AT89C51ED2-RDTIM datasheet(97/137 Pages) ATMEL

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AT89C51RD2/ED2

4235E–8051–04/04

Table 68. Program Lock Bits

Note:

U: Unprogrammed or "one" level.

P: Programmed or "zero" level.

X: Do not care

WARNING: Security level 2 and 3 should only be programmed after Flash and code

verification.

These security bits protect the code access through the parallel programming interface.

They are set by default to level 4. The code access through the ISP is still possible and

is controlled by the "software security bits" which are stored in the extra Flash memory

accessed by the ISP firmware.

To load a new application with the parallel programmer, a chip erase must first be done.

This will set the HSB in its inactive state and will erase the Flash memory. The part ref-

erence can always be read using Flash parallel programming modes.

Default Values

The default value of the HSB provides parts ready to be programmed with ISP:

•

BLJB: Programmed force ISP operation.

•

X2: Unprogrammed to force X1 mode (Standard Mode).

•

XRAM: Unprogrammed to valid XRAM

•

LB2-0: Security level four to protect the code from a parallel access with maximum

security.

Software Registers

Several registers are used in factory and by parallel programmers. These values are

used by Atmel ISP.

These registers are in the "Extra Flash Memory" part of the Flash memory. This block is

also called "XAF" or eXtra Array Flash. They are accessed in the following ways:

•

Commands issued by the parallel memory programmer.

•

Commands issued by the ISP software.

•

Calls of API issued by the application software.

Several software registers are described in Table 69.

Program Lock Bits

Protection Description

Security

Level

LB0

LB1

LB2

No program lock features enabled.

MOVC instruction executed from external program memory is disabled

from fetching code bytes from internal memory, EA is sampled and

latched on reset, and further parallel programming of the on chip code

memory is disabled.

ISP and software programming with API are still allowed.

Same as 2, also verify code memory through parallel programming

interface is disabled.

Same as 3, also external execution is disabled (Default).

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