AT25DF321A datasheet(25/51 Pages) RENESAS

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3686I–DFLASH–02/2019

AT25DF321A

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 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 35

). 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 t

LOCK. 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

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

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