-"We don't have hardware to run the code in."
-"Every part of the code is hardware dependant."
I noticed I wrote about these approximately a year ago for the first time (I, II, and III). That was more on the theoretical side. This time I have a bit more experience on the subject. We have found Alistair Cocburn's article on hexagonal architecture valuable in philosophizing this issue. Many firmware developers are not familiar with design patterns, but a good starting point is above mentioned article. Writing adapters for your hardware, grouping them to ports, keeping the number of them low, and only allowing them to be responsible for the lowest-level of hardware interfacing, will help you isolate hardware dependant parts. Here is a picture of hexagonal architecture for the case project we are working on.
Then there is a philosophical issue of testing the adapters, or wrappers. In many cases binary input/output handlings are just like accessors, getters and setters. In TDD literature it is generally agreed that these do not need unit tests. When writing hardware wrappers we need to think a bit harder. We could understand the unit tests for hardware wrappers as our firmware/hardware interface specification. Then for sure it makes sense to write these tests. My thinking goes along with "to be able to judge what to unit test and what not to unit test, you have to be able to unit test everything".