After many years with the AVR, I am starting with ARM, specifically ATSAMC21J18AU. The JTAGICE3 works fine with AVRs, but fails to recognize the ARM part (using SWD interface.)
The "Tools | Device Programming | Apply" command appears to succeed, but when I click "Device Signature", it returns:
Timestamp: 2019-09-10 13:55:01.203
ModuleName: TCF (TCF command: Device:startSession failed.)
Unexpected Chip Identifier 0x11010400 (expected 0x1101050f).
Since both 1 and 0 bits are seen, it appears the wiring is correct.
The Atmel Studio software is newly installed (using the web installer), Atmel Studio 7.0 1931; ATSAMC21_DFP.1.2.176
I have seen several similar queries on the boards, the usual answer is to update the Studio tools. But these are newly downloaded, afaik all latest versions.
Any further suggestions?