Understanding the Makefile

The Makefile for this project is given below.

# Compiler
CC:=avr-gcc

# Source files
SRC_DIR := ./src/

# Microcontroller
MCU := atmega328p

# Clock frequency (16MHz clock chosen)
## To give user input F_CPU, write 'make hex_build "F_CPU=12E6"' for 12MHz clock
F_CPU = 16E6

# Library and other source files
## Directory
DEVICE_DIR := ./inc/$(MCU)/
UTIL_DIR := ./inc/lib/

## Library files location
DEVICE_LIB := $(shell find $(DEVICE_DIR) -name '*.h')
UTIL_LIB := $(shell find $(UTIL_DIR) -name '*.h')

## C files location
DEVICE_C := $(shell find $(DEVICE_DIR) -name '*.c')
UTIL_C := $(shell find $(UTIL_DIR) -name '*.c')

## Collecting all C files and O file names
DEPEN_C := $(DEVICE_C) $(UTIL_C)
DEPEN_O := $(patsubst %.c,%.o,$(DEPEN_C))


# Build and object directory
BUILD_DIR := ./build/
OBJ_DIR := $(BUILD_DIR)obj/

# C standard
STD_C := gnu99

# AVRDUDE Programmer
PROGRAMMER := arduino # Try with avrftdi to avoid using arduino bootloader

# USB Port Number
PORT_NO := /dev/ttyACM0 # works for arduino programmers
#PORT_NO ?= $(shell bash -c 'read -p "USB Port number: " port_no; echo $$port_no')

#Flags
CCFLAGS:= -std=$(STD_C)
CCFLAGS+= -Wall -mmcu=$(MCU) -F_CPU=$(F_CPU)

# Build All files
all: obj_build elf_build hex_build

# Build all dependency source files to object files
$(DEPEN_O): $(DEPEN_C)
	$(CC) $(CCFLAGS) -c -o $(OBJ_DIR)$(notdir $@) $(patsubst %.o,%.c,$@)

# Build main object file and dependency object files
obj_build: $(SRC_DIR)main.c $(DEPEN_O)
	$(CC) $(CCFLAGS) $(SRC_DIR)main.c -c $(shell find $(OBJ_DIR) -name '*.o') -o $(BUILD_DIR)main.o
	echo "\nDone Object file creation"

# Build ELF file
elf_build: $(BUILD_DIR)main.o obj_build
	$(CC) $(CCFLAGS) $(BUILD_DIR)main.o $(shell find $(OBJ_DIR) -name '*.o') -o $(BUILD_DIR)main.elf

# Build Intel Hex file format for avrdude flashing
hex_build: obj_build elf_build $(BUILD_DIR)main.elf
	avr-objcopy -j .text -j .data -O ihex $(BUILD_DIR)main.elf $(BUILD_DIR)main.hex
	
# lsusb -t -v

# Used for Flashing Microcontroller!
flash_mcu: hex_build
	avrdude -v -p $(MCU) -c $(PROGRAMMER) -P $(PORT_NO) -U flash:w:./build/main.hex:i

# find the programmers using avrdude -c randomdata

clean:
	rm ./build/main.*

• The Makefile is made generic as possible. Listed below as the generalisations:
  • Target micro-controller can be changed just by changing the variable name MCU.
  • DEVICE_DIR is stores the particular micro-controller’s memory map header file and relevant declarations in the header file.
  • UTIL_DIR contains the header file and source files for the different peripherals such as the LCD display, Motor driver etc.
  • DEVICE_LIB and UTIL_LIB both are made to specify the header file locations for tje compiler.
  • User defined libraries and functions can be present in the lib folder. This can also be changed by changing the UTIL_DIR variable.
  • The programmer by default is set arduino using the variable PROGRAMMER.
  • The USB port is also assigned using the variable PORT_NO.
  • The project by default uses C99 standard which is can be changed by changing the STD_C variable value.
  • The processor clock frequency is specified with the flag F_CPU. This was first not present, but as the delay function was used at multiple places, it was cumbersome to define it in each file, hence it was made as a flag that can be accepted from the user.
    • To give a different clock frequency, the user can run make as follows

      make hex_build "F_CPU=12E6"

• By default the project assumes the main.c file under src directory is where the user would write the main function code.

• make all would generate the hex file for flashing. It contains the build, link and object copy stages.

• CC_FLAGS
  • --mmcu flag is used to tell the compiler which is the target micro-controller.
  • -std is used to specify the C standard.
  • -c is the flag used to indicate the source C file
  • -L is the flag used to indicate the user defined library file location
  • -Wall ensures that all warnings from the Compiler are printed out (for easy debugging).

• $(DEPEN_O): $(DEPEN_C)
  • This is used to create the object files of the library source files that are present under the inc directory.
  • All the object files for inc directory are stored in the obj directory under the build directory.
  • This would then be linked if required in the next step.

• make obj_build would generate the main.o object file in the BUILD_DIR.
  • $(CC) $(CCFLAGS) $(SRC_DIR)main.c -c $(shell find $(OBJ_DIR) -name '*.o') -o $(BUILD_DIR)main.o
    • This would first generate all the required object files from the inc director, and link then to create a final object file named main.o in the build directory.

• make elf_build also works similar. It generates the elf file from the object file generated for the target MCU.

• make hex_build generates the Intel compatible hex file format which can be used to flash the micro-controller.
  • avr-objcopy -j .text -j .data -O ihex $(BUILD_DIR)main.elf $(BUILD_DIR)main.hex
    • -j flags are used to mention which segments of the elf file we wish to join (Needs to be verified!) ⇒ here we join the text and data segment.
    • -o ihex is used to specify that we require the output in hex file format.

• make flash_mcu uses the avrdude to flash the micro-controller.
  • avrdude -v -p $(MCU) -c $(PROGRAMMER) -P $(PORT_NO) -U flash:w:./build/main.hex:i
    • -p is used to specify the part (or micro-controller target).
    • -c is used to specify the programmer (arduino here as it comes with a bootloader).
    • -P is used to specify the USB port that the arduino board is connected to. Usually its /dev/ttyACM0.
    • -U is used to specify the hex file for flashing.

References

1. https://utat-ss.readthedocs.io/en/master/getting-started/hello-world.html

2. https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html

3. https://github.com/avrdudes/avrdude

4. https://www.ladyada.net/learn/avr/avrdude.html

5. https://avrdudes.github.io/avrdude/7.2/avrdude_3.html#Option-Descriptions

6. https://unix.stackexchange.com/questions/759887/how-to-compile-every-c-file-separately-and-then-link-all-objects-with-a-makefile

7. https://stackoverflow.com/questions/59446839/get-filename-from-in-makefile

8. https://www.gnu.org/software/make/manual/html_node/Wildcard-Function.html