I’ve just (belatedly) added Doxygen commentary to the firmware I’ve written for the OpenEffects Project hardware. It was half a day’s work, but it was reasonably painless to install Doxygen and Graphviz dot, even starting from scratch as I was.
I had blanched at the idea of manually diagramming the class hierarchy and arranging the comments so they made sense. Doxygen handles it well enough that I didn’t have to struggle (much).