If you enable its internal 20k pull-up resistor, it will hang at around 1.7V instead of the expected 5V because the onboard LED and series resistor pull the voltage level down, meaning it always returns LOW. NOTE: Digital pin 13 is harder to use as a digital input than the other digital pins because it has an LED and resistor attached to it that's soldered to the board on most boards. Prior to Arduino 1.0.1, it was possible to configure the internal pull-ups in the following manner:Ģ digitalWrite ( pin, HIGH ) // turn on pullup resistors This works in the other direction as well, and an output pin that is left in a HIGH state will have the pullup resistors set if switched to an input with pinMode(). Consequently, a pin that is configured to have pullup resistors turned on when the pin is an INPUT, will have the pin configured as HIGH if the pin is then switched to an OUTPUT with pinMode(). The pullup resistors are controlled by the same registers (internal chip memory locations) that control whether a pin is HIGH or LOW. If LEDs in a project seem to be working, but very dimly, this is likely what is going on. The pullup resistors provide enough current to dimly light an LED connected to a pin that has been configured as an input. In the case of a simple switch, this causes the pin to read HIGH when the switch is open, and LOW when the switch is pressed. When connecting a sensor to a pin configured with INPUT_PULLUP, the other end should be connected to ground. For the exact value, consult the datasheet of the microcontroller on your board. On the Arduino Due, it is between 50kΩ and 150kΩ. On most AVR-based boards, the value is guaranteed to be between 20kΩ and 50kΩ. The value of this pullup depends on the microcontroller used. This effectively inverts the behavior of the INPUT mode, where HIGH means the sensor is off, and LOW means the sensor is on. These built-in pullup resistors are accessed by setting the pinMode() as INPUT_PULLUP. There are 20K pullup resistors built into the Atmega chip that can be accessed from software. Properties of Pins Configured as INPUT_PULLUP A 10K resistor is a good value for a pullup or pulldown resistor. This can be done by adding a pullup resistor (to +5V), or a pulldown resistor (resistor to ground) on the input. Often it is useful to steer an input pin to a known state if no input is present. Pullup Resistors with pins configured as INPUT This also means however, that pins configured as pinMode(pin, INPUT) with nothing connected to them, or with wires connected to them that are not connected to other circuits, will report seemingly random changes in pin state, picking up electrical noise from the environment, or capacitively coupling the state of a nearby pin. This means that it takes very little current to move the input pin from one state to another, and can make the pins useful for such tasks as implementing a capacitive touch sensor, reading an LED as a photodiode, or reading an analog sensor with a scheme such as RCTime. Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin. Pins configured this way are said to be in a high-impedance state. Properties of Pins Configured as INPUTĪrduino (Atmega) pins default to inputs, so they don't need to be explicitly declared as inputs with pinMode() when you're using them as inputs. While the title of this document refers to digital pins, it is important to note that vast majority of Arduino (Atmega) analog pins, may be configured, and used, in exactly the same manner as digital pins. This document explains the functioning of the pins in those modes. The pins on the Arduino can be configured as either inputs or outputs.
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