Datasheet

335

7766F–AVR–11/10

ATmega16/32U4

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

1 Reset Vector = Application Reset (address 0x0000)

0 Reset Vector = Boot Loader Reset (see Table 27-8 on page 344)

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