Arduino UNO by design has come out be a complex system, needing three different flash programs to make it run. I will show where these three are located and method to update them. Below excerpt is from Atmega8u2 /Atmega16u2 datasheet.
When the HWBE fuse is enable the PD7/HWB pin is configured as input during reset and sampled during reset rising edge. When PD7/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 .
Hence starting with Atmega8u2 / Atmega16u2 that is used as USB to UART, when it is first connected to the USB port of your PC, it is read as power on by the Atmega8u2 / Atmega16u2 and bootloader is skipped and it enters application section (i.e usb to uart) even though your HWB pin is pulled low (for UNO Rev3). However if you reset Atmega8u2 / Atmega16u2 it is read as reset by the uC and during the rising edge of reset signal the uC enters into the bootloader mode.
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So below are the configuration to flash different parts of devices on UNO
| Device | Section | Source | Method |
|---|---|---|---|
| Atmega8u2 / 16u2 | Bootloader | usbdfu | Through ISP Header at top right |
| Atmega8u2 / 16u2 | Application Section | usbserial | Reset Atmega8U2 by shorting reset to gnd to enter bootloader. |
| Atmega328 | Bootloader | Optiboot | Through ISP Header at the bottom |
Actually, the usbdfu and usbserial are combined together in Arduino distribution as 1 hex file and can be downloaded in one go though Atmega8u2 / Atmega16u2 6 pin ISP port and using AVR ISP.
.....cont
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