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 Figure 1 should be used.

Figure 1. debugWIRE (SPI) Header Pinout

Note: The debugWIRE interface can not be used as a programming interface. This means that the SPI interface must also be available (as shown in Figure 1) 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 wire-AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway between target and debugger.