15

AT25DF256

DS-25DF256–035D–2/2017

10.

Security Commands

10.1 Program OTP Security Register

The device contains a specialized OTP (One-Time Programmable) Security Register that can be used for purposes such

as unique device serialization, system-level Electronic Serial Number (ESN) storage, locked key storage, etc. The OTP

Security Register is independent of the main Flash memory array and is comprised of a total of 128 bytes of memory

divided into two portions. The first 64 bytes (byte locations 0 through 63) of the OTP Security Register are allocated as a

one-time user-programmable space. Once these 64 bytes have been programmed, they cannot be erased or

reprogrammed. The remaining 64 bytes of the OTP Security Register (byte locations 64 through 127) are factory

programmed by Adesto and will contain a unique value for each device. The factory programmed data is fixed and

cannot be changed.

The user-programmable portion of the OTP Security Register does not need to be erased before it is programmed. In

addition, the Program OTP Security Register command operates on the entire 64-byte user-programmable portion of the

OTP Security Register at one time. Once the user-programmable space has been programmed with any number of bytes

the user-programmable space cannot be programmed again; therefore, it is not possible to only program the first two

bytes of the register and then program the remaining 62 bytes at a later time.

Before the Program OTP Security Register 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”. To program the OTP Security Register, the CS

pin must first be asserted and an opcode of 9Bh must be clocked into the device followed by the three address bytes

denoting the first byte location of the OTP Security Register to begin programming at. Since the size of the user-

programmable portion of the OTP Security Register is 64 bytes, the upper order address bits do not need to be decoded

by the device. Therefore, address bits A23-A6 will be ignored by the device and their values can be either a logical “1” or

Table 9-2.

Hardware and Software Locking

WP

BPL

Locking

BPL Change Allowed

BP0 and Protection Status

0

0

Can be modified from 0 to 1

BP0 bit unlocked and modifiable using the Write

Status Register command. Memory array can be

protected and unprotected freely.

0

1

Hardware

Locked

Locked

BP0 bit locked in current state. The Write Status

Register command will have no affect. Memory

array is locked in current protected or unprotected

state.

1

0

Can be modified from 0 to 1

BP0 bit unlocked and modifiable using the Write

Status Register command. Memory array can be

protected and unprotected freely.

1

1

Can be modified from 1 to 0

BP0 bit unlocked and modifiable using the Write

Status Register command. Memory array can be

protected and unprotected freely.

Table 10-1. OTP Security Register

Security Register

Byte Number

0

1

. . .

62

63

64

65

. . .

126

127

One-Time User Programmable

Factory Programmed by Adesto