When connecting to most Cortex-M part, by default, the debugger will typically attempt to connect using SWD. If you need to connect via JTAG, then you must make a debugger configuration change as follows:
Right-click on your project in Project Explorer
Select Launch Configurations -> Edit Current
in the Debug Configurations dialog, select the Debug or Release configuration for your project
click on the Debugger tab
in the table under Script Values, scroll the list and select the option Debug Protocol
Select "JTAG" from the drop-down list
Press Apply and then Debug
The debugger will now connect using JTAG instead of SWD.
Alternatively, you can use the Debug options template dropdown to select between JTAG and SWD debug templates.
Note: The first time that you connect to a target over JTAG, a pop up dialog will appear for you to select which device on the JTAG scan chain to connect to, the selected device will be remembered for subsequent debug operations.
Since SWD only supports connections to a one core system, JTAG must be used when debugging Multicore applications. For more information about Multicore debug, see the following FAQS: