29.8 Using the JTAG Programming Capabilities
Programming of AVR parts via JTAG is performed via the four-pin JTAG port, TCK, TMS, TDI, and TDO. These are the only pins that need to be controlled/observed to perform JTAG programming (in addition to power pins). It is not required to apply 12V externally. The JTAGEN fuse must be programmed and the JTD bit in the MCUCSR Register must be cleared to enable the JTAG Test Access Port.
The JTAG programming capability supports:
- Flash programming and verifying
- EEPROM programming and verifying
- Fuse programming and verifying
- Lock bit programming and verifying
The Lock bit security is exactly as in Parallel Programming mode. If the Lock bits LB1 or LB2 are programmed, the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a security feature that ensures no back-door exists for reading out the content of a secured device.
The details on programming through the JTAG interface and programming specific JTAG instructions are given in the section Programming Via the JTAG Interface.