Programming an ATmega32u4 through USB

Question

I'm designing my first circuit with an ATmega 32u4 processor. Since I read about the BOOTRST and HWBE fuses I'm confused about the programming of the chip.

If

• I use a brand new, untouched processor with factory default settings,
• my only access point to the built device are the four USB lines (Vcc,D-,D+,GND),
• I have access neither to the RESET pin nor to the HWB pin

then will it be possible to upload a program to the device with the help of the built in USB boot loader?

Answer 1

No - take a look at page 3 of this PDF. All entry to the DFU bootloader is done via reset (which you can't access) or a jump from user code (which you don't have on there because it's a blank IC).

However, how are you intending to configure the device in the first place? You'll want to set your fuses correctly, right, which means access to the ICSP pins, which means access to the reset pin...

But in short, without access to the reset pin this is a no-go.

• mTThomas: Thank you! I hoped, that it will be possible to assembly the designed device to the full end, and after that, when only the USB cable sticks out, to perform the full calibration and programming in one step. Sadly, as you write, it will be not possible. Thank you for the answare.