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

C++

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

// 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

C++

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

// 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

C++

// 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);

// 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

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);

// 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

C++

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);
    }
}

// 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 an encodered mechanism

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