4.2.2 Connecting Atmel AVR Dragon Probe to 6-pins SPI Header using a 6-pin Cable

When the DWEN fuse is programmed, there is only need for the GND, VTref, and RESET lines to be able to use the debugWIRE interface. However, to ease the task of changing between SPI programming mode and debugWIRE mode, it is recommended to use debugWIRE with all six lines connected. The SPI pins will not be driven by the Dragon when running debugWIRE, but pull-up resistors will still be active.