Scheduling Functions at Custom Frequencies

TimedRobot’s addPeriodic() method allows one to run custom methods at a rate faster than the default TimedRobot periodic update rate (20 ms). Previously, teams had to make a Notifier to run feedback controllers more often than the TimedRobot loop period of 20 ms (running TimedRobot more often than this is not advised). Now, users can run feedback controllers more often than the main robot loop, but synchronously with the TimedRobot periodic functions, eliminating potential thread safety issues.

The addPeriodic() (Java) / AddPeriodic() (C++) method takes in a lambda (the function to run), along with the requested period and an optional offset from the common starting time. The optional third argument is useful for scheduling a function in a different timeslot relative to the other TimedRobot periodic methods.

Note

The units for the period and offset are seconds in Java. In C++, the units library can be used to specify a period and offset in any time unit.

Java

public class Robot extends TimedRobot {
    private Joystick m_joystick = new Joystick(0);
    private Encoder m_encoder = new Encoder(1, 2);
    private Spark m_motor = new Spark(1);
    private PIDController m_controller = new PIDController(1.0, 0.0, 0.5, 0.01);
    public Robot() {
        addPeriodic(() -> {
            m_motor.set(m_controller.calculate(m_encoder.getRate()));
        }, 0.01, 0.005);
    }
    @Override
    public teleopPeriodic() {
        if (m_joystick.getRawButtonPressed(1)) {
            if (m_controller.getSetpoint() == 0.0) {
                m_controller.setSetpoint(30.0);
            } else {
                m_controller.setSetpoint(0.0);
            }
        }
    }
}

C++ (Header)

class Robot : public frc::TimedRobot {
 private:
  frc::Joystick m_joystick{0};
  frc::Encoder m_encoder{1, 2};
  frc::Spark m_motor{1};
  frc::PIDController m_controller{1.0, 0.0, 0.5, 10_ms};
  Robot();
  void TeleopPeriodic() override;
};

C++ (Source)

void Robot::Robot() {
  AddPeriodic([&] {
    m_motor.Set(m_controller.Calculate(m_encoder.GetRate()));
  }, 10_ms, 5_ms);
}
void Robot::TeleopPeriodic() {
  if (m_joystick.GetRawButtonPressed(1)) {
    if (m_controller.GetSetpoint() == 0.0) {
      m_controller.SetSetpoint(30.0);
    } else {
      m_controller.SetSetpoint(0.0);
    }
  }
}

The teleopPeriodic() method in this example runs every 20 ms, and the controller update is run every 10 ms with an offset of 5 ms from when teleopPeriodic() runs so that their timeslots don’t conflict (e.g., teleopPeriodic() runs at 0 ms, 20 ms, 40 ms, etc.; the controller runs at 5 ms, 15 ms, 25 ms, etc.).