Use a more Modern CMake approach

[itavero]

First of all, thanks for this project. It's my first time starting an ATSAMD project from scratch. (I typically use NXP MCUs and to be honest, their SDK/tooling is a lot more "developer friendly" than what Microchip seems to be offering. However this did help to simplify some things.)

One thing I did notice, is that the CMake files generated are rather "old school" CMake, rather than Modern CMake (see this gist and this online "course" for a bit of background).

I've split off the Atmel Start specific instructions from toolchain.cmake into a separate CMakeLists.txt and defined the Atmel Start code as a separate static library with all the include paths and compile/link options set as PUBLIC options.
Rather than setting global options/variables, everything Atmel specific should be more or less linked to the atstart target defined in the generated CMakeLists.txt

I've never developer with Go before, so I still need to figure out how I can install my changes on my system in order to test them. That's why I marked the PR as a Draft.

[aurimasniekis]

I am not sure what's wrong with CMake files, but the linker doesn't link the libatstart.a file correctly which results in bad elf file...

Did it work for you?

[itavero]

I'm still waiting for my prototypes to arrive, unfortunately.
I've used a similar process with SDKs from other manufacturers.

Do you know what is wrong with the ELF file?
Are you missing specific things?

[aurimasniekis]

here is a comparison of the master branch, and your pull request version. Looks like somehow main function is missing or etc https://gist.github.com/aurimasniekis/35da388137d9cde7fbca8c613852802c

[aurimasniekis]

One clue I found is that there is a startup c file with reset handler in lib so I moved it out and now at least the MCU starts, but I am getting stuck in DummyHandler for some reason...

[aurimasniekis]

Okay, I have figured out the issue. The first one is that ${ATMEL_START_DIR}/samd21a/gcc/gcc/startup_samd21.c can't be in the library as it's main handler need to be moved to ur main CMakeFiles.txt second is that some flags are missing from linker.

add_executable(example ${ATMEL_START_DIR}/samd21a/gcc/gcc/startup_samd21.c main.c)

target_link_options(example PUBLIC -Wl,--section-start=.text=0 -mthumb -mcpu=cortex-m0plus -D__SAMD21G18A__)

[aurimasniekis]

I am not sure if there is a nice way to specify that startup_samd21.c file without adding it to add_executable

[itavero]

I don't have a computer near me now, but I think that perhaps certain symbols in that startup should perhaps be made weak. That way you can override them if you don't want the implementation provided be the SDK.

I might have some time tomorrow to have a look at it.

[aurimasniekis]

This is my current solution including fix of 0 value clocks generated because of how Atmel Start generates YAML file aurimasniekis@6025d08

[aurimasniekis]

I also just noticed that moving ASF into lib causes a lot of other issues, like STDIO redirect doesn't find replacements and etc things.

[aurimasniekis]

I replaced the library approach with simple adding target sources to target it now works with all the things which weren't working before. aurimasniekis@2a8bdab

[itavero]

Finally got around to assembling my prototypes so I could continue with this.
Unfortunately I can't even remember how I installed my own fork of the atstart command on my MacBook and it seemed to have disappeared somehow.
Guess I'll be downloading and editing files manually until I finally have time to learn and understand this Go stuff.