4.5.1.5.3 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