AT89C51ED2-RDTUM datasheet(100/137 Pages) ATMEL

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4235K–8051–05/08

AT89C51RD2/ED2

24.6.4

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 conditions 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 24-4).

Figure 24-4. Hardware conditions typical sequence during power-on.

The on-chip bootloader boot process is shown Figure 24-5.

VCC

PSEN

RST

Table 24-6.

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.

AT89C51ED2-RDTUM Datasheet(PDF) 100 Page - ATMEL Corporation