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Wednesday, November 25, 2009

C is for cookie

The robobuilder kit comes with a suite of tools which can be used to easily create poses and moves. Unfortunately, the overall impression they gave me was that they were useful for little more than making the robot dance. There were some useful features, such as being able to record the current pose of the robot and save it to a file, so I will probably play around with them later.

One of the most attractive features of the robobuilder system is that the firmware can be reprogrammed in C. If that doesn't send shivers down your spine, the rest of this post will probably be over your head. Go watch tv until the grown-ups are finished talking.

The tools to customise the robobuilder firmware are available for download. It comes with a 70 page PDF tutorial which walks you through the process of using the CodeVisionAVR IDE to customise the example builds included. The CodeVision IDE is not free, but I would recommend following the official procedure if: a) You are new to the C language or microprocessor programming and b) you use Windows.

If, like me, you fall into neither of the above categories, I suggest you save your money and have a go at using the avr-gcc compiler and/or WinAVR (which is just a Windows implementation of avr-gcc).

But first, a short rant.

When did motherboard manufacturers stop including serial ports?! I know that they are an aging technology, but they are damn useful. And you, robobuilder - you knew this all along but you didn't ship with a USB adapter or anything. I'm very disappointed. So here I am, forced to write code on the only computer in the house with a serial port - an old Dell Latitude D600. It runs Ubuntu 9.04 and was being used as a media centre PC, but I've since commandeered it in the name of the robot uprising. FYI, all of the robobuilder tools and software will run under WINE.

If you are compiling your code in linux, you will need to install avr-gcc and avr-libc. For reasons unknown, avr-gcc will not detect that avr-libc is installed and include it's headers when you compile. If you get an error message like:

error: avr/io.h: No such file or directory

You may need to copy the contents of /usr/lib/avr/include to /usr/lib/gcc/avr/4.3.2/include.

This robosavvy thread contains a port of the official robobuilder example source files which use the standard avr-gcc headers (also, comments in english!). Details about converting headers to gcc can be found at the avr-libc page. This is the skeleton code you need to edit. Use the command:

avr-gcc -mmcu=atmega128 -I. -g -Wall -Os -c main.c

to compile the source. I do recommend the -Os flag, since there is only 128kB of flash memory on the chip. You may still get some error messages at this stage - I did.

The names of header files are case sensitive in gcc, so check that the names of the #includes match the filenames:

#include "macro.h"
#include "main.h"
#include "comm.h"
#include "dio.h"
#include "math.h"

This should get rid of the remaining compile errors. Next you must link the files together using:

avr-gcc -o main.elf -mmcu=atmega128 main.o comm.o dio.o

If you get linker errors at this stage, check that you used the same -mmcu values for both compiling and linking (otherwise, gcc will try to link to code intended for different platforms).The final step is to convert the instructions to hex using the command:

avr-objcopy -j .text -O ihex main.elf main.hex

The .hex file created from this step is the actual firmware which will be written to the atmega128 chip. To upload it to the robot, simply run the "RBC Firmware update tool", and connect the robot using the (shudder) serial cable. Select the appropriate .hex file, and click the "Click Here" button. Now, press the reset button on the robot, and let the download finish.

To start the custom program, press the PF1 button. BEWARE! If you've made a mistake in your code, your robot will spaz out like an epileptic kid during a fire drill! To prevent damage itself and nearby life forms, either hold it by the head or make sure it is on the floor before testing it. Also note that unless you've specifically prevented such actions, the robot has no idea of it's own kinematics - meaning that it is quite likley to try and put it's arms through it's body when trying to scratch it's back. Be ready to kill the power if you want to prevent damage to the servos.

Now that I have some skeleton code to play with, over the next few days I'll experiment with various useful functions of the chip, like loading pre-built motions and getting data from the IR remote.

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