AT25DF161_13 datasheet(25/52 Pages) ETC2

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AT25DF161

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 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

(see “Write Status Register Byte 2” on page 34). 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 deasserted, 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 lockdown 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

3687H–DFLASH–5/2013

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