Blink led at Arduino pin 13. Pure C version.

hello_world/firmware/Makefile

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+# Name: Makefile
+# Author: <insert your name here>
+# Copyright: <insert your copyright message here>
+# License: <insert your license reference here>
+
+# This is a prototype Makefile. Modify it according to your needs.
+# You should at least check the settings for
+# DEVICE ....... The AVR device you compile for
+# CLOCK ........ Target AVR clock rate in Hertz
+# OBJECTS ...... The object files created from your source files. This list is
+#                usually the same as the list of source files with suffix ".o".
+# PROGRAMMER ... Options to avrdude which define the hardware you use for
+#                uploading to the AVR and the interface where this hardware
+#                is connected.
+# FUSES ........ Parameters for avrdude to flash the fuses appropriately.
+
+DEVICE     = atmega328p
+CLOCK      = 16000000L
+BAUD 	   = 57600
+SERIALPORT = /dev/tty.usb*
+PROGRAMMER = arduino
+OBJECTS    = main.o
+#FUSES      = -U hfuse:w:0xd9:m -U lfuse:w:0x24:m
+
+# Tune the lines below only if you know what you are doing:
+AVRDUDE = avrdude -c $(PROGRAMMER) -b $(BAUD) -P $(SERIALPORT) -p $(DEVICE)
+COMPILE = avr-gcc -Wall -Os -DF_CPU=$(CLOCK) -mmcu=$(DEVICE)
+
+# symbolic targets:
+all:	main.hex
+
+.c.o:
+	$(COMPILE) -c $< -o $@
+
+.S.o:
+	$(COMPILE) -x assembler-with-cpp -c $< -o $@
+# "-x assembler-with-cpp" should not be necessary since this is the default
+# file type for the .S (with capital S) extension. However, upper case
+# characters are not always preserved on Windows. To ensure WinAVR
+# compatibility define the file type manually.
+
+.c.s:
+	$(COMPILE) -S $< -o $@
+
+flash:	all
+	$(AVRDUDE) -U flash:w:main.hex:i
+
+fuse:
+	$(AVRDUDE) $(FUSES)
+
+# Xcode uses the Makefile targets "", "clean" and "install"
+install: flash fuse
+
+# if you use a bootloader, change the command below appropriately:
+load: all
+	bootloadHID main.hex
+
+clean:
+	rm -f main.hex main.elf $(OBJECTS)
+
+# file targets:
+main.elf: $(OBJECTS)
+	$(COMPILE) -o main.elf $(OBJECTS)
+
+main.hex: main.elf
+	rm -f main.hex
+	avr-objcopy -j .text -j .data -O ihex main.elf main.hex
+# If you have an EEPROM section, you must also create a hex file for the
+# EEPROM and add it to the "flash" target.
+
+# Targets for code debugging and analysis:
+disasm:	main.elf
+	avr-objdump -d main.elf
+
+cpp:
+	$(COMPILE) -E main.c

hello_world/firmware/main.c

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+#include <avr/io.h>
+#include <util/delay.h>
+
+int main(void)
+{
+    /* Make PORTB5 (Arduino pin 13) an output. */
+    DDRB |= _BV(PORTB5);
+
+    for(;;){
+        /* Toggle state of PORTB5 (Arduino pin 13). */
+        PORTB ^= _BV(PORTB5);
+        _delay_ms(500);
+    }
+    
+    /* Never reached. */
+    return 0;
+}