Project using arm_math.h on nRF52 results in Linker Error

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For some reason unknown to me, arduino-nRF5/cores/nRF5/SDK/components/toolchain/CMSIS/Include at master · sandeepmistry/arduino-nRF5 · GitHub gives you the arm_math.h and related header files, but doesn’t actually include the library file where the functions are implemented…

You can either:

  • compile the CMSIS-DSP library from the source files (here)
  • tell the linker to link against the statically compiled library

I’ll do the latter because it’s easier. The libraries are compiler per Cortex flavour, so you have the Cortex-M3, M4, M4F (M4 + FPU) etc. Your nrf52_dk board has a nRF52832 which is in turn a "The ARM® Cortex®-M4 processor with floating-point unit (FPU) " (here, here), i.e. an M4F. However, PlatformIO compiles the firmware using soft-float instead of hard-float (-mcpu=cortex-m4 -mfloat-abi=softfp -mfpu=fpv4-sp-d16, here), which means we actually have to use the link-compatible M4 type. See here for an explanation on float-ABIs.

To make the project compile, do:

  • download libarm_cortexM4l_math.a from here
  • place the file in your project folder
  • add the line build_flags = -l arm_cortexM4l_math to the the build environment of the [env:nrf52_dk]
  • observe [SUCCESS] Took 10.10 seconds at the end of the compilation :slight_smile:
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