I have a S6 v1.2 with a Mini 12864 v2.1 connected. Marlin 2.0-bugfix has been loaded using STM32 programmer - so far, no problem.
Then I wanted to try to upgrade the firmware using the SDcard on the 12864. When a SDcard is inserted in the slot of the Mini12864, it looks as if the board isn't booting at all. Waiting 2 minutes and then rebooting changes nothing.
Is this a known issue ? I am not sure whether this is a circuit issue (something pulls a pin down, refusing the CPU to start up), or it is as simple as something with the bootloader - which has no knowledge of the connected display, where contract and LED backlight needs to be set, if something should be visible ?