ATmega16U4 datasheet(336/408 Pages) ATMEL

336 / 408 page

336

7766D–AVR–01/09

ATmega16U4/ATmega32U4

Note:

1. “1” means unprogrammed, “0” means programmed

Note:

1. “1” means unprogrammed, “0” means programmed

27.5

Entering the Boot Loader Program

The bootloader can be executed with three different conditions:

27.5.1

Regular application conditions.

A jump or call from the application program. This may be initiated by a trigger such as a com-

mand received via USART, SPI or USB.

27.5.2

Boot Reset Fuse

The Boot Reset Fuse (BOOTRST) can be programmed so that the Reset Vector is pointing to

the Boot Flash start address after a reset. In this case, the Boot Loader is started after a reset.

After the application code is loaded, the program can start executing the application code. Note

that the fuses cannot be changed by the MCU itself. This means that once the Boot Reset Fuse

Table 27-2.

Boot Lock Bit0 Protection Modes (Application Section)(1)

BLB0 Mode

BLB02

BLB01

Protection

No restrictions for SPM or (E)LPM accessing the

Application section.

SPM is not allowed to write to the Application section.

SPM is not allowed to write to the Application section, and

(E)LPM executing from the Boot Loader section is not

allowed to read from the Application section. If Interrupt

Vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

(E)LPM executing from the Boot Loader section is not

allowed to read from the Application section. If Interrupt

Vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

Table 27-3.

Boot Lock Bit1 Protection Modes (Boot Loader Section)

(1)

BLB1 Mode

BLB12

BLB11

Protection

No restrictions for SPM or (E)LPM accessing the Boot

Loader section.

SPM is not allowed to write to the Boot Loader section.

SPM is not allowed to write to the Boot Loader section,

and (E)LPM executing from the Application section is not

allowed to read from the Boot Loader section. If Interrupt

Vectors are placed in the Application section, interrupts

are disabled while executing from the Boot Loader section.

(E)LPM executing from the Application section is not

allowed to read from the Boot Loader section. If Interrupt

Vectors are placed in the Application section, interrupts

are disabled while executing from the Boot Loader section.

ATmega16U4 Datasheet(PDF) 336 Page - ATMEL Corporation

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