User guide

Atmel AVR2054: Serial Bootloader User Guide [APPLICATION NOTE]

8390D−WIRELESS−03/2015

3.6 Bootloader USB DFU specifics

The embedded bootloader for Atmel SAM3S microcontrollers operates in a different way than shown on Figure 3-2 to

conform to certain features of the SAM3S microcontroller and development boards, on which it is hosted.

Embedded bootloader for SAM3S uses Device Firmware Upgrade (DFU) standard. A DFU component should be

included in the application (see Section 3.6.2.1). The procedure of uploading a new firmware image does not change for

the user (see Section 4.1.2). The remaining part of this section describes implementation details of SAM3S embedded

bootloader and is intended for users who will possibly modify the embedded bootloader code.

3.6.1 Changing the reset vector

The Atmel SAM3S MCU does not contain a special bootloader section in the flash memory

. The fuse settings do not

indicate whether to start execution from the application section or from the bootloader section and where this section

starts. So if the bootloader code is first programmed at the end of the flash memory and then the application code is

loaded into the remaining part of the flash memory, the MCU will start code execution from the last loaded code after

reset (that is, the application code).

In order for the embedded bootloader to work correctly it must receive control first after reset to be able to communicate

with the host software and upload a new device image before the application code is executed.

To achieve this, the embedded bootloader for SAM3S, during uploading, reassigns the reset vector value (the MCU

component that stores the address to start from reset), setting it to its own starting address. Thus the MCU will always

start execution from the embedded bootloader, which, in turn, will pass control to the application section upon

completion.

3.6.2 DFU impact

Another feature involves notifying embedded bootloader that the PC tool is ready to transfer a new firmware image to the

device. For the Atmel AVR and XMEGA MCUs the device must be reset before the new software upload can take place.

Upon reset the COM port assigned to the device on the PC is still enabled. The bootloader receives control and waits for

the uploading requests from the PC tool. However, to reset a USB stick with SAM3S it should be removed and re-

attached to the PC. This may cause the PC to assign a different COM port to the device and stop the serial

communication with the device.

To prevent serial port reassignment when using SAM3S, the USB Device Firmware Upgrade components based on [11]

should be used. DFU components should be included in both the embedded bootloader and the application code (see

Section 3.6.1). The DFU component inserted in the application code should always be enabled along with other

application components. Upon receiving a command indicating that a new firmware image is ready, it writes a special

byte into the program memory and initiates a device reset. After reset, the DFU component included in the embedded

bootloader checks this status byte and enters the new firmware upload mode.

If the embedded bootloader is the only image loaded in the program memory, it will constantly be ready to receive and

upload a new firmware image from the host.

To program a device that contains the embedded bootloader with DFU support (such as the embedded bootloader for

Atmel SAM3S) you may also use the third-party software, since DFU components provided with this software package

closely adhere to the DFU standard.

3.6.2.1 Adding DFU support to an application

The DFU class for the application is implemented on top of the USB driver. Set the

DFU_SUPPORT parameter to 1 in

configuration.h file to enable DFU support:

A dedicated bootloader ROM section is included by individual manufactures for their individual use.