Even installing the GHI bootloader interface USB Drivers the COM port didn’t appear after a reset with LDR0 and LDR1 pressed.
So I went with the solution explained here: http://docs.ghielectronics.com/software/tinyclr/loaders/intro.html#sam-ba-bootloader
and finally I could install the bootloader and firmware!!!
I used SAM-BA 2.18 as the 2.12 version shipped with the GHI package was always hanging up during the erase phase.
Thanks a lot for your help.
Looking forward to finding a better description of various situations an step-by-step guides because I lost a long time looking for solutions jumping from the new to the legacy old website with broken links and specifications that are not very clear for new users.
Best regards,
Vittorio