This lesson reads the Start and Stop buttons on the control panel. The code counts down from 10 to 0 when the Start button is pressed, and can be stopped at any time with the Stop button, which resets the countdown back to 10 seconds.

Constant.h

#pragma once
#define _USE_MATH_DEFINES
#include <math.h>
// Defines a namespace to hold constant values
namespace constant
{
  static constexpr int START_BUTTON = 8;
  static constexpr int STOP_BUTTON = 11;
}

Robot.h

#pragma once
#include <frc/TimedRobot.h>
#include <frc2/command/Command.h>
#include <frc/DigitalInput.h>
#include <frc/Timer.h>
#include "RobotContainer.h"
#include "Constants.h"
class Robot : public frc::TimedRobot {
public:
  void RobotInit() override;
  void RobotPeriodic() override;
  void DisabledInit() override;
  void DisabledPeriodic() override;
  void AutonomousInit() override;
  void AutonomousPeriodic() override;
  void TeleopInit() override;
  void TeleopPeriodic() override;
  void TestPeriodic() override;
private:
  frc::DigitalInput startButton{constant::START_BUTTON};
  frc::DigitalInput stopButton{constant::STOP_BUTTON};
  frc::Timer countdownTimer;
  bool countdownActive = false;
  bool startWasPressed = false;
  bool stopWasPressed = false;
  const double countdownLength = 10.0;
};

Robot.cpp

#include "Robot.h"
#include <frc/smartdashboard/SmartDashboard.h>
#include <cmath>
void Robot::RobotInit()
{
  countdownActive = false;
  countdownTimer.Stop();
  countdownTimer.Reset();
  frc::SmartDashboard::PutNumber("Countdown", 10);
  frc::SmartDashboard::PutString("Countdown Status", "Waiting");
}
void Robot::RobotPeriodic()
{
  bool startPressed = !startButton.Get();
  bool stopPressed = !stopButton.Get();
  if (startPressed && !startWasPressed && !countdownActive)
  {
    countdownActive = true;
    countdownTimer.Reset();
    countdownTimer.Start();
    frc::SmartDashboard::PutString("Countdown Status", "Running");
  }
  if (stopPressed && !stopWasPressed)
  {
    countdownActive = false;
    countdownTimer.Stop();
    countdownTimer.Reset();
    frc::SmartDashboard::PutNumber("Countdown", 10);
    frc::SmartDashboard::PutString("Countdown Status", "Stopped");
  }
  if (countdownActive)
  {
    double timeLeft = countdownLength - countdownTimer.Get();
    if (timeLeft > 0)
    {
      frc::SmartDashboard::PutNumber("Countdown", std::ceil(timeLeft));
    }
    else
    {
      countdownActive = false;
      countdownTimer.Stop();
      frc::SmartDashboard::PutNumber("Countdown", 0);
      frc::SmartDashboard::PutString("Countdown Status", "Finished");
    }
  }
  startWasPressed = startPressed;
  stopWasPressed = stopPressed;
}
void Robot::DisabledInit()
{
  countdownActive = false;
  countdownTimer.Stop();
  countdownTimer.Reset();
  frc::SmartDashboard::PutNumber("Countdown", 10);
  frc::SmartDashboard::PutString("Countdown Status", "Disabled");
}
void Robot::DisabledPeriodic() {}
void Robot::AutonomousInit()
{
  countdownActive = false;
  countdownTimer.Stop();
  countdownTimer.Reset();
  frc::SmartDashboard::PutNumber("Countdown", 10);
  frc::SmartDashboard::PutString("Countdown Status", "Ready");
}
void Robot::AutonomousPeriodic() {}
void Robot::TeleopInit()
{
  countdownActive = false;
  countdownTimer.Stop();
  countdownTimer.Reset();
  frc::SmartDashboard::PutNumber("Countdown", 10);
  frc::SmartDashboard::PutString("Countdown Status", "Ready");
}
void Robot::TeleopPeriodic() {}
void Robot::TestPeriodic() {}
#ifndef RUNNING_FRC_TESTS
int main() { return frc::StartRobot<Robot>(); }
#endif