4

A/T89C51AC2 UART Bootloader

4231C–CAN–03/05

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).

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.

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

2LB2

P

To lock the chip (see datasheet)

1LB1

U

0LB0

U

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