Alright so the LPC1768 on your SKR 1.3 is alive and well.
You can further verify this if you connect to the telnet interface (e.g. via ncat or telnet.exe) on the commandline (127.0.0.1 port 4444) and try to use commands for e.g. reading out the flash banks and dumping the flash content to a file (docs, reset halt, flash banks, flash info 0, flash read_bank 0 dump.bin,…). But this already strongly indicates that the MCU is alive and well.
But the SKR 1.4 doesn’t seem to be responding over the SWD interface 
. You can try a few things:
- making sure that the
TRSTsignal is connected as well to the STLink V2 (I think it’sNRSTthere) - omititing the
-c "reset_config none separate"switch from the invocation - trying different reset configs as listed here
- is the
(not)RESETline permanently low? (holds the MCU in reset, should be high by default for “not in reset”) - disconnecting every external device from the motherboard, like motors, headbeds, LCDs and whatnot. The measured voltage
3.151845seems a bit low for a 3.3V target, as if there’s a siginificant load / power draw in the system. - is the board externally powered? (I think the STLinkv2 will first try to measure the voltage on its VCC_TARGET pin, and if there’s no voltage present, will try to supply power - that may not be enough for the whole target motherboard; should use USB supply + the J15 jumper to VUSB maybe.)
- using a multimeter, is the 3.3V rail of the board and especially on the microcontroller stable? (e.g. measurable via the C14 capacitor in the schematic)
- does any chip get hot? are there burn marks on the PCB?
If the chip gets a proper power supply on all pins but still decides do nothing and not be connected to via its SWD interface, then the chip may be actually dead. These chips are listed for 4-5€ on ebay (lpc1768fbd100 LQFP100) so if you have a hot-air soldering gun and the skills, desoldering the chip and placing a new one might restore the motherboard.