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Open source ARM Debugger
By Nandox7

Been searching but can't find any documentation or reference to it.
Is there any specific reason/limitation for the lack of support of xMega devices (i.e.: ATxmega256A3BU) over JTAG?

Seems long time ago USBPROG 5.0 (that now seems gone) used to support xMega devices but just over PDI
not over JTAG.