Content originally posted in LPCWare by bavarian on Mon May 23 03:16:46 MST 2016
Sounds tricky and confusing ;-)
Let me summarize this situation:
[list]
[*] It was initially working with one board + MCU
[*] With a new board and a new MCU it was not working
[*] You checked all differences and made crosschecks by changing/replacing the MCUs
[*] You now don't get it working again on both PCB versions
[/list]
What could be the reasons:
[list]
[*] JTAG/SWD port damaged: check the voltage on the JTAG/SWD pins, if one of the pins is on an unclear level somewhere between HIGH and LOW, then the pin might be damaged. This could happen for example when connecting the powered JTAG debugger to the powered board. You could have have a huge voltage potential difference due to the different power supply domains and the current spike destroyed the pin. This can be avoided by connecting first GND1 to GND2, but when connecting the 10-pin debugger connector this cannot be ensured, one of the JTAG i/f pins could have been the first one.
[*] You have a firmware inside the chip which starts running after power-on and prevents somehow the access via JTAG/SWD. This could be solved by going into ISP bootmode P2_7 on LOW level) and access the chip via UART0 and the FlashMagic tool. You can also try to send a "?" to UART0, the chip should respond with an "OK".
[*] Your debugger box is damaged
[*] Last but not least ... you have overseen a hardware modification on your boards which prevents the chip to start correctly
[/list]
Some more notes:
[list]
[*] The 3.3V on the debugger connector is not really required
[*] For SWD you should have a pullup at least on the SWDIO pin
[*] External oscillator is not required, the chip starts with the internal 12MHz RC oscillator
[*] If you have USB, then you could try to boot in DFU mode and access the chip using the LPCScrypt tool
[/list]
Hope this includes some helpful information,
NXP Support Team