3.3.4.6 AVR debugWIRE Interface
The debugWIRE interface is 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 bidirectional half-duplex asynchronous communication with the debugger tool.
The debugWIRE interface can not be used as a programming interface. This means that the SPI interface must also be available (as shown in 3.3.4.2 AVR SPI Interface) 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) bidirectional I/O pin with pull-up enabled and becomes the communication gateway between target and debugger.