Rainbow Electronics AT45DB021D - Manual (Page 15)

15
3638K–DFLASH–11/2012
AT45DB021D
7.1.4 Various Aspects About the Sector Protection Register
The Sector Protection Register is subject to a limit of 10,000 erase/program cycles. Users are encouraged to
carefully evaluate the number of times the Sector Protection Register will be modified during the course of the
applications’ life cycle. If the application requires that the Sector Protection Register be modified more than the
specified limit of 10,000 cycles because the application needs to temporarily unprotect individual sectors (sector
protection remains enabled while the Sector Protection Register is reprogrammed), then the application will need
to limit this practice. Instead, a combination of temporarily unprotecting individual sectors along with disabling
sector protection completely will need to be implemented by the application to ensure that the limit of 10,000 cycles
is not exceeded.
8. Security Features
8.1 Sector Lockdown
The device incorporates a Sector Lockdown mechanism that allows each individual sector to be permanently
locked so that it becomes read only. This is useful for applications that require the ability to permanently protect a
number of sectors against malicious attempts at altering program code or security information. Once a sector is
locked down, it can never be erased or programmed, and it can never be unlocked.
To issue the Sector Lockdown command, the CS pin must first be asserted as it would be for any other command.
Once the CS pin has been asserted, the appropriate 4-byte opcode sequence must be clocked into the device in
the correct order. The 4-byte opcode sequence must start with 3DH and be followed by 2AH, 7FH, and 30H. After
the last byte of the command sequence has been clocked in, then three address bytes specifying any address
within the sector to be locked down must be clocked into the device. After the last address bit has been clocked in,
the CS pin must then be deasserted to initiate the internally self-timed lockdown sequence.
The lockdown sequence should take place in a maximum time of t
P
, during which time the Status Register will
indicate that the device is busy. If the device is powered-down before the completion of the lockdown sequence,
then the lockdown status of the sector cannot be guaranteed. In this case, it is recommended that the user read the
Sector Lockdown Register to determine the status of the appropriate sector lockdown bits or bytes and reissue the
Sector Lockdown command if necessary.
Table 8-1. Sector Lockdown
Figure 8-1. Sector Lockdown
Command Byte 1 Byte 2 Byte 3 Byte 4
Sector Lockdown 3DH 2AH 7FH 30H
Opcode
Byte 1
Opcode
Byte 2
Opcode
Byte 3
Opcode
Byte 4
CS
Address
Bytes
Address
Bytes
Address
Bytes
Each transition
represents 8 bits
SI