4.3.1.5.3 Understanding debugWIRE and the DWEN Fuse

When enabled, the debugWIRE interface takes control of the device’s RESET pin, which makes it mutually exclusive to the SPI interface, which also needs this pin. When enabling and disabling the debugWIRE module, follow one of these two approaches:

Let the software front-end take care of things (recommended)
Set and clear DWEN manually (exercise caution, advanced users only!)

Important: When manipulating DWEN manually, the SPIEN fuse must remain set to avoid having to use High-Voltage programming.

Figure 4-1. Understanding debugWIRE and the DWEN Fuse