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Ok I got the point
After a little further reading I saw that a flat battery might force the PW1 into a secure low power mode without auxiliary like eMMC.
To prevent low power mode I added (in parallel to the battery) a fixed voltage source - which did not change the behaviour of the PW1-board.
Next step from my side will be to build an u-boot from source and try to enable debugging (if I remember this correctly there were some disabled switches in the code) to see if I get more information from U-Boot on the serial console before it is halting the board and to find out more about MMC switch: -19
cheers
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