跨多个包含文件使用函数

I am currently programming a µC in C, and for making things easier to read I separated all different connection functions (like SPI, TWI, GPIO) in separate function-/include-files. Thus I have now two files for all functions regarding TWI, two files for GPIO aso.. Now I want to use functions for SPI in my GPIO-functions, and the other way round. The easiest way would be to put simply all functions in a big file, but I want to avoid that in order to increase readibility. But I also can not include the GPIO-include file into the SPI-include file, after I want to use the functions from both files in the other file, too.I am currently programming a µC in C, and for m