The answer to your last question: Yes, the previous firmware can be restored, if you saved it before the update using DfuSe (in the window on the left) or if you can find the previous firmware somewhere, then with that. You can see a picture on my website. DFU mode will remain on the device until you upload a dfu that was written for this device. Then you can log in the next time you turn it on.
When the device is in DFU mode, it will remain in this mode until you have uploaded a usable firmware. Even if you turn it off and on again.
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Gyula HA3HZ ( )
HA3HZ