Digital Inputs - Software

Note

This section covers digital inputs in software. For a hardware guide to digital inputs, see Digital Inputs - Hardware.

The roboRIO’s FPGA supports up to 26 digital inputs. 10 of these are made available through the built-in DIO ports on the RIO itself, while the other 16 are available through the MXP breakout port.

Digital inputs read one of two states - “high” or “low.” By default, the built-in ports on the RIO will read “high” due to internal pull-up resistors (for more information, see Digital Inputs - Hardware). Accordingly, digital inputs are most-commonly used with switches of some sort. Support for this usage is provided through the DigitalInput class (Java, C++).

The DigitalInput class

A DigitalInput can be initialized as follows:

JAVA

// Initializes a DigitalInput on DIO 0
DigitalInput input = new DigitalInput(0);

C++

// Initializes a DigitalInput on DIO 0
frc::DigitalInput input{0};

Reading the value of the DigitalInput

The state of the DigitalInput can be polled with the get method:

JAVA

// Gets the value of the digital input.  Returns true if the circuit is open.
input.get();

C++

// Gets the value of the digital input.  Returns true if the circuit is open.
input.Get();

Creating a DigitalInput from an AnalogInput

Note

An AnalogTrigger constructed with a port number argument can share that analog port with a separate AnalogInput, but two AnalogInput objects may not share the same port.

Sometimes, it is desirable to use an analog input as a digital input. This can be easily achieved using the AnalogTrigger class (Java, C++).

An AnalogTrigger may be initialized as follows. As with AnalogPotentiometer, an AnalogInput may be passed explicitly if the user wishes to customize the sampling settings:

JAVA

// Initializes an AnalogTrigger on port 0
AnalogTrigger trigger0 = new AnalogTrigger(0);

// Initializes an AnalogInput on port 1 and enables 2-bit oversampling
AnalogInput input = new AnalogInput(1);
input.setAverageBits(2);

// Initializes an AnalogTrigger using the above input
AnalogTrigger trigger1 = new AnalogTrigger(input);

C++

// Initializes an AnalogTrigger on port 0
frc::AnalogTrigger trigger0{0};

// Initializes an AnalogInput on port 1 and enables 2-bit oversampling
frc::AnalogInput input{1};
input.SetAverageBits(2);

// Initializes an AnalogTrigger using the above input
frc::AnalogTrigger trigger1{input};

Setting the trigger points

Note

For details on the scaling of “raw” AnalogInput values, see Analog Inputs - Software.

To convert the analog signal to a digital one, it is necessary to specify at what values the trigger will enable and disable. These values may be different to avoid “dithering” around the transition point:

JAVA

// Sets the trigger to enable at a raw value of 3500, and disable at a value of 1000
trigger.setLimitsRaw(1000, 3500);

// Sets the trigger to enable at a voltage of 4 volts, and disable at a value of 1.5 volts
trigger.setLimitsVoltage(1.5, 4);

C++

// Sets the trigger to enable at a raw value of 3500, and disable at a value of 1000
trigger.SetLimitsRaw(1000, 3500);

// Sets the trigger to enable at a voltage of 4 volts, and disable at a value of 1.5 volts
trigger.SetLimitsVoltage(1.5, 4);

Using DigitalInputs in code

As almost all switches on the robot will be used through a DigitalInput. This class is extremely important for effective robot control.

Limiting the motion of a mechanism

Nearly all motorized mechanisms (such as arms and elevators) in FRC® should be given some form of “limit switch” to prevent them from damaging themselves at the end of their range of motions. A short example is given below:

JAVA

Spark spark = new Spark(0);

// Limit switch on DIO 2
DigitalInput limit = new DigitalInput(2);

public void autonomousPeriodic() {
    // Runs the motor forwards at half speed, unless the limit is pressed
    if(!limit.get()) {
        spark.set(.5);
    } else {
        spark.set(0);
    }
}

C++

// Motor for the mechanism
frc::Spark spark{0};

// Limit switch on DIO 2
frc::DigitalInput limit{2};

void AutonomousPeriodic() {
    // Runs the motor forwards at half speed, unless the limit is pressed
    if(!limit.Get()) {
        spark.Set(.5);
    } else {
        spark.Set(0);
    }
}

Homing a mechanism

Limit switches are very important for being able to “home” a mechanism with an encoder. For an example of this, see Homing a Mechanism.