DebugWIRE^TM is a One-wire, bi-directional interface for all new AVRs with 16 kByte and less flash memory. It enables On-Chip debugging at the real target device.
This debugging interface offers full execution and program flow control.
In addition this presents an unlimited number of breakpoints, adjustment of I/O pins, monitor functionality and programming of nonvolatile memories.
Additional pins are not required as the reset pin is used for connectivity.

The AT90JTAGICE-MK2 is the dedicated tool , which gives the possibility to debug all AVR devices with JTAG or debugWIRE interface.