Compile c to hex

avr-gcc -Os -std=c11 -mmcu=atmega324pa *.c -o out.elf
avr-objcopy -j .text -j .data -O ihex out.elf out.hex

Flash to atmega324a with pololu programmer

sudo avrdude -v -c stk500v2 -p m324pa -F -U flash:w:out.hex -P /dev/ttyACM0

Serial connection with putty
sudo putty /dev/ttyUSB0 -serial -sercfg 19200,8,n,1,N