AT89C51ED2-RDTIM datasheet(105/137 Pages) ATMEL

105 / 137 page

105

AT89C51RD2/ED2

4235E–8051–04/04

Functional Description

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 software security bits.

From level 2, one cannot read and write anything.

Table 73. Software Security Byte Behavior

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

Bootloader Info

Read-only access allowed

Erase Block

Allowed

Not allowed

Full Chip Erase

Allowed

Blank Check

Allowed

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