I've been wanting to get back into some interesting embedded systems and electronics work - so I figured I'd dust off my old Arduino. In the process I ended up having to re-learn how to program it using C without using the Arduino IDE (it's not my favourite thing), which is what I'm going to go into here.
I downloaded CrossPack which contains GCC cross compiler for the AVR architecture, as well as a handful of other useful tools (like avrdude, which we'll use to flash the Arduino). Once that's setup we can start off with a pretty simple program which rapidly blinks the arduino's onboard LED, which is sort of the Hello World of the Arduino community:
To build this using GCC we just need to call avr-gcc, making sure to specify the processor (using -mmcu), define the processor frequency (the F_CPU macro)
This creates an ELF binary. One odd thing is that pretty much everyone building C code for Arduino seems to include the "-R .eeprom" switch which removes the ".eeprom" section from the output binary. However I've not ever seen this section does not exist in my ELF binary - see below:
Anyway, perhaps there's something else at play that I've not understood so I've kept this switch in (I'm making a mental note to explore this later). Again many other places seem to insist on using the avr-objcopy utility to produce intel hex file before uploading to the Arduino - so often you'll see the below:
However again this is not a necessary step it seems, since the utility we use to upload the binary to the Arduino - avrdude - supports ELF so you can skip it. When we want to upload the binary to the Arduino we'll use avrdude, but first we need to know the programming device. For me this is just the USB type A to type B cable that comes with the arduino, which in the example below is /dev/tty.usbmodem1421:
I've found it useful to create a little Makefile to tie these steps all together:
Finally to tie this together in a slightly more existing example than blink.c, I've created a little program that will flash the LED attached to pin 13 with a message encoded in morse - "hello world" or ".... . .-.. .-.. --- / .-- --- .-. .-.. -..":
The code is on github at https://github.com/smcl/arduino_morse and only requires a bare Arduino and USB cable.