Owner manual

Rev. F - 15 February, 2001 52
T89C51RD2
8.4.2.1. Boot Loader Lock Bit (BLLB)
One bit of the HSB is used to secure by hardware the internal boot loader sector against software reprogramming.
When the BLLB is cleared, any attempt to write in the boot loader segment (Address FC00h to FFFFh) will have
no effect. This protection applies for software writing only.
Boot Loader Jump Bit (BLJB)
One bit of the HSB, the BLJB bit, is used to force the boot address:
• When this bit is set the boot address is 0000h.
• When this bit is reset the boot address is FC03h. By default, this bit is cleared and the ISP is enabled.
8.4.2.2. FLASH memory lock bits
The three lock bits provide different levels of protection for the on-chip code and data, when programmed according
to Table 29.
Table 27. Hardware Security Byte (HSB)
7 6 5 4 3 2 1 0
SB BLJB BLLB - - LB2 LB1 LB0
Bit
Number
Bit
Mnemonic
Description
7 SB
Safe Bit
This bit must be cleared to secure the content of the HSB. Only security level can be increased.
6 BLJB
Boot loader Jump Bit
Set to force hardware boot address at 0000h. (unless previously force by hardware conditions as
described in the chapter 9.6).
Clear to force hardware boot address at FC03h (default).
5 BLLB
Boot loader Lock Bit
Set to allow programming and writing of the boot loader segment.
Clear to forbid software programming and writing of the boot loader segment (default). This protection protect
only ISP or IAP access; protection through parallel access is done globally by the lock bits LB2-0.
4 -
Reserved
Do not clear this bit.
3 -
Reserved
Do not clear this bit.
2-0 LB2-0
User Memory Lock Bits
See Table 29