Last time I wrote about how to build a firmware image for the pico 2. This time the plan is to set up debugging using openocd, and hook it into my development environment

This process uses the raspberry pi debug probe. The rest of the text assumes that it’s connected to both your computer and you debug target, and permissions are set up properly. This should mostly be a matter of creating the udev file /etc/udev/rules.d/20-debug-probe.rules with something like the following content:

ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="000c", MODE="660", GROUP="plugdev", TAG+="uaccess"

Ideally we don’t want to have to be root to be able use the debug probe, and the above configuration should make sure of that.

Now, after some experimentation I found a couple of gotchas, that might make the setup process different in the future. For the time being, the latest stable release of OpenOCD, 0.12.0, does not properly support the pico. The problem is that stopping the execution causes sleep functions to freeze. This meant that I instead had to build OpenOCD from source, which causes some other headache. Long story short: libgpiod 2.x is not backward compatible with previous versions, and OpenOCD uses 1.x.

There is a github issue describing what to look for, and what the underlying problem is with using latest stable version of OpenOCD

So normally, you would start with installing OpenOCD

sudo dnf install openocd

Now instead we might as well clone the openocd fork made by the Raspberry pi foundation. When we build OpenOCD we need to disable some warnings, since the recent version of gcc on Fedora causes some build errors due to newer warnings. When building openocd you also need to make sure that some dependencies are available.

sudo dnf install libtool libusb1-devel hidapi-devel

Then clone, configure, compile and install. The steps below will install in $HOME/local

git clone --recursive openocd openocd-rbpi
cd openocd-rbpi
./bootstrap
CFLAGS=-Wnoerror ./configure --prefix=$HOME/local \
                             --program-suffix=-pico \
                             --enable-cmsis-dap \
                             --disable-linuxgpiod
make -j$(nproc)
make install

If you install OpenOCD using the options above you will have a local install called openocd-pico.

For this example we will build the blink example for the original pico. So using the steps from the previous post, build using the following commands:

cmake -DCMAKE_BUILD_TYPE=Debug -B build -DPICO_BOARD=pico
cmake --build build

The Debug part is important, as that allows gdb to read debug symbols.

Now you can flash the device using openocd using the following commands:

export OOCDS=$HOME/local/share/openocd/scripts
openocd-pico \
  -f $OODCS/interface/cmsis-dap.cfg \
  -f $OODCS/target/rp2040.cfg       \
  -c "adapter speed 5000"    \
  -c "program build/blink.elf verify reset exit"

If this worked, and your device started blinking the onboard LED, you should have everything working, and now it’s “only software”.

For getting things working with gdb you want to launch OpenOCD like this:

export OOCDS=$HOME/local/share/openocd/scripts
openocd-pico \
  -f $OODCS/interface/cmsis-dap.cfg \
  -f $OODCS/target/rp2040.cfg       \
  -c "adapter speed 5000"

You should have log output stating that OpenOCD is listening on port 3333. Launch gdb, load debug symbols, and connect to the debug server

gdb build/blink.elf

In gdb, connecto the server

target extended-remote :3333
break main
run

You should now be able to run gdb as normal. And you can again do the same flash operations by doing monitor commands

monitor program build/blink.elf verify reset exit