SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
Using a general purpose I/O line is simpler, as the line can be dedicated to this function, but it does require a free line available. It will also require a pull up or down and a test point to override the pull up or pull down.
However, it is often useful to have a free I/O line available for development. It is very useful for instrumenting code. It can be used to indicate internal events to monitor timing and trigger oscilloscopes. Since the line is only checked at reset for the backdoor, it can then be used for other functions once the program is started.