AT89C51RB2/RC2
Bootloader Functionality
Introduction
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 an output port in normal operating mode (running user application or boor-
loader code) after reset, it is recommended 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 38).
Figure 38. Hardware conditions typical sequence during power-on.
VCC
PSEN
RST
The on-chip bootloader boot process is shown in Figure 39.
Purpose
The Hardware Conditions force the bootloader execution whatever BLJB, BSB
and SBV values.
Hardware Conditions
The Boot Loader Jump Bit forces the application execution.
BLJB = 0 => Boot loader execution.
BLJB = 1 => Application execution.
The BLJB is a fuse bit in the Hardware Byte.
BLJB
That can be modified by hardware (programmer) or by software (API).
Note:
The BLJB test is perform by hardware to prevent any program execution.
The Software Boot Vector contains the high address of custumer bootloader
stored in the application.
SBV = FCh (default value) if no custumer bootloader in user Flash.
SBV
Note:
The costumer bootloader is called by JMP [SBV]00h instruction.
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4180E–8051–10/06
ATMEL [ ATMEL ]
