This is the fourth part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the firstsecond and third part please do it now. These parts contain a lot information which lead to this point of the tutorial.

The third part ended with step 18, planing the final firmware. There a decision was made about the language and style of the firmware. This article will focus on the code of the firmware itself.

Step 19: Write a Preliminary Firmware

In order to be able to do some final tests with the prototypes and be able to work on the final PCB, I need a firmware which is is very close to the final one. In the Atmel Studio, I start a new C++ project in a new folder.

The first thing I do is checking the chosen compiler options for the project. Everything looks reasonable, I just add the option --std=c++11 to the C++ compiler options to get the latest language features.

In a section below I will describe all modules I wrote and will point details about the functions. I obviously did not wrote the whole firmware sequentially in that order, instead I use a incremental approach to develop the software:

  1. Create empty frameworks for all modules.
    • Create a header and implementation file for each module with the correct name.
    • Add the header comments, the namespace, #pragma once and the #include for the own header file.
    • At this point, each module should be ready, so I can easily add new functions to each module.
  2. Start with the hardware module.
    • Write the initialisation for the hardware, like CPU speed, port directions and other important stuff.
    • Layout the interface for the hardware module and prepare empty implementation blocks to be filled with code.
    • At each place where code is missing, I write a comment // FIXME!! to be reminded that there is something missing.
  3. Start the logic module.
    • Write the main entry point of the logic.
    • Call this entry point in the main() method of the firmware.
    • Add the hardware initialisation to the logic.

At this point, I have the structure of the firmware prepared as planed. This structure will lead me through the development process. Continue Reading