3

7618C–AVR–07/08

• Boot Reset Fuse The Boot Reset Fuse (BOOTRST) can be programmed so that the Reset

Vector points to the Boot Flash section start address after reset. Once the user code is

loaded, a bootloader command (“start application”) 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 is programmed, the Reset Vector will always point to the Bootloader Reset and

the fuse can only be changed through the serial or parallel programming interface. The

BOOTRST fuse is not active in the default factory configuration.

• External Hardware conditions The Hardware Boot Enable fuse (HWBE) can be

programmed so that upon special hardware conditions under reset, the bootloader execution

is forced after reset.

These different conditions are summarized in Figure 3-1 on page 3.

Figure 3-1.

Boot Process

4.

Protocol

4.1

Device Firmware Upgrade Introduction

Device Firmware Upgrade (DFU) is the mechanism implemented to perform device firmware

modifications. Any USB device can exploit this capability by supporting the requirements speci-

fied in this document.

Because it is unpractical for a device to concurrently perform both DFU operations and its nor-

mal run-time activities, these normal activities must cease for the duration of the DFU

operations. Doing so means that the device must change its operating mode; i.e., a printer is not

a printer while it is undergoing a firmware upgrade; it is a PROM programmer. However, a

BOOTRST = 0

Start Bootloader

Yes

PC = 0000h

No

PC = boot loader section

Ext Hardware

conditions

No

Yes

Application

Running

"Software activation (jump)"

Reset