Reset extension phase
When nRESET is released and the SWCLK/TCK pin is low, the MCU enters the so-called "Reset extension phase". In this phase the CPU does not execute any code.
When manually toggling the reset pin (e.g. in a production test), it is necessary to pull the SWCLK/TCK pin up, before releasing nRESET:
This ensures that the reset behaves as expected.