we are using a STM32H743VIT6 on a custom board with a JLink debugger. Out of the blue the processor jumps in a state where it isn't possible to flash the ECU anymore. The board is running but nether JMem nor our IDE (uVision) are able to access or detect the controller. Has anyone else encountered this behaviour so far? Google wasn't helpful either.
It is almost impossible to archive unless you enable RDP (which is very hard to archive if it was not the intention of the programmer).
You probably have screw-up the board design. You should have pull-up resistors on the debug lines and NRST connected to the programmer.
If you do not have NRST available simple solder the wire to the NRST, and when the programming probe connects to the uC, connect it to the GND.
If the NRST line is connected to the programmer you need to select nn the configuration "Connect under Reset"