Arduino pin pullup

Digital Pins

The pins on the Arduino can be configured as either inputs or outputs. This document explains the functioning of the pins in those modes. 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.

Properties of Pins Configured as INPUT

Arduino (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. Pins configured this way are said to be in a high-impedance state. 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. 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.

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.

Pullup Resistors with pins configured as INPUT

Often it is useful to steer an input pin to a known state if no input is present. This can be done by adding a pullup resistor (to +5V), or a pulldown resistor (resistor to ground) on the input. A 10K resistor is a good value for a pullup or pulldown resistor.

Properties of Pins Configured as INPUT_PULLUP

There are 20K pullup resistors built into the Atmega chip that can be accessed from software. These built-in pullup resistors are accessed by setting the pinMode() as INPUT_PULLUP. This effectively inverts the behavior of the INPUT mode, where HIGH means the sensor is off, and LOW means the sensor is on.

The value of this pullup depends on the microcontroller used. On most AVR-based boards, the value is guaranteed to be between 20kО© and 50kО©. On the Arduino Due, it is between 50kО© and 150kО©. For the exact value, consult the datasheet of the microcontroller on your board.

When connecting a sensor to a pin configured with INPUT_PULLUP, the other end should be connected to ground. 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.

The pullup resistors provide enough current to dimly light an LED connected to a pin that has been configured as an input. If LEDs in a project seem to be working, but very dimly, this is likely what is going on.

The pullup resistors are controlled by the same registers (internal chip memory locations) that control whether a pin is HIGH or LOW. 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(). 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().

Prior to Arduino 1.0.1, it was possible to configure the internal pull-ups in the following manner:

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. 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. If you must use pin 13 as a digital input, set its pinMode() to INPUT and use an external pull down resistor.

Properties of Pins Configured as OUTPUT

Pins configured as OUTPUT with pinMode() are said to be in a low-impedance state. This means that they can provide a substantial amount of current to other circuits. Atmega pins can source (provide positive current) or sink (provide negative current) up to 40 mA (milliamps) of current to other devices/circuits. This is enough current to brightly light up an LED (don’t forget the series resistor), or run many sensors, for example, but not enough current to run most relays, solenoids, or motors.

Short circuits on Arduino pins, or attempting to run high current devices from them, can damage or destroy the output transistors in the pin, or damage the entire Atmega chip. Often this will result in a «dead» pin in the microcontroller but the remaining chip will still function adequately. For this reason it is a good idea to connect OUTPUT pins to other devices with 470О© or 1k resistors, unless maximum current draw from the pins is required for a particular application.

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constants

Description

Constants are predefined expressions in the Arduino language. They are used to make the programs easier to read. We classify constants in groups:

Defining Logical Levels: true and false (Boolean Constants)

There are two constants used to represent truth and falsity in the Arduino language: true , and false .

false

false is the easier of the two to define. false is defined as 0 (zero).

true is often said to be defined as 1, which is correct, but true has a wider definition. Any integer which is non-zero is true, in a Boolean sense. So -1, 2 and -200 are all defined as true, too, in a Boolean sense.

Note that the true and false constants are typed in lowercase unlike HIGH , LOW , INPUT , and OUTPUT .

Defining Pin Levels: HIGH and LOW

When reading or writing to a digital pin there are only two possible values a pin can take/be-set-to: HIGH and LOW .

The meaning of HIGH (in reference to a pin) is somewhat different depending on whether a pin is set to an INPUT or OUTPUT . When a pin is configured as an INPUT with pinMode() , and read with digitalRead() , the Arduino (ATmega) will report HIGH if:

a voltage greater than 3.0V is present at the pin (5V boards)

a voltage greater than 2.0V is present at the pin (3.3V boards)

A pin may also be configured as an INPUT with pinMode() , and subsequently made HIGH with digitalWrite() . This will enable the internal 20K pullup resistors, which will pull up the input pin to a HIGH reading unless it is pulled LOW by external circuitry. This can be done alternatively by passing INPUT_PULLUP as argument to the pinMode() function, as explained in more detail in the section «Defining Digital Pins modes: INPUT, INPUT_PULLUP, and OUTPUT» further below.

When a pin is configured to OUTPUT with pinMode() , and set to HIGH with digitalWrite() , the pin is at:

5 volts (5V boards)

3.3 volts (3.3V boards)

In this state it can source current, e.g. light an LED that is connected through a series resistor to ground.

The meaning of LOW also has a different meaning depending on whether a pin is set to INPUT or OUTPUT . When a pin is configured as an INPUT with pinMode() , and read with digitalRead() , the Arduino (ATmega) will report LOW if:

a voltage less than 1.5V is present at the pin (5V boards)

a voltage less than 1.0V (Approx) is present at the pin (3.3V boards)

When a pin is configured to OUTPUT with pinMode() , and set to LOW with digitalWrite() , the pin is at 0 volts (both 5V and 3.3V boards). In this state it can sink current, e.g. light an LED that is connected through a series resistor to +5 volts (or +3.3 volts).

Defining Digital Pins modes: INPUT, INPUT_PULLUP, and OUTPUT

Digital pins can be used as INPUT , INPUT_PULLUP , or OUTPUT . Changing a pin with pinMode() changes the electrical behavior of the pin.

Pins Configured as INPUT

Arduino (ATmega) pins configured as INPUT with pinMode() are said to be in a high-impedance state. Pins configured as INPUT make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 Megohms in front of the pin. This makes them useful for reading a sensor.

If you have your pin configured as an INPUT , and are reading a switch, when the switch is in the open state the input pin will be «floating», resulting in unpredictable results. In order to assure a proper reading when the switch is open, a pull-up or pull-down resistor must be used. The purpose of this resistor is to pull the pin to a known state when the switch is open. A 10 K ohm resistor is usually chosen, as it is a low enough value to reliably prevent a floating input, and at the same time a high enough value to not draw too much current when the switch is closed. See the Digital Read Serial tutorial for more information.

If a pull-down resistor is used, the input pin will be LOW when the switch is open and HIGH when the switch is closed.

If a pull-up resistor is used, the input pin will be HIGH when the switch is open and LOW when the switch is closed.

Pins Configured as INPUT_PULLUP

The ATmega microcontroller on the Arduino has internal pull-up resistors (resistors that connect to power internally) that you can access. If you prefer to use these instead of external pull-up resistors, you can use the INPUT_PULLUP argument in pinMode() .

See the Input Pullup Serial tutorial for an example of this in use.

Pins configured as inputs with either INPUT or INPUT_PULLUP can be damaged or destroyed if they are connected to voltages below ground (negative voltages) or above the positive power rail (5V or 3V).

Pins Configured as OUTPUT

Pins configured as OUTPUT with pinMode() are said to be in a low-impedance state. This means that they can provide a substantial amount of current to other circuits. ATmega pins can source (provide current) or sink (absorb current) up to 40 mA (milliamps) of current to other devices/circuits. This makes them useful for powering LEDs because LEDs typically use less than 40 mA. Loads greater than 40 mA (e.g. motors) will require a transistor or other interface circuitry.

Pins configured as outputs can be damaged or destroyed if they are connected to either the ground or positive power rails.

Defining built-ins: LED_BUILTIN

Most Arduino boards have a pin connected to an on-board LED in series with a resistor. The constant LED_BUILTIN is the number of the pin to which the on-board LED is connected. Most boards have this LED connected to digital pin 13.

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Arduino INPUT_PULLUP Explained (pinMode)

What is the Arduino INPUT_PULLUP option for the pinMode function?

In this tutorial I will show you different examples, using an Arduino board and a simple push button, to explain what INPUT_PULLUP does, and how to use it in your Arduino programs.

And… Let’s get started!

Table of Contents

Quick recap about pinMode

With Arduino you can use digital pins to either read (binary) data from a sensor, or write (binary) data to an actuator.

It’s quite simple. Either you set the pin as:

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  • OUTPUT: this is to write data to an actuator, for example an LED.
  • INPUT: in this case you’re going to read data from the sensor. The value you’ll get will be HIGH or LOW (binary).

And… There’s also a 3rd option: INPUT_PULLUP. This option is the same as INPUT (you read data from the sensor), but in addition to that, an internal pull up resistor – between 20k and 50k Ohm – is enabled, to keep the signal HIGH by default.

What does that mean?

Well, let’s see with 3 different circuits doing the same thing: reading data from a push button.

The problem: floating pin

Let’s consider this circuit.

This is quite simple: you plug one leg of the push button to the ground (GND), and another one – on the other side of the button – to a digital pin.

Let’s write a simple code to print the push button’s value on the Serial Monitor.

Nothing fancy here, the 2 importants parts are:

  • pinMode(BUTTON_PIN, INPUT); : we set pin 4 to INPUT so we can read data from the button.
  • digitalRead(BUTTON_PIN) : this will give us the current state of the button, either LOW or HIGH.

If we run this program, and open the Serial Plotter (Tools > Serial Plotter, or CTRL+SHIFT+L), here is what we get, without pressing the button.

When we press the button, the value is always LOW, but when we release it it’s quite random: sometimes HIGH, sometimes LOW, and it moves a lot.

We see this because the voltage for the button is floating between 0 and 5V.

If the voltage is below a certain amount of V, the Arduino will read LOW. And if it is above a certain amount of V, the Arduino will read HIGH. As there is no internal or external voltage reference for the push button, the value will oscillate a lot in a random way.

And as you can foresee, we can’t rely on this data to take decisions inside our Arduino program.

What we need to do is to “force” the default state (button not pushed) to be close to HIGH or LOW, which will make it quite stable. Then, when we press the button the state will simply go to the opposite of the default state.

Using Arduino INPUT_PULLUP

Let’s use the exact same circuit, but this time with INPUT_PULLUP instead of INPUT for the pinMode function.

If you run this code and open the Serial Plotter, you’ll see that the default value is 1 (HIGH). When you press the button the state directly goes to 0 (LOW) and comes back to HIGH when you release the button.

Well now it’s much better. Problem solved!

When you set the mode to INPUT_PULLUP, an internal resistor – inside the Arduino board – will be set between the digital pin 4 and VCC (5V). This resistor – value estimated between 20k and 50k Ohm – will make sure the state stays HIGH. When you press the button, the states becomes LOW.

Using an external resistor instead of Arduino INPUT_PULLUP

Pull up resistor

Instead of using the internal pull up resistor from your Arduino board, you could decide to create the circuit yourself and add an external pull up resistor.

Your circuit will look like this.

Here I have simply added a 10k Ohm resistor between one leg of the button (same side as the data side – digital pin 4) and VCC (5V).

Now, with this circuit you don’t need to enable the internal pull up anymore. So, in your program use pinMode(BUTTON_PIN, INPUT); instead of pinMode(BUTTON_PIN, INPUT_PULLUP); .

When you run the program you will have the same result: the default state for the button is HIGH, and when you press it, its states goes to LOW.

Pull down resistor

This is another option you can choose, which is also a quite popular one: add a pull down resistor. Thus, the default button’s state will be LOW, and when you press it it will become HIGH.

Contrary to the pull up resistor, you can’t set this up with just the code, you’ll have to use an external resistor.

Here’s the circuit.

The circuit is quite similar to the previous one, but pay attention to the differences:

  • The 10k Ohm resistor is between one leg and the ground (GND).
  • The wire for digital pin 4 is on the same side as the ground.
  • The other side of the button is connected to VCC (5V) directly.

When you run the program using pinMode(BUTTON_PIN, INPUT); , you’ll get:

Great! Now the default value when the button is not pressed is LOW. And in this example when I pressed the button the state rose to HIGH.

Conclusion – Arduino INPUT_PULLUP recap

In this tutorial you’ve seen how to properly use pull up and pull down resistors for your Arduino sensors, and when to use the INPUT_PULLUP option for the pinMode function.

To recap, you have 3 choices, depending on the default state you want for the button:

  1. Add an external pull down resistor, so the default state is LOW.
  2. Add an external pull up resistor, so the default state is HIGH.
  3. Use the Arduino internal pull up resistor. The behavior will be the same as for option no 2.

There is no better or worse choice, it depends on the available hardware components you have and some requirements specific to your project. Also it’s a matter of preference: do you want the default state (when not pressed) to be LOW or HIGH? – knowing that this can easily be corrected on the software side.

The most important thing to pay attention to is not to have a floating state for any of your component: this will make any measurement wrong.

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