The debugWIRE interface was developed by Atmel for use on low pin-count devices. Unlike the JTAG interface which uses four pins, debugWIRE makes use of just a single pin (/RESET) for bi-directional half-duplex asynchronous communication with the debugger tool.
When designing an application PCB which includes an Atmel AVR with the debugWIRE interface, the pinout shown in the figure below should be used.
The debugWIRE interface can not be used as a programming interface. This means that the SPI interface must also be available (as shown in AVR SPI Pinout) in order to program the target.
When launching a debug session using debugWIRE, flash will be programmed using the debugWIRE interface. This is not an option which can be considered for factory programming.
When the debugWIRE enable (DWEN) fuse is programmed and lock-bits are un-programmed, the debugWIRE system within the target device is activated. The /RESET pin is configured as a wire-AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway between target and debugger.