AT25DF081A datasheet(25/54 Pages) ETC2

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8715C–SFLSH–1/2013

AT25DF081A

10.

Security Commands

10.1 Sector Lockdown

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.

Issuing the Sector Lockdown command to a particular sector address will set the corresponding Sector Lockdown

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 “Write Status Register Byte 2” on page 36). To issue the Sector Lockdown command, the

CS pin must first be 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 following 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 deas-

serted, the Sector Lockdown Register corresponding 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 tLOCK. In 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 lock-

down state can be frozen so that no further modifications to the Sector Lockdown Registers can be made (see

“Freeze Sector Lockdown State” below). If the Sector Lockdown command is disabled or if the sector lockdown

state is frozen, then any attempts to issue the Sector Lockdown command will be ignored, and the device will reset

the WEL bit in the Status Register back to a logical “0” and return to the idle state once the CS pin has been

deasserted.

Table 10-1.

Sector Lockdown Register Values

Value

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.

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