Just found something in iMX35 manual:
To determine the active serial port (either UART or USB), MCU ROM polls the UART and USB status register for 32 seconds. If there is no activity on either of the ports, within the predefined polling loop time,then the ROM powers down the device, using the watchdog (WDOG). In the USB/UART bootloader validcase, the WDOG is serviced periodically. If the communication between the host and Baseboard IC (BB IC) hangs for more than 32 seconds or the processor goes into an endless loop, then the WDOG expires,and powers down the device.
Could someone with a functioning K3 please post how long it takes until the dwonload mode kicks in ?