Datasheet

348
7593L–AVR–09/12
AT90USB64/128
Note: 1. “1” means unprogrammed, “0” means programmed.
29.5.3 External hardware conditions
The Hardware Boot Enable Fuse (HWBE) can be programmed (see Table 29-5) so that upon
special hardware conditions under reset, the boot loader 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 Figure
29-3).
Figure 29-3. Boot process description.
Table 29-4. Boot reset fuse
(1)
.
BOOTRST Reset address
1 Reset Vector = Application reset (address 0x0000)
0 Reset Vector = Boot loader reset (see Table 29-8 on page 357)
Table 29-5. Hardw are 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 boot loader execution after reset
HWBE
BOOTRST ?
Ext. hardware
conditions ?
Reset vector = Application reset Reset vector = Boot loader reset
?
RESET
ALE/HWB
t
SHRH
t
HHRH