4.2.4 debugWIRE
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 AVR with the debugWIRE interface, the pinout shown in Figure 4-6 should be used.
The debugWIRE interface cannot be used as a programming interface. This means that the SPI interface must also be available (as shown in Figure 4-7) in order to program the target.
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 wired-AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway between target and debugger.