AT89C51ED2-RDTIM datasheet(102/137 Pages) ATMEL

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102

AT89C51RD2/ED2

4235E–8051–04/04

Bootloader Functionality

The bootloader can be activated by two means: Hardware conditions or regular boot

process.

The Hardware conditions (EA = 1, PSEN = 0) during the Reset# falling edge force the

on-chip bootloader execution. This allows an application to be built that will normally

execute the end user’s code but can be manually forced into default ISP operation.

As PSEN is a an output port in normal operating mode after reset, user application

should take care to release PSEN after falling edge of reset signal. The hardware condi-

tions are sampled at reset signal falling edge, thus they can be released at any time

when reset input is low.

To ensure correct microcontroller startup, the PSEN pin should not be tied to ground

during power-on (See Figure 43).

Figure 43. Hardware conditions typical sequence during power-on.

The on-chip bootloader boot process is shown Figure 44.

VCC

PSEN

RST

Table 72. Bootloader Process Description

Purpose

Hardware Conditions

The Hardware Conditions force the bootloader execution whatever

BLJB, BSB and SBV values.

BLJB

The Boot Loader Jump Bit forces the application execution.

BLJB = 0 => Bootloader execution

BLJB = 1 => Application execution

The BLJB is a fuse bit in the Hardware Byte.

It can be modified by hardware (programmer) or by software (API).

Note: The BLJB test is performed by hardware to prevent any

program execution.

SBV

The Software Boot Vector contains the high address of customer

bootloader stored in the application.

SBV = FCh (default value) if no customer bootloader in user Flash.

Note: The customer bootloader is called by JMP [SBV]00h

instruction.

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