3.10.10.3 Physical Interface

When the debugWIRE enable (DWEN) fuse is programmed and lock bits are unprogrammed, the debugWIRE system within the target device is activated. The NRESET port pin is configured as a wire-AND (open-drain) bi-directional I/O pin with a pull-up resistor enabled and becomes the communication gateway between target and emulator.

The following figure shows the schematic of a target MCU with debugWIRE enabled, and the emulator connector. The system clock is not affected by debugWIRE.

Figure 3-93. debugWIRE Setup

When designing a system where the debugWIRE is used, the following observations must be made for correct operation:

  • Pull-up resistor on the NRESET (PC0) pin must be in the range of 10 kΩ to 20 kΩ. However, the external pull-up resistor is optional.
  • Connecting the NRESET (PC0) pin directly to VS will not work.
  • Capacitors inserted onto the NRESET (PC0) pin must be disconnected when using debugWIRE.
  • All external reset sources must be disconnected.