[vvanders]

That's assuming that you have a functional ICE[1] and aren't dealing with a timing issue in another subsystem :).

ICE pins are usually configured with the exact registers this article talks about. It's common for UART/SPI serial port configuration to be a part of enabling ICE. Some chips also let you disable the ICE pins so that you can use them to pick the cheapest chip possible.

[1] https://en.wikipedia.org/wiki/In-circuit_emulation

[alxlaz]

Why are you doing any kind of professional work, at such a low level, without an ICE?

[__david__]

Because ICEs are overrated. Many systems can't be debugged by single stepping (think servos or anything with physical hardware being controlled). ICEs rarely work well—every single one I've ever used was completely unreliable and required lots of fiddling to make it work. And then the next day you had to start the whole fiddling process over again. In the end they aren't very productive except for very specific types of bugs (they are invaluable in the very beginning of a project when you are bringing up a board). Once everything is generally up and running I find them to be pretty useless.