Pin Modes

To setup digital pins for one of the various input or output modes you can use the pinMode(pin, mode) function which takes the pin number (as given in the pinout card) as first and the mode as second parameter. The following modes are available on all Teensy models:

Mode	Description
OUTPUT	Low impedance output mode. Pin can be set or cleared using digitalWriteFast
OUTPUT_OPENDRAIN	Operate the pin in open drain mode. Please note this Teensy specific quirk of open-drain mode.
INPUT	High impedance input mode. Pin can be read with digitalReadFast. This mode is set per default. Reading an open pin gives undefined values.
INPUT_PULLUP	Adds a pull up resistor (~10k?) to the pin. Open pin reads HIGH
INPUT_PULLDOWN	Adds a pull down resistor (~10k?) to the pin. Open pin reads LOW
INPUT_DISABLE	Disables the pin

Here an example demonstrating how to read and write digital pins. It also shows the effect of PULLUP and PULLDOWN input modes on open pins. (Examples can be downloaded here)

constexpr int inputPin = 0;

void setup()
{
    while (!Serial && millis() < 1000) {}

    pinMode(LED_BUILTIN, OUTPUT);

    pinMode(inputPin, INPUT_PULLUP);
    Serial.println("Using INPUT_PULLUP:");
    printDigitalVal(inputPin, digitalReadFast(inputPin));

    pinMode(inputPin, INPUT_PULLDOWN);
    Serial.println("\nUsing INPUT_PULLDOWN:");
    printDigitalVal(inputPin, digitalReadFast(inputPin));
}

void loop()
{
    digitalWriteFast(LED_BUILTIN, !digitalReadFast(LED_BUILTIN)); // toggle pin
    delay(150);
}

// Helpers ----------------------------------------------------------

void printDigitalVal(int pin, int val)
{
    Serial.printf("Pin %d value: %s\n", pin, val == 0 ? "LOW" : "HIGH");
}