DebugWIRE
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debugWIRE is a serial communications protocol, designed by Atmel. It is used for on-chip debugging of Atmel AVR microprocessors.
Protocol
debugWIRE is designed as 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 lacks support for single-step execution of code.
Programmer models
debugWIRE is supported by three programmer models, made by Atmel: AVR Dragon, AVR JTAGICE mkII and AVR One.
debugWire specifications
debugWire use a 1 wire open drain serial connection similar to RS232. 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. [1]