pinMode() configures the specified pin to behave either as an input (with or without an internal weak pull-up or pull-down resistor), or an output.

void pinMode(uint16_t pin, PinMode mode)

int button = D0;                      // button is connected to D0
int LED = D1;                         // LED is connected to D1

void setup()
  pinMode(LED, OUTPUT);               // sets pin as output
  pinMode(button, INPUT_PULLDOWN);    // sets pin as input

void loop()
  // blink the LED as long as the button is pressed
  while(digitalRead(button) == HIGH) {
    digitalWrite(LED, HIGH);          // sets the LED on
    delay(200);                       // waits for 200mS
    digitalWrite(LED, LOW);           // sets the LED off
    delay(200);                       // waits for 200mS

pinMode() takes two arguments:

  • pin: the pin you want to set the mode of (A0, A1, D0, D1, TX, RX, etc.). The type pin_t can be used instead of uint16_t to make it more obvious that the code accepts a pin number in your code.

  • mode: the mode to set to pin to:

    • INPUT digital input (the default at power-up)
    • INPUT_PULLUP digital input with a pull-up resistor to 3V3
    • INPUT_PULLDOWN digital input with a pull-down to GND
    • OUTPUT an output (push-pull)
    • OUTPUT_OPEN_DRAIN an open-drain or open-collector output. HIGH (1) leaves the output in high impedance state, LOW (0) pulls the output low. Typically used with an external pull-up resistor to allow any of multiple devices to set the value low safely.

You do not need to set the pinMode() to read an analog value using analogRead as the pin will automatically be set to the correct mode when analogRead is called.

When porting code from Arudino, pin numbers are numbered (0, 1, 2, ...) in Arduino code. Pin D0 has a value of 0, but it's best to use Particle pin names like D0 instead of just 0. This is especially true as the numeric value of A0 varies depending on the device and how many digital pins it has. For example, on the Argon, A0 is 19 but on the Photon it's 10.

Gen 3 Devices:

  • Make sure the signal does not exceed 3.3V. Gen 3 devices (Tracker SoM as well as Argon, Boron, Xenon, and the B Series SoM) are not 5V tolerant in any mode (digital or analog).

  • INPUT_PULLUP and INPUT_PULLDOWN are approximately 13K on Gen 3 devices.

If you are using the Particle Ethernet FeatherWing you cannot use the pins for GPIO as they are used for the Ethernet interface:

Argon, Boron, Xenon B Series SoM Ethernet FeatherWing Pin

When using the FeatherWing Gen 3 devices (Argon, Boron, Xenon), pins D3, D4, and D5 are reserved for Ethernet control pins (reset, interrupt, and chip select).

When using Ethernet with the Boron SoM, pins A7, D22, and D8 are reserved for the Ethernet control pins (reset, interrupt, and chip select).

By default, on the B Series SoM, the Tinker application firmware enables the use of the bq24195 PMIC and MAX17043 fuel gauge. This in turn uses I2C (D0 and D1) and pin A6 (PM_INT). If you are not using the PMIC and fuel gauge and with to use these pins for other purposes, be sure to disable system power configuration. This setting is persistent, so you may want to disable it with your manufacturing firmware only.

B Series SoM Power Manager Usage
A6 PM_INT (power manager interrupt)

Gen 2 Devices:

  • When using INPUT_PULLUP or INPUT_PULLDOWN make sure a high level signal does not exceed 3.3V.
  • INPUT_PULLUP does not work as expected on TX on the P1, Electron, and E Series and should not be used.
  • INPUT_PULLDOWN does not work as expected on D0 and D1 on the P1 because the P1 module has hardware pull-up resistors on these pins.
  • INPUT_PULLUP and INPUT_PULLDOWN are approximately 40K on Gen 2 devices
  • On the P1, D0 and D1 have 2.1K hardware pull-up resistors to 3V3.

On Gen 2 devices (Photon, P1, Electron, and E Series), GPIO pins are 5V tolerant if all of these conditions are met:

  • Digital input mode (INPUT) (the ADC is not 5V tolerant)
  • Not using INPUT_PULLDOWN or INPUT_PULLUP (internal pull is not 5V tolerant)
  • Not using pins A3 or A6 (the DAC pins are not 5V tolerant, even in INPUT mode)
  • Not using pins D0 and D1 on the P1 only as there is a pull-up to 3V3 on the P1 module only

Also beware when using pins D3, D5, D6, and D7 as OUTPUT controlling external devices on Gen 2 devices. After reset, these pins will be briefly taken over for JTAG/SWD, before being restored to the default high-impedance INPUT state during boot.

  • D3, D5, and D7 are pulled high with a pull-up
  • D6 is pulled low with a pull-down
  • D4 is left floating

The brief change in state (especially when connected to a MOSFET that can be triggered by the pull-up or pull-down) may cause issues when using these pins in certain circuits. You can see this with the D7 blue LED which will blink dimly and briefly at boot.