You must know everything that your device has to do, right down to the smallest subfunction, before you attempt to write the first line of code. Further, in a device that has any sort of critical function, you must be certain to exclude everything that you're tempted to include if it's not absolutely necessary for the proper function of the device.

You must start with a complete and thoroughly detailed requirements analysis which results in a detailed set of specifications, and design your product in accordance with those specifications, including, in this case, all the regulatory issues that you must address. You must design a series of tests that verifies that you have met each and every one of the requirements through rigorous testing methods. Those tests should prove that you meet all your requirements, including the specified mean-time between failures over the entire operating voltage, temperature, and mechanical stress range that you must meet.

You'll find, I believe, that the combination of hardware design, firmware design, prototyping, coding and debugging is about 5% of the entire task. The biggest part, by far, is the documentation, which must be complete before you start building hardware or writing code. That way, if you "lose your way" along the path, you can refer back to the documents to find your way.

RE