This project implements an application that synchronizes two machines using GPIO signals. This README describes how to configure the hardware and software in order to run gsync. For a more in depth discussion of the project, check out the GPIO Driven Synchronization blog post. For project documentation, see the section Building the Docs and More.

You can run gsync on any single board computer supporting Linux. We used two BeagleBone Black (BBB) computers as our test platform. The rest of this guide will assume you are using two BBBs. Below is a list of the hardware needed to run gsync:

• 2x BeagleBone Black
• Breadboard
• Breadboard Jumper Wires
• 2x 470 Ohm Through-hole Resistors
• 2x LED (optional)
• Oscilliscope (optional)

Below is the wiring configuration used during testing.

P9_15 and P9_23 are used as the GPIO TX/RX lines. Be sure to cross the GPIO lines as shown in the diagram. P9_2 is GND. If you choose to use different GPIOs, consult the header pin table to ensure you pick unallocated GPIOs that are muxed correctly!

To get the best synchronization results, you will want to configure your BBBs to run the latest Linux kernel with the PREEMPT_RT patch applied. The bbb_kernel_builder project can help you get up and running with a PREEMPT_RT patched kernel. See my Building and Deploying a Real-Time Kernel to the BeagleBone Black post for more info. For help and guidance in configuring the kernel for real-time, check out the section on kernel configuration in Real-Time Linux App Development.

There's a good number of tweaks needed to make an application successfully run in near real-time on a Linux system. The gsync source and launch scripts cover most of the bases. You will however want to make one additional system level tweak: disable /proc/sys/kernel/sched_rt_runtime_us. You can read about why this configuration is important in the section on scheduling policies in Real-Time Linux App Development. This setting needs to be configured after every boot. To do so, we can write a systemd service as described below.

1. Create the service file /etc/systemd/system/disable-sched-rt-runtime-us.service with the following contents:

[Unit]
Description=Disable RT process runtime limiting

[Service]
ExecStart=/bin/sh -c "echo -1 > /proc/sys/kernel/sched_rt_runtime_us"

[Install]
WantedBy=multi-user.target

2. Enable the service so that it is active on boot:

systemctl enable disable-sched-rt-runtime-us

3. Either reboot the board or start the service directly using systemctl:

systemctl start disable-sched-rt-runtime-us

gsync includes a build script that supports multiple build options. Run build.sh -h to see all the options. Unless you are building the project directly on the BBB we, recommend following the containerized build instructions outlined below to cross compile gsync for the BBB's armv7 architecture. To follow these instructions, you will need to install Docker on your development PC.

1. Build the cross compilation image:

cd docker && docker build . -t gsync:latest

2. Run the continerized_build.sh script to launch the container:

cd scripts/ && ./containerized_build.sh

3. From within the container shell, run the build.sh project build script with the cross compilation flag set:

./build.sh -c

4. Verify the gtimer and gsync executables were installed on the host PC under /path/to/gsync/bin/.

There are many ways to get files from a host PC to the BBB. The steps below assume you will be moving the necessary binaries and scripts using SSH. To deploy the project to BBB follow, these steps:

1. Copy the gtimer and gsync binaries from bin to a location on the BBB:

scp bin/* [email protected]:/root/

2. Copy the run.sh and kill.sh scripts to the same location on the BBB as in (1):

scp scripts/run.sh [email protected]:/root/
scp scripts/kill.sh [email protected]:/root/

3. Repeat steps (1) - (2) for BBB number 2.

Assuming you have followed the steps in all the preceding sections, kicking off synchronization is as simple as executing the run.sh script previously installed on each of the BBBs. To kill the application, run the kill.sh script.

By default, gsync runs at a frequency of 1 Hz with a coupling constant of 0.5. In this configuration, you can verify the boards have synced when the LEDs in the circuit flash in unison. If you want to run at higher frequencies or tune the coupling constant, edit the FREQ_HZ and COUPLING_CONST variables in the run.sh script. Be sure all changes are applied to both boards!

At higher frequencies, you will want to use an oscilliscope to measure the time delay between the start of the gsync program on each board. Replacing the LEDs with jumper wires connected to oscilliscope probes will allow you to take this measurement.

This project uses Doxygen for source documentation. You can build the project docs by running build.sh -d. HTML documentation will be output to docs/gsync.

If you're interested in modifying the source, looking at the design artifacts first might be helpful. Included in the docs/images folder are pictures of the circuit as well as diagrams showing the high level SW design and gtimer/gsync state machines.