2 Issues found 1st you need to include the sub::imp_spi on the config.cmake 2nd in config.cmake sub::pin should be compiled after the spi.c as you are using pin funtion directi on spi.c Now compiling ;)

ked/csl/interfaces/spi.h

@@ -20,9 +20,8 @@
 extern "C" {
 #endif
 
+#include "hardwareDescription.h"
 #include "pin.h"
-#include "spi.h"
-#include <stdint.h>
 
 // TODO: when everything worksmove this into imp.spi.c
 //#include "hardwareDescription.h"

ked/csl/stm32f042/Src/imp_spi.c

@@ -1,5 +1,5 @@
 #include "spi.h"
-#include "hardwareDescription.h"
+
 #define SPI_BASE	((SPI_TypeDef *)spiBase_Addr_List[spi_hw_ch])
 
 void spiReset(spiCH_t spi_hw_ch)

ked/csl/stm32f042/config.cmake

@@ -71,10 +71,11 @@ set (MAIN_FLAGS ${C_FLAGS})
 set (MAIN_DEFS ${C_DEFS})
 
 list(APPEND EXTRA_LIBS sub::startup)
-list(APPEND EXTRA_LIBS sub::pin) 
 list(APPEND EXTRA_LIBS sub::delay)
-list(APPEND EXTRA_LIBS sub::spi)
 list(APPEND EXTRA_LIBS sub::timer)
 list(APPEND EXTRA_LIBS sub::usart)
 list(APPEND EXTRA_LIBS sub::init)
 list(APPEND EXTRA_LIBS sub::max7219)
+list(APPEND EXTRA_LIBS sub::spi)
+list(APPEND EXTRA_LIBS sub::imp_spi)
+list(APPEND EXTRA_LIBS sub::pin) 

ked/device_drivers/max7219/CMakeLists.txt

@@ -1,6 +1,5 @@
-# https://www.youtube.com/watch?v=abuCXC3t6eQ
 add_library(MAX7219 max7219.c)
 target_compile_options(MAX7219 PRIVATE ${C_FLAGS})
 target_compile_definitions(MAX7219 PRIVATE ${C_DEFS})
-target_include_directories(MAX7219 PUBLIC ${INTERFACES_DIR} ${CSL_INCLUDES})
+target_include_directories(MAX7219 PUBLIC . ${INTERFACES_DIR} ${CSL_INCLUDES})
 add_library(sub::max7219 ALIAS MAX7219)

main.c

@@ -5,7 +5,6 @@
 #include "ascii.h"
 #include "timer.h"
 #include "spi.h"
-
 #include "max7219.h"
 //#include "ked/device_drivers/max7219/max7219.h"