AT89C51AC2(2005) データシートの表示（PDF） - Atmel Corporation
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AT89C51AC2 Datasheet PDF : 29 Pages
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Mapping and Default Value of
Hardware Security Byte
The 4 Most Significant Byte (MSB) of the Hardware Byte can be read/written by soft-
ware (this area is called Fuse bits). The 4 Least Significant Byte (LSB) can only be read
by software and written by hardware in parallel mode (with parallel programmer
devices).
Bit Position Mnemonic Default Value Description
7
X2B
U
To start in x1 mode
6
BLJB
P
To map the boot area in code area between F800h-FFFFh
5
Reserved
U
4
Reserved
U
3
Reserved
U
2
LB2
P
1
LB1
U
To lock the chip (see datasheet)
0
LB0
U
Note: U: Unprogram = 1
P: Program = 0
Security
The bootloader has Software Security Byte (SSB) to protect itself from user access or
ISP access.
The Software Security Byte (SSB) protects from ISP accesses. 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)
In this level it is impossible to write in the Flash memory, BSB and SBV.
The Bootloader returns an error message.
From level 1, one can write only level 2.
• level 2: RD_WR_SECURITY (FCh)
Level 2 forbids all read and write accesses to/from the Flash memory.
The Bootloader returns an error message.
Only a full chip erase command can reset the software security bits.
Level 0
Level 1
Level 2
Flash/EEPROM Any access allowed
Read only access allowed All access not allowed
Fuse bit
Any access allowed
Read only access allowed All access not allowed
BSB & SBV & EB Any access allowed
Read only access allowed All access not allowed
SSB
Any access allowed
Write level2 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
4 A/T89C51AC2 UART Bootloader
4231C–CAN–03/05
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