Using ATmega644 on JTAG Debugger

Overview

This debugger was implemented and designed for the ATmega644 which utilizes its JTAG interface for communication as it sets breakpoints and access registers and memory in order to control program execution.

Explanation

JTAG stands for Joint Test Action Group that is commonly used to program hardware devices and to debug embedded systems. Four signals are used via 4 port pins by the JTAG standard including TCK, TMS, TDI, and TDO.

There are two ATmega644 MCUs used in this project. The target microcontroller that is debugged is programmed with any arbitrary code by the user while the debugger microcontroller is where the actual debugging firmware is contained. Through a command window on HyperTerminal, the user is able to interface with the debugger which makes the debugging MCU be able to control the target through the JTAG port. A series of bits to the JTAG interface is being sent by the code running on the debugger. The module responsible for executing AVR instructions and setting breakpoints is an On-Chip debugger unit that is contained in the ATmega644. This is because all the necessary functions of the debugger cannot be performed by alone by the JTAG interface.