Content originally posted in LPCWare by mch0 on Sun Apr 10 06:07:46 MST 2016
I have managed to kill one target as well (just JTAG/SWD) over maybe one or two weeks.
As in your case the remainder of the chip (target) is not affected, I can still reprogram it by using a bootloader. Just JTAG/SWD isn't operational ony more.
However, in may case I am 99% sure that it was my fault: I did not protect the targets's JTAG port against beeing driven by the probe while not powered.
Normally this condiction is avoided by using Vtarget to turn off any signals from the probe, but I omitted that on my probe (hence my fault) since I knew both the target an the probe are working at the same VDDIO.
If your setup also inadvertently can enter that "mode" then I suspect that you overloaded the input protection circuit of TDI and it died after some time due to excessive stress.
Since I can't change the current batch of targets (i.e. cannot add theVtarget pin) I inserted a 2k2 resistor in series for the debug lines. This limits the current that can still flow if I forget to power up the target but does not affect debugging (I have very short lines). As a secondary fix I added external schottky diodes connecting the debug lines to VDD on the target HW. These should help to protect the internal diodes, too.
As of now I have not lost any more targets of that batch and I have done A LOT of debugging with several pairs of target/probes now.
Mike