ULINK keeps controller in reset when not connected to PC

Hi everybody,

I have a custom pcb with a 20-pin standard Cortex-M debug connector. The schematic of the debug interface is equal to the one on page 26 in this document:

www.mouser.com/.../Board_Users_Manual_CPU_Board_XMC4500_General_Purpo-255431.pdf

The debuggers are ULINK2 and ULINKpro. When the ULINK is connected to the pcb, but not plugged into the PC it keeps the controller on the board in reset state. Only when the ULINK is powered the controller starts running.
Why is that? Is it due to the schematic? What has to be done to prevent this behaviour?

Regards, Niclas

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