The first of the Uncle Bob's Three Rules of tdd states:
1. You are not allowed to write any production code unless it is to make a failing unit test pass.
A few recent discussions among embedded developers revealed that this rule has caused some confusion among fellow beginners of embedded tdd. So, let me first point you to another tip from the same source(Uncle Bob advises to do fiddling around on things that you are not sure of how they work). I will share a short story:
I've been writing a proof-of-concept driver for serial to Ethernet controller. It was not sure if the serial port on uC could be configured to work with the MAC controller, or if it would be possible to use the DMA controller to manage longer transfers. I needed the proof-of-concept for the hardware team fast. I learned how to use the controller by trial and error, glueing together several bits from application notes and examples and running them in combination of two evaluation kits. Needless to say it was a mess. It even turned out I couldn't get the job done without a few circuits from my hardware pals. It would have been really awkward and laborous to have tests written during this fast paced back and forth experimenting based on a sample code which of course didn't come with tests. All this was done in C, with the tools for C.
After I knew which bits worked and which didn't, I wanted to illustrate this learning in tests. I harnessed the quickly hammered code with tests and then massaged the tests and code hand in hand into better shape. In retrospective, I should have treated the original code as a throw away prototype (aka code from Spike). I thought I would be faster by continuing to work with the code I had. Sad, but lacking the discipline made my overall cycle time propably massively longer. I believe that this is more of a rule.
It might be from the first XP book, I'm not sure, but when I was first introduced to agile methods people always listed the last rule; rules are just rules. Based on this experience I do believe it is pragmatic to fiddle things around without tests. But when you start sculpting the solution towards production code, you should take a fresh start and drive test-first based on your newly acquired knowledge. I know I will next time.