debugWIRE is a serial communications protocol, designed by Atmel. It is used for on-chip debugging of Atmel AVR microprocessors.
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 you have 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. The single undocumented hardware breakpoint can be used by selecting run-to-cursor.
Programmer models 
debugWIRE is supported by four programmer models, made by Atmel: AVR Dragon, AVR JTAGICE mkII, AVR JTAGICE 3 and AVR One.
debugWIRE specifications 
debugWIRE use a 1 wire open drain serial connection similar to RS-232. The default baud used is AVR clock/128. Sending a BREAK will return 0x55 which is used to detect the baud. 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 0x07 to reset the AVR. 0xF0 returns the PC, 0xF3 the debugWIRE ID.