Tuna-i3-Plus VS mdrivlib

Other than the fact that 1, 2, and 3 are perfectly representable as integers in IEEE-754, there are a number of languages that have fixed-point types, C and C++ offer specific forms as compiler extensions, and it really isn't hard to implement in C++ using a templated class (though you get better ABI semantics for it in Clang with [[trivial_abi]]). Here's a crude implementation in C++17 for AVR, complete with mismatched tabs/spaces.

I did some similar things with my old libtuna for AVR, mainly with intsz and with how fixed-point values are handled.

Being able to mark a register RW vs RO vs WO and have the compiler check your access is great. Have you also considered the case where a register bit is cleared by writing a 1 to it (or to another register)? I've also implemented my own approach for a general register class (bits and pieces stolen from various other projects): https://github.com/4ms/mdrivlib/blob/main/drivers/register_access.hh I use types for registers, rather than structs, which lets me do RegisterSetClear to handle the case where different registers set and clear a bit.