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.. _ar_digital_pullup:

1.9 Digital Input Pull-Up
=========================

Overview
---------

When using some switch input devices, some pull-up or pull-down
resistors are often used to keep the level of corresponding pins at
certain value on the condition that the device is not working. Such as
in 1.4 Digital Read, a 10k resistor is used to make the pin be connected
to GND under the condition that the button is not pressed down. If we
have used a lot of switch input components and want to simplify the
circuit, we can set the pin mode to「INPUT_PULLUP」in the code so that
the pin reads the high level in the suspended state.

Components Required
-------------------

.. image:: img/list_1.9.png

* :ref:`cpn_mega2560`
* :ref:`cpn_breadboard`
* :ref:`cpn_wires`
* :ref:`cpn_button`



Fritzing Circuit
----------------

In this example, we use pin 2 to read the signal of button. The internal
pull-up in pin 2 is valid, so if the button isn’t pressed, HIGH is read
in pin 2; when the button is pressed, LOW is read.

.. image:: img/image53.png


Schematic Diagram
-----------------

.. image:: img/image411.png


Code
----

.. note::

    * You can open the file ``1.9_digitalInputPullup.ino`` under the path of ``sunfounder_vincent_kit_for_arduino\code\1.9_digitalInputPullup`` directly.
    * Or copy this code into Arduino IDE.




.. raw:: html

    <iframe src=https://create.arduino.cc/editor/sunfounder01/223d1c70-2644-4b9c-9f69-74e18394631f/preview?embed style="height:510px;width:100%;margin:10px 0" frameborder=0></iframe>

After the codes are uploaded to the Mega2560 board, you can open the serial port monitor to view the read values of the pin. When the Button is pressed, the serial port monitor will display "0", and the "1" will be displayed when the button is released.

Code Analysis
-------------

Run the serial communication in setup() and set the data rate to 9600.

.. code-block:: arduino

    Serial.begin(9600);

Configure pin 2 as an input and enable the internal pull-up resistor.

.. code-block:: arduino

    pinMode(2, INPUT_PULLUP);

Read the level state from the digital pin 2 by using digitalRead() statement in loop() and declare a variable to store it.

.. code-block:: arduino

    int buttonState = digitalRead(2);

Print the values stored by variables on the serial port monitor.

.. code-block:: arduino

    Serial.println(buttonState);

Phenomenon Picture
------------------

.. image:: img/image55.jpeg