Sunday, January 5, 2014

Update: AVR-LCD Assembly Project

Well, my holiday break has been mostly uneventful, besides the fact that I received a Raspberry Pi (Model A) as a gift, so I'm looking forward to getting into that soon. In the meantime, though, I've finally had the chance to dig back into the AVR-LCD assembly project.
   After a couple weeks of on-and-off work, I'm pretty much at exactly the same place. Except that I've managed to switch the compiler to avr-gcc. Exciting! For a while, the conversion broke the code in ways that confounded me until I stopped being dumb and actually took a look at the listing produced by the compiler, which converts the register defines into plain hexadecimal. Apparently, avr-gcc doesn't do you the favor of subtracting the IO-memory-space-offset from special function register addresses used with IO-specific instructions while the Atmel compiler does. When using IN, OUT, SBI, CBI, etc., the _SFR_IO_ADDR() macro can be used to subtract the offset. Once that was inserted to all of the appropriate places, the program worked properly. Hello again, world!
   The online avr-libc manual became a valuable resource for me during the conversion by providing code samples and a Makefile template. I also took some time to play around with Simulavr. Since that simulator only supports a limited number of AVR devices (not including the ATMega328P), I needed to make my assembly compatible with the ATMega328. Fortunately, all this required was replacing the CALL instructions with RCALL. With the help of this page, I was then able to put together a command to run the simulator and output a VCD file to verify the functionality. The command is recorded in a script that is up on the project's GitHub page (avr-gcc branch) along with the updated assembly source and Makefile.

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