Datasheet

103
4235K–8051–05/08
AT89C51RD2/ED2
24.8 Functional Description
24.8.1 Software Security Bits (SSB)
The SSB protects any Flash access from ISP command.
The command "Program Software Security Bit" can only write a higher priority level.
There are three levels of security:
• level 0: NO_SECURITY (FFh)
This is the default level.
From level 0, one can write level 1 or level 2.
• level 1: WRITE_SECURITY (FEh)
For this level it is impossible to write in the Flash memory, BSB and SBV.
The Bootloader returns ’P’ on write access.
From level 1, one can write only level 2.
• level 2: RD_WR_SECURITY (FCh
The level 2 forbids all read and write accesses to/from the Flash/EEPROM memory.
The Bootloader returns ’L’ on read or write access.
Only a full chip erase in parallel mode (using a programmer) or ISP command can reset the soft-
ware security bits.
From level 2, one cannot read and write anything.
Table 24-7. Software Security Byte Behavior
24.8.2 Full Chip Erase
The ISP command "Full Chip Erase" erases all user Flash memory (fills with FFh) and sets
some bytes used by the bootloader at their default values:
• BSB = FFh
Level 0 Level 1 Level 2
Flash/EEPROM Any access allowed Read-only access allowed Any access not allowed
Fuse Bit Any access allowed Read-only access allowed Any access not allowed
BSB & SBV Any access allowed Read-only access allowed Any access not allowed
SSB Any access allowed Write level 2 allowed Read-only access allowed
Manufacturer Info Read-only access allowed Read-only access allowed Read-only access allowed
Bootloader Info Read-only access allowed Read-only access allowed Read-only access allowed
Erase Block Allowed Not allowed Not allowed
Full Chip Erase Allowed Allowed Allowed
Blank Check Allowed Allowed Allowed