debugWIRE is designed as a simpler alternative to JTAG, aimed at processors with limited resources. It is supported by most modern 8-bit AVRs. By using debugWIRE one has full read and write access to all memory and full control over the execution flow. It supports single-step, run-to-cursor, step-out, and software break instructions. A breakpoint (run-to-cursor) will be set by reprogramming the BREAK opcode into the FLASH storage of the AVR. Care must be taken due to the limited guaranteed write cycles (10000) of the device.
debugWIRE is supported by all modern debuggers from Atmel. This includes the Atmel-ICE, the JTAGICE3, the AVR Dragon and the JTAGICE mkII.
debugWIRE is activated by setting (programming to zero) of the fuse DWEN (debugWIRE enable), normally located in the high fuse byte. After this action the /RESET pin function isn't available anymore and the ISP protocol cannot be used. debugWIRE can be disabled with e.g. JTAGICE mkII by sending a special reset command that disables temporarily the debugWIRE function and reenables /RESET and also ISP until next power down cycle. debugWIRE is not able to program the fuses of a device. If you want to disable debugWIRE permanently you have to disable the function temporarily and program the fuse using the now available ISP function. debugWIRE uses a 1 wire open drain serial connection. The default baud rate used is AVR clock/128. Sending a BREAK 0x00 will return 0x55 which is used to detect the baud rate. The AVR will return a BREAK when a breakpoint is hit or a collision is detected on the debugWIRE connection. 0x06 is used to disable debugWIRE and reenable /RESET and ISP, 0x07 resets the AVR, 0xF0 returns the PC, 0xF3 the debugWIRE ID.