ATmega16U4 datasheet(337/408 Pages) ATMEL

337 / 408 page

337

7766D–AVR–01/09

ATmega16U4/ATmega32U4

is programmed, the Reset Vector will always point to the Boot Loader Reset and the fuse can

only be changed through the serial or parallel programming interface.

Note:

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

27.5.3

External Hardware conditions

The Hardware Boot Enable Fuse (HWBE) can be programmed (See Table 27-5) so that upon

special hardware conditions under reset, the bootloader execution is forced after reset.

Note:

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

When the HWBE fuse is enable the ALE/HWB pin is configured as input during reset and sam-

pled during reset rising edge. When ALE/HWB pin is ‘0’ during reset rising edge, the reset vector

will be set as the Boot Loader Reset address and the Boot Loader will be executed (See Figures

27-3).

Figure 27-3. Boot Process Description

Table 27-4.

Boot Reset Fuse(1)

BOOTRST

Reset Address

Reset Vector = Application Reset (address 0x0000)

Reset Vector = Boot Loader Reset (see Table 27-8 on page 346)

Table 27-5.

Hardware Boot Enable Fuse(1)

HWBE

Reset Address

1ALE/HWB pin can not be used to force Boot Loader execution after reset

0ALE/HWB pin is used during reset to force bootloader execution after reset

HWBE

BOOTRST ?

Ext. Hardware

Conditions ?

Reset Vector = Application Reset

Reset Vector =Boot Lhoader Reset

RESET

ALE/HWB

SHRH

HHRH

ATmega16U4 Datasheet(PDF) 337 Page - ATMEL Corporation