Thursday, January 9, 2014

Developing STM32 microcontroller code on Linux (Part 4 of 8, building openocd)

The first post of this series covered the steps to build and run code for the STM32. The second post covered how to build a cross-compiler for the STM32. The third post covered how to build a debugger for the STM32. This post is going to cover building OpenOCD for your development environment.

As mentioned in the introductory post, we need OpenOCD so we can take binaries that we build and upload them onto the STM32. OpenOCD is a highly configurable tool and understands a number of different protocols. For our purposes, we really only need it to understand STLinkV2, which is what the STM32 uses. Also note that unlike previous posts, this post does not need or build a cross-compiled tool. That's because OpenOCD itself runs on our development machine, so we just need to do a normal compile. As before, I'm going to compile a certain version of OpenOCD (0.7.0). Newer or older versions may work, but your mileage may vary.

As before, we start out by exporting some environment variables:

$ export TOPDIR=~/cross-src
$ export TARGET=arm-none-eabi
$ export PREFIX=~/opt/cross
$ export BUILDPROCS=$( getconf _NPROCESSORS_ONLN )
$ export PATH=$PREFIX/bin:$PATH
The TOPDIR environment variable is the directory in which the sources are stored. The TARGET environment variable is the architecture that we want our compiler to emit code for. For ARM chips without an operating system (like the STM32), we want arm-none-eabi. The PREFIX environment variable is the location we want our cross-compile tools to end up in; feel free to change this to something more suitable. The BUILDPROCS environment variable is the number of processors that we can use; we will use all of them while building to substantially speed up the build process. Finally, we need to add the location of the cross-compile binaries to our PATH so that later building stages can find it.

Now we are ready to start. Let's fetch openocd:

$ cd $TOPDIR
$ wget http://downloads.sourceforge.net/project/openocd/\
openocd/0.7.0/openocd-0.7.0.tar.gz
To start the compile, we first need to install a dependency:

$ sudo yum install libusbx-devel
Now let's unpack and build openocd:

$ tar -xvf openocd-0.7.0.tar.gz
$ cd openocd-0.7.0
$ ./configure --enable-stlink --prefix=$PREFIX
$ make
$ make install
Here we are unpacking, configuring, building, and installing OpenOCD. The configure flags require a bit of explanation. The --enable-stlink flag means to enable support for STLink and STLinkV2, which is what we need for this board. The --prefix flag tells the build system to install OpenOCD to our ~/opt/cross location. This isn't strictly correct; this isn't a cross compile tool. However, it is convenient to have everything in one place, so we install it there.

Assuming everything went properly, we should now have a openocd binary in ~/opt/cross/bin. There will also be a bunch of configuration files installed to ~/opt/cross/share/openocd. These are important as these are pre-canned configuration files provided by OpenOCD. While it is possible to create your own from scratch, the syntax is baroque and it is a lot more work than you would think. Luckily OpenOCD already comes with scripts for STLinkV2 and STM32, so we'll just use those.

In order to have a working configuration, we are going to start creating our "project" directory. This is where the code that eventually runs on the STM32 is going to be placed. I'm going to call my directory ~/stm32-project; feel free to change it for your project. So we do:

$ mkdir ~/stm32-project
$ cd ~/stm32-project
$ cat <<EOF > stm32-openocd.cfg
source [find interface/stlink-v2.cfg]
source [find target/stm32f3x_stlink.cfg]
reset_config srst_only srst_nogate
EOF
Here we create the project directory, cd into it, and then create the configuration file for OpenOCD. The configuration file deserves a bit of explanation. First, we tell it to "find" the stlink-v2.cfg configuration file. Where it looks depends on the PREFIX we configured, so in our case it is going to look through ~/opt/cross/share/openocd for that file (where it should find it). Next we tell OpenOCD to "find" the stm32f3x_stlink.cfg file. Again, that file is located in ~/opt/cross/share/openocd, and it again should find it. Note that if you have a different STM32 chip, you should substitute f3x with whatever version of the chip you have. Finally there is a line about reset_config srst_only, and srst_nogate. I honestly don't know what those are for, though they seem to be necessary.

That's it for OpenOCD. Everything should be built, configured, and ready to go.

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