the opcode of 33h must be clocked into the device followed by three address bytes
designating any address within the 64-Kbyte sector to be locked down. After the three address
bytes have been clocked in, a confirmation byte of D0h must also be clocked in immediately fol-
lowing the three address bytes. Any additional data clocked into the device after the first byte of
data will be ignored. When the CS pin is deasserted, the Sector Lockdown Register correspond-
ing to the sector addressed by A23-A0 will be set to the logical “1” state, and the sector itself will
then be permanently locked down from program and erase operations within a time of t
addition, the WEL bit in the Status Register will be reset back to the logical “0” state.
The complete three address bytes and the correct confirmation byte value of D0h must be
clocked into the device before the CS pin is deasserted, and the CS pin must be deasserted on
an even byte boundary (multiples of eight bits); otherwise, the device will abort the operation.
When the device aborts the Sector Lockdown operation, the state of the corresponding Sector
Lockdown Register as well as the SLE bit in the Status Register will be unchanged; however, the
WEL bit in the Status Register will be reset to a logical “0”.
As a safeguard against accidental or erroneous locking down of sectors, the Sector Lockdown
command can be enabled and disabled as needed by using the SLE bit in the Status Register.
In addition, the current sector lockdown state can be frozen so that no further modifications to
Value
0
1
Sector Lockdown Status
Sector is not locked down and can be programmed and erased. This is the default state.
Sector is permanently locked down and can never be programmed or erased again.
Sector Lockdown Register Values
39). To issue the Sector Lockdown command, the CS pin must first be
“Write Status Reg-
8693A–DFLASH–8/10
LOCK
. In