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AUnit Tests

An adjustable, compact, event-driven button library for Arduino platforms.

This library provides classes which accept inputs from a mechanical button connected to a digital input pin on the Arduino. The library should be able to handle momentary buttons, maintained buttons, and switches, but it was designed primarily for momentary (aka push) buttons.

The library is named "AceButton" because:

  • many configurations of the button are adjustable, either at compile-time or run-time
  • the library is optimized to create compact objects which take up a minimal amount of static memory
  • the library detects changes in the button state and sends events to a user-defined EventHandler callback function

Most of the features of the library can be accessed through 2 classes, using either a callback function or an interface:

  • AceButton (class)
  • ButtonConfig (class)
  • EventHandler (typedef for callback function)
  • IEventHandler (interface)

The AceButton class contains the logic for debouncing and determining if a particular event has occurred.

The ButtonConfig class holds various timing parameters, the event handler, code for reading the button, and code for getting the internal clock.

The EventHandler is a user-defined callback function with a specific signature which is registered with the ButtonConfig object. When the library detects interesting events, the callback function is called by the library, allowing the client code to handle the event.

The IEventHandler is an interface (pure abstract class) that provides an alternative to the EventHandler. Instead of using a callback function, an object of type IEventHandler can be used to handle the button events.

The supported events are:

  • AceButton::kEventPressed
  • AceButton::kEventReleased
  • AceButton::kEventClicked
  • AceButton::kEventDoubleClicked
  • AceButton::kEventLongPressed
  • AceButton::kEventRepeatPressed
  • AceButton::kEventLongReleased (v1.8)
  • AceButton::kEventHeartBeat (v1.10)

The basic ButtonConfig class assumes that each button is connected to a single digital input pin. In some situations, the number of buttons that we want is greater than the number of input pins available. This library provides 2 subclasses of ButtonConfig which may be useful:

  • EncodedButtonConfig
    • Supports binary encoded buttons, to read 2^N - 1 buttons using N pins (e.g. 7 buttons using 3 digital pins).
  • LadderButtonConfig
    • Supports 1-8 buttons (maybe more) on a single analog pin through a resistor ladder. The analogRead() method is used to read the different voltage levels corresponding to each button.

Both EncodedButtonConfig and LadderButtonConfig support all events listed above (e.g. kEventClicked and kEventDoubleClicked).

Version: 1.10.1 (2023-05-25)


Table of Contents


Here are the high-level features of the AceButton library:

  • debounces the mechanical contact
  • supports both pull-up and pull-down wiring
  • event-driven through a user-defined EventHandler callback function
  • event-driven through an object-based IEventHandler (>= v1.6)
  • supports the following event types:
    • kEventPressed
    • kEventReleased
    • kEventClicked
    • kEventDoubleClicked
    • kEventLongPressed
    • kEventRepeatPressed
    • kEventLongReleased
    • kEventHeartBeat
  • adjustable configurations at runtime or compile-time
    • timing parameters
    • digitalRead() button read function can be overridden
    • millis() clock function can be overridden
  • small memory footprint
    • each AceButton consumes 17 bytes (8-bit) or 20 bytes (32-bit)
    • each ButtonConfig consumes 20 bytes (8-bit) or 24 bytes (32-bit)
    • one System ButtonConfig instance created automatically by the library
    • 970-2180 bytes of flash memory for the simple case of 1 AceButton and 1 ButtonConfig, depending on 8-bit or 32-bit processors
  • supports multiple buttons on shared pins using various circuits
  • only 13-15 microseconds (on 16MHz ATmega328P) per polling call to AceButton::check()
  • extensive testing
    • thoroughly unit tested using AUnit
    • Tier 1 support includes: Arduino AVR (UNO, Nano, Micro etc), SAMD21 (Seeed XIAO M0), STM32 (Blue Pill), SAMD51 (Adafruit ItsyBitsy M4), ESP8266, and ESP32

Compared to other Arduino button libraries, I think the unique or exceptional features of the AceButton library are:

  • many supported event types (e.g. LongPressed and RepeatPressed)
  • able to distinguish between Clicked and DoubleClicked
  • small memory usage
  • thorough unit testing
  • support for multiple buttons using Binary Encoding or a Resistor Ladder


Here is a simple program (see examples/HelloButton) which controls the builtin LED on the Arduino board using a momentary button connected to PIN 2.

#include <AceButton.h>
using namespace ace_button;

const int BUTTON_PIN = 2;
const int LED_ON = HIGH;
const int LED_OFF = LOW;

AceButton button(BUTTON_PIN);

void handleEvent(AceButton*, uint8_t, uint8_t);

void setup() {

void loop() {

void handleEvent(AceButton* /*button*/, uint8_t eventType,
    uint8_t /*buttonState*/) {
  switch (eventType) {
    case AceButton::kEventPressed:
      digitalWrite(LED_BUILTIN, LED_ON);
    case AceButton::kEventReleased:
      digitalWrite(LED_BUILTIN, LED_OFF);

(The button and buttonState parameters are commented out to avoid an unused parameter warning from the compiler. We can't remove the parameters completely because the method signature is defined by the EventHandler typedef.)


The latest stable release is available in the Arduino IDE Library Manager. Search for "AceButton". Click install.

The development version can be installed by cloning the GitHub repository (, checking out the develop branch, then manually copying over the contents to the ./libraries directory used by the Arduino IDE. (The result is a directory named ./libraries/AceButton.)

The master branch contains the tagged stable releases.

External Dependencies

The core of the library is self-contained and has no external dependencies.

The some programs in examples/ may depend on:

The unit tests under tests depend on:

Source Code

The source files are organized as follows:

  • src/AceButton.h - main header file
  • src/ace_button/ - all implementation files
  • src/ace_button/testing/ - internal testing files
  • tests/ - unit tests which require AUnit
  • examples/ - example sketches



The following example sketches are provided:

  • Basic Single Button
    • HelloButton
      • minimal program that reads a switch and control the built-in LED
    • SingleButton
      • single button wired with an internal pull-up resistor
    • SingleButtonPullDown
      • same as SingleButton but with an external pull-down resistor
    • SingleButtonUsingIEventHandler
      • same as SingleButton using an object-based IEventHandler
    • Stopwatch
      • measures the speed of AceButton:check() with a start/stop/reset
      • button uses kFeatureLongPress
  • Multiple Buttons
    • TwoButtonsUsingOneButtonConfig
      • two buttons using one ButtonConfig
    • TwoButtonsUsingTwoButtonConfigs
      • two buttons using two ButtonConfigs
    • ThreeButtonsUsingOneButtonConfig
      • three buttons using one ButtonConfig
      • used as a reference for ThreeButtonsUsingOneButtonConfigFast (below)
    • TunerButtons
      • implements 5 radio buttons (tune-up, tune-down, and 3 presets)
      • shows multiple ButtonConfig and EventHandler instances
      • shows an example of how to use getId()
      • uses kFeatureLongPress, kFeatureRepeatPress, kFeatureSuppressAfterLongPress, and kFeatureSuppressAfterRepeatPress
    • ArrayButtons
      • shows how to define an array of AceButton and initialize them using the init() method in a loop
    • SimultaneousButtons
      • detecting simultaneous Pressed and Released of 2 buttons using a custom IEventHandler
  • Distinguishing Click versus Double-Click
  • Distinguishing Pressed and LongPressed
  • CapacitiveButton
  • HeartBeat
    • demo of activating the new (v1.10) kEventHeartBeat feature, and using it to generate 2 custom events: kCustomEventLongPressed (similar to kEventLongPressed) and kCustomEventLongReleased (no built-in equivalent)
  • Binary Encoded Buttons
    • Encoded4To2Buttons
      • demo of Encoded4To2ButtonConfig class to decode M=3 buttons with N=2 pins
    • Encoded8To3Buttons
      • demo of Encoded8To3ButtonConfig class to decode M=7 buttons with N=3 pins
    • Encoded16To4Buttons
      • demo of general M-to-N EncodedButtonConfig class to handle M=15 buttons with N=4 pins
  • Resistor Ladder Buttons
    • LadderButtonCalibrator
      • print out the value returned by analogRead() for various buttons
      • useful to compare the expected values of the resistor ladder versus the actual values returned by the function
    • LadderButtons
      • demo of 4 buttons on a single analog pin using analogRead()
    • LadderButtonsTiny
      • 2 buttons on the RESET/A0 pin of an ATtiny85 microcontroller
      • avoids wasting the RESET pin, saving the other pins for other purposes
  • digitalWriteFast
  • Benchmarks
    • These are internal benchmark programs. They were not written as examples of how to use the library.
    • AutoBenchmark
      • generates the timing stats (min/average/max) for the AceButton::check() method for various types of events (idle, press/release, click, double-click, and long-press)
    • MemoryBenchmark
      • determines the amount of flash memory consumed by various objects and features of the library


There are 2 classes and one typedef that a user will normally interact with:

  • AceButton (class)
  • ButtonConfig (class)
  • EventHandler (typedef)

Advanced usage is supported by:

  • EncodedButtonConfig - binary encoded buttons supporting 2^N-1 buttons on N digital pins
  • LadderButtonConfig - resistor ladder buttons using analog pins
  • IEventHandler - use a callback object instead of a callback function

We explain how to use these below.

Include Header and Use Namespace

Only a single header file AceButton.h is required to use this library. To prevent name clashes with other libraries that the calling code may use, all classes are defined in the ace_button namespace. To use the code without prepending the ace_button:: prefix, use the using directive:

#include <AceButton.h>
using namespace ace_button;

If you are dependent on just AceButton, the following might be sufficient:

#include <AceButton.h>
using ace_button::AceButton;

Pin Wiring and Initialization

The ButtonConfig class supports the simplest wiring. Each button is connected to a single digital input pin, as shown below. In the example below, 3 buttons labeled S0, S1 and S2 are connected to digital input pins D2, D3, and D4:

Direct Digital

An Arduino microcontroller pin can be in an OUTPUT mode, an INPUT mode, or an INPUT_PULLUP mode. This mode is controlled by the pinMode() method.

By default upon boot, the pin is set to the INPUT mode. However, this INPUT mode puts the pin into a high impedance state, which means that if there is no wire connected to the pin, the voltage on the pin is indeterminate. When the input pin is read (using digitalRead()), the boolean value will be a random value. If you are using the pin in INPUT mode, you must connect an external pull-up resistor (connected to Vcc) or pull-down resistor (connected to ground) so that the voltage level of the pin is defined when there is nothing connected to the pin (i.e. when the button is not pressed).

The INPUT_PULLUP mode is a special INPUT mode which tells the microcontroller to connect an internal pull-up resistor to the pin. It is activated by calling pinMode(pin, INPUT_PULLUP) on the given pin. This mode is very convenient because it eliminates the external resistor, making the wiring simpler.

The 3 resistors Rc1, Rc2 and Rc3 are optional current limiting resistors. They help protect the microcontroller in the case of misconfiguration. If the pins are accidentally set to OUTPUT mode, then pressing one of the buttons would connect the output pin directly to ground, causing a large amount of current to flow that could permanently damage the microcontroller. The resistance value of 220 ohms (or maybe 330 ohms) is high enough to keep the current within safety limits, but low enough compared to the internal pullup resistor that it is able to pull the digital pin to a logical 0 level. These current limiting resistors are good safety measures, but I admit that I often get lazy and don't use them when doing quick experiments.

The AceButton library itself does not call the pinMode() function. The calling application is responsible for calling pinMode(). Normally, this happens in the global setup() method but the call can happen somewhere else if the application requires it. The reason for decoupling the hardware configuration from the AceButton library is mostly because the library does not actually care about the specific hardware wiring of the button. It does not care whether an external resistor is used, or the internal resistor is used. It only cares about whether the resistor is a pull-up or a pull-down.

See for additional information about the I/O pins on an Arduino.

AceButton Class

The AceButton class looks like this (not all public methods are shown):

namespace ace_button {

class AceButton {
    static const uint8_t kEventPressed = 0;
    static const uint8_t kEventReleased = 1;
    static const uint8_t kEventClicked = 2;
    static const uint8_t kEventDoubleClicked = 3;
    static const uint8_t kEventLongPressed = 4;
    static const uint8_t kEventRepeatPressed = 5;
    static const uint8_t kEventLongReleased = 6;
    static const uint8_t kEventHeartBeat = 7;

    static const uint8_t kButtonStateUnknown = 127;

    static __FlashStringHelper eventName(uint8_t e);

    explicit AceButton(uint8_t pin = 0, uint8_t defaultReleasedState = HIGH,
        uint8_t id = 0);
    explicit AceButton(ButtonConfig* buttonConfig, uint8_t pin = 0,
        uint8_t defaultReleasedState = HIGH, uint8_t id = 0);
    void init(uint8_t pin = 0, uint8_t defaultReleasedState = HIGH,
        uint8_t id = 0);
    void init(ButtonConfig* buttonConfig, uint8_t pin = 0,
        uint8_t defaultReleasedState = HIGH, uint8_t id = 0);

    ButtonConfig* getButtonConfig();
    void setButtonConfig(ButtonConfig* buttonConfig);
    void setEventHandler(ButtonConfig::EventHandler eventHandler);

    uint8_t getPin();
    uint8_t getDefaultReleasedState();
    uint8_t getId();

    void check();


Each physical button will be handled by an instance of AceButton. At a minimum, the instance needs to be told the pin number of the button. This can be done through the constructor:

const uint8_t BUTTON_PIN = 2;

AceButton button(BUTTON_PIN);

void setup() {

Or we can use the init() method in the setup():

AceButton button;

void setup() {

Both the constructor and the init() function take 3 optional parameters as shown above:

  • pin: the I/O pin number assigned to the button
  • defaultReleasedState: the logical value of the button when it is in its default "released" state (HIGH using a pull-up resistor, LOW for a pull-down resistor)
  • id: an optional, user-defined identifier for the button, for example, an index into an array with additional information

The pin must be defined either through the constructor or the init() method. But the other two parameters may be optional in many cases.

Sampling Rate

To read the state of the button, the AceButton::check() method should be called from the loop() method periodically. Roughly speaking, this should be about 4 times faster than the value of getDebounceDelay() so that the various event detection logic can work properly. For example, for the default debounce delay is 20 ms, AceButton::check() should be called every 5 ms. I have successfully experimented with using a sampling delay as large as 10 ms, but I recommend about 5 ms in most cases.

You could call the AceButton::check() method directly in the global loop() function like this:

void loop() {

This would sample the button as fast as possible on your particular microprocessor, perhaps as fast as 10,000 or 100,000 times a second, depending on the other code that is in the loop() function.

Most of the time, a high sampling rate is not a problem except for 2 things:

  • Calling the AceButton::check() has a small overhead and your processor could be doing other things during that time.
  • If you use Resistor Ladder Buttons described below, on an ESP8266, you will trigger a bug that causes the WiFi to disconnect if you sample the analogRead() function more than a 1000 times/second.

If you want to limit the sampling rate, see the example code in Rate Limit CheckButtons. The code relies on using a static variable to implement a non-blocking delay, like this:

AceButton button;

void checkButtons() {
  static uint16_t prev = millis();

  // DO NOT USE delay(5) to do this.
  // The (uint16_t) cast is required on 32-bit processors, harmless on 8-bit.
  uint16_t now = millis();
  if ((uint16_t) (now - prev) >= 5) {
    prev = now;

void loop() {

Compiler Error On Pin 0

If you attempt to use Pin 0 in the AceButton() constructor:

AceButton button(0);

you may encounter a compile-time error such as this:

error: call of overloaded 'AceButton(int)' is ambiguous

The solution is to explicitly cast the 0 to a uint8_t type, or to assign it explicitly to a uint8_t const, like this:

// Explicit cast
AceButton button((uint8_t) 0);

// Or assign to a const first.
static const uint8_t PIN = 0;
AceButton button(PIN);

See Issue #40 for details.

ButtonConfig Class

The core concept of the AceButton library is the separation of the button (AceButton) from its configuration (ButtonConfig).

  • The AceButton class has the logic for debouncing and detecting the various events (Pressed, Released, etc), and the various bookkeeping variables needed to implement the logic. These variables are associated with the specific instance of that AceButton.
  • The ButtonConfig class has the various timing parameters which control how much time is needed to detect certain events. This class also has the ability to override the default methods for reading the pin (readButton()) and the clock (getClock()). This ability allows unit tests to be written.

The class looks like this (not all public methods are shown):

namespace ace_button {

class ButtonConfig {
    static const uint16_t kDebounceDelay = 20;
    static const uint16_t kClickDelay = 200;
    static const uint16_t kDoubleClickDelay = 400;
    static const uint16_t kLongPressDelay = 1000;
    static const uint16_t kRepeatPressDelay = 1000;
    static const uint16_t kRepeatPressInterval = 200;
    static const uint16_t kHeartBeatInterval = 5000;

    typedef uint16_t FeatureFlagType;
    static const FeatureFlagType kFeatureClick = 0x01;
    static const FeatureFlagType kFeatureDoubleClick = 0x02;
    static const FeatureFlagType kFeatureLongPress = 0x04;
    static const FeatureFlagType kFeatureRepeatPress = 0x08;
    static const FeatureFlagType kFeatureSuppressAfterClick = 0x10;
    static const FeatureFlagType kFeatureSuppressAfterDoubleClick = 0x20;
    static const FeatureFlagType kFeatureSuppressAfterLongPress = 0x40;
    static const FeatureFlagType kFeatureSuppressAfterRepeatPress = 0x80;
    static const FeatureFlagType kFeatureSuppressClickBeforeDoubleClick = 0x100;
    static const FeatureFlagType kFeatureHeartBeat = 0x200;
    static const FeatureFlagType kFeatureSuppressAll = (
        | kFeatureSuppressAfterDoubleClick
        | kFeatureSuppressAfterLongPress
        | kFeatureSuppressAfterRepeatPress
        | kFeatureSuppressClickBeforeDoubleClick);

    typedef void (*EventHandler)(AceButton* button, uint8_t eventType,
        uint8_t buttonState);

    ButtonConfig() = default;

    uint16_t getDebounceDelay() const;
    uint16_t getClickDelay() const;
    uint16_t getDoubleClickDelay() const;
    uint16_t getLongPressDelay() const;
    uint16_t getRepeatPressDelay() const;
    uint16_t getRepeatPressInterval() const;
    uint16_t getHeartBeatInterval() const;

    void setDebounceDelay(uint16_t debounceDelay);
    void setClickDelay(uint16_t clickDelay);
    void setDoubleClickDelay(uint16_t doubleClickDelay);
    void setLongPressDelay(uint16_t longPressDelay);
    void setRepeatPressDelay(uint16_t repeatPressDelay);
    void setRepeatPressInterval(uint16_t repeatPressInterval);
    void setHeartBeatInterval(uint16_t heartBeatInterval);

    virtual unsigned long getClock();
    virtual int readButton(uint8_t pin);

    bool isFeature(FeatureFlagType features) const;
    void setFeature(FeatureFlagType features);
    void clearFeature(FeatureFlagType features);
    void resetFeatures();

    void setEventHandler(EventHandler eventHandler);
    void setIEventHandler(IEventHandler* eventHandler);

    static ButtonConfig* getSystemButtonConfig();


The ButtonConfig (or a customized subclass) can be created and assigned to one or more AceButton instances using dependency injection through the AceButton(ButtonConfig*) constructor. This constructor also accepts the same (pin, defaultReleasedState, id) parameters as init(pin, defaultReleasedState, id) method. Sometimes it's easier to set all the parameters in one place using the constructor. Other times, the parameters are not known until the AceButton::init() method can be called from the global setup() method.

const uint8_t PIN1 = 2;
const uint8_t PIN2 = 4;

ButtonConfig buttonConfig;
AceButton button1(&buttonConfig, PIN1);
AceButton button2(&buttonConfig, PIN2);

void setup() {
  pinMode(PIN1, INPUT_PULLUP);
  pinMode(PIN2, INPUT_PULLUP);

Another way to inject the ButtonConfig dependency is to use the AceButton::setButtonConfig() method but it is recommended that you use the constructor instead because the dependency is easier to follow.

System ButtonConfig

A single instance of ButtonConfig called the "System ButtonConfig" is automatically created by the library at startup. By default, all instances of AceButton are automatically assigned to this singleton instance. We explain in the Single Button Simplifications section below how this simplifies the code needed to handle a single button.

Configuring the EventHandler

The ButtonConfig class provides a number of methods which are mostly used internally by the AceButton class. The one method which is expected to be used by the calling client code is setEventHandler() which assigns the user-defined EventHandler callback function to the ButtonConfig instance. This is explained in more detail below in the EventHandler section below.

Timing Parameters

Here are the methods to retrieve the timing parameters:

  • uint16_t getDebounceDelay(); (default: 20 ms)
  • uint16_t getClickDelay(); (default: 200 ms)
  • uint16_t getDoubleClickDelay(); (default: 400 ms)
  • uint16_t getLongPressDelay(); (default: 1000 ms)
  • uint16_t getRepeatPressDelay(); (default: 1000 ms)
  • uint16_t getRepeatPressInterval(); (default: 200 ms)

The default values of each timing parameter can be changed at run-time using the following methods:

  • void setDebounceDelay(uint16_t debounceDelay);
  • void setClickDelay(uint16_t clickDelay);
  • void setDoubleClickDelay(uint16_t doubleClickDelay);
  • void setLongPressDelay(uint16_t longPressDelay);
  • void setRepeatPressDelay(uint16_t repeatPressDelay);
  • void setRepeatPressInterval(uint16_t repeatPressInterval);

Hardware Dependencies

The ButtonConfig class has 2 methods which provide hooks to its external hardware dependencies:

  • virtual unsigned long getClock();
  • virtual int readButton(uint8_t pin);

By default these are mapped to the underlying Arduino system functions respectively:

  • millis()
  • digitalRead()

Unit tests are possible because these methods are virtual and the hardware dependencies can be swapped out with fake ones.

Multiple ButtonConfig Instances

We have assumed that there is a 1-to-many relationship between a ButtonConfig and the AceButton. In other words, multiple buttons will normally be associated with a single configuration. Each AceButton has a pointer to an instance of ButtonConfig. So the cost of separating the ButtonConfig from AceButton is 2 bytes in each instance of AceButton. Note that this is equivalent to adding virtual methods to AceButton (which would add 2 bytes), so in terms of static RAM size, this is a wash.

The library is designed to handle multiple buttons, and it assumes that the buttons are normally grouped together into a handful of types. For example, consider the buttons of a car radio. It has several types of buttons:

  • the tuner buttons (2, up and down)
  • the preset buttons (6)
  • the AM/FM band button (1)

In this example, there are 9 buttons, but only 3 instances of ButtonConfig would be needed.

EventHandler Typedef

The event handler is a callback function that gets called when the AceButton class determines that an interesting event happened on the button. The advantage of this mechanism is that all the complicated logic of determining the various events happens inside the AceButton class, and the user will normally not need to worry about the details.

EventHandler Signature

The event handler is defined in the ButtonConfig class and has the following signature:

class ButtonConfig {
    typedef void (*EventHandler)(AceButton* button, uint8_t eventType,
        uint8_t buttonState);

The event handler is registered with the ButtonConfig object, not with the AceButton object, although the convenience method AceButton::setEventHandler() is provided as a pass-through to the underlying ButtonConfig (see the Single Button Simplifications section below):

ButtonConfig buttonConfig;

void handleEvent(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void setup() {

The motivation for this design is to save static memory. If multiple buttons are associated with a single ButtonConfig, then it is not necessary for every button of that type to hold the same pointer to the EventHandler function. It is only necessary to save that information once, in the ButtonConfig object.

Pro Tip 1: Comment out the unused parameter(s) in the handleEvent() method to avoid the unused parameter compiler warning:

void handleEvent(AceButton* /*button*/, uint8_t eventType,
    uint8_t /*buttonState*/) {

The Arduino sketch compiler can get confused with the parameters commented out, so you may need to add a forward declaration for the handleEvent() method before the setup() method:

void handleEvent(AceButton*, uint8_t, uint8_t);

Pro Tips 2: The event handler can be an object instead of just a function pointer. An object-based event handler can be useful in more complex applications with numerous buttons. See the section on Object-based Event Handler in the Advanced Topics below.

EventHandler Parameters

The EventHandler function receives 3 parameters from the AceButton:

  • button
    • pointer to the AceButton instance that generated this event
    • can be used to retrieve the getPin() or the getId()
  • eventType
    • the type of this event given by the various AceButton::kEventXxx constants
  • buttonState
    • the HIGH or LOW button state that generated this event

The button pointer should be used only to extract information about the button that triggered the event. It should not be used to modify the button's internal variables in any way within the eventHandler. The logic in AceButton::check() assumes that those internal variable are held constant, and if they are changed by the eventHandler, unpredictable results may occur. (I should have made the button be a const AceButton* but by the time I realized this, there were too many users of the library already, and I did not want to make a breaking change to the API.)

If you are using only a single button, then you should need to check only the eventType.

It is not expected that buttonState will be needed very often. It should be sufficient to examine just the eventType to determine the action that needs to be performed. Part of the difficulty with this parameter is that it has the value of LOW or HIGH, but the physical interpretation of those values depends on whether the button was wired with a pull-up or pull-down resistor. Use the helper function button->isReleased(buttonState) to translate the raw buttonState into a more meaningful determination if you need it.

One EventHandler Per ButtonConfig

Only a single EventHandler per ButtonConfig is supported. An alternative would have been to register a separate event handler for each of the 8 kEventXxx events. But each callback function requires 2 bytes of memory (on 8-bit processors, or 4 bytes on 32-bit processors) and it was assumed that in most cases, the calling client code would be interested in only a few of these event types, so it seemed wasteful to allocate 16 or 32 bytes when most of these would be unused. If the client code really wants separate event handlers, it can be easily emulated by invoking them through the main event handler:

void handleEvent(AceButton* button, uint8_t eventType, uint8_t buttonState) {
  switch (eventType) {
    case AceButton::kEventPressed:
      handleEventPressed(button, eventType, buttonState);
    case AceButton::kEventReleased:
      handleEventReleased(button, eventType, buttonState);

EventHandler Tips

The Arduino runtime environment is single-threaded, so the EventHandler is called in the middle of the AceButton::check() method, in the same thread as the check() method. It is therefore important to write the EventHandler code to run somewhat quickly, so that the delay doesn't negatively impact the logic of the AceButton::check() algorithm. Since AceButton::check() should run approximately every 5 ms, the user-provided EventHandler should run somewhat faster than 5 ms. Given a choice, it is probably better to use the EventHandler to set some flags or variables and return quickly, then do additional processing from the loop() method.

Sometimes it is too convenient or unavoidable to perform a long-running operation inside the event handler (e.g. making an HTTP). This is fine, I have done this occasionally. Just be aware that the button scanning operation will not work during that long-running operation.

Speaking of threads, the API of the AceButton Library was designed to work in a multi-threaded environment, if that situation were to occur in the Arduino world.

Event Types

The supported events are defined by a list of integer (uint8_t) constants in AceButton.h:

  • AceButton::kEventPressed (always enabled, cannot be suppressed)
  • AceButton::kEventReleased (default: enabled)
  • AceButton::kEventClicked (default: disabled)
  • AceButton::kEventDoubleClicked (default: disabled)
  • AceButton::kEventLongPressed (default: disabled)
  • AceButton::kEventRepeatPressed (default: disabled)
  • AceButton::kEventLongReleased (default: disabled, autoenabled by kFeatureSuppressAfterLongPress, new for v1.8)

These values are sent to the EventHandler in the eventType parameter.

Two of the events are enabled by default, four are disabled by default but can be enabled by using a Feature flag described below.

During development and debugging, it is useful to print a human-readable version of these integer constants. The AceButton::eventName(e) is a static function which returns a string for each event constant, and can be used like this:

void handleEvent(AceButton* button, uint8_t eventType, uint8_t buttonState) {

ButtonConfig Feature Flags

There are 9 flags defined in ButtonConfig which can control the behavior of AceButton event handling:

  • ButtonConfig::kFeatureClick
  • ButtonConfig::kFeatureDoubleClick
  • ButtonConfig::kFeatureLongPress
  • ButtonConfig::kFeatureRepeatPress
  • ButtonConfig::kFeatureSuppressAfterClick
  • ButtonConfig::kFeatureSuppressAfterDoubleClick
  • ButtonConfig::kFeatureSuppressAfterLongPress
  • ButtonConfig::kFeatureSuppressAfterRepeatPress
  • ButtonConfig::kFeatureSuppressClickBeforeDoubleClick
  • ButtonConfig::kFeatureSuppressAll

These constants are used to set or clear the given flag:

// Get the current config.
ButtonConfig* config = button.getButtonConfig();

// Set a specific feature

// Clear a specific feature

// Test for a specific feature
if (config->isFeature(ButtonConfig::kFeatureLongPress)) {

// Clear all features

The meaning of these flags are described below.

Event Activation

Of the various event types, the following are disabled by default:

  • AceButton::kEventClicked
  • AceButton::kEventDoubleClicked
  • AceButton::kEventLongPressed
  • AceButton::kEventRepeatPressed
  • AceButton::kEventLongReleased
  • AceButton::kEventHeartBeat

To receive these events, call ButtonConfig::setFeature() with the following corresponding flags:

  • ButtonConfig::kFeatureClick
  • ButtonConfig::kFeatureDoubleClick
  • ButtonConfig::kFeatureLongPress
  • ButtonConfig::kFeatureRepeatPress
  • ButtonConfig::kFeatureSuppressAfterLongPress
    • suppresses kEventReleased after a LongPress, but turns on kEventLongReleased as a side effect
  • ButtonConfig::kFeatureHeartBeat

like this:

ButtonConfig *config = button.getButtonConfig();

To disable these events, call ButtonConfig::clearFeature() with one of these flags, like this:

ButtonConfig *config = button.getButtonConfig();

Enabling kFeatureDoubleClick automatically enables kFeatureClick, because we need to have a Clicked event before a DoubleClicked event can be detected.

It seems unlikely that both LongPress and RepeatPress events would be useful at the same time, but both event types can be activated if you need it.

Event Suppression

Event types can be considered to be built up in layers, starting with the lowest level primitive events: Pressed and Released. Higher level events are built on top of the lower level events through various timing delays. When a higher level event is detected, it is sometimes useful to suppress the lower level event that was used to detect the higher level event.

For example, a Clicked event requires a Pressed event followed by a Released event within a ButtonConfig::getClickDelay() milliseconds (200 ms by default). The Pressed event is always generated. If a Clicked event is detected, we could choose to generate both a Released event and a Clicked event, and this is the default behavior.

However, many times, it is useful to suppress the Released event if the Clicked event is detected. The ButtonConfig can be configured to suppress these lower level events. Call the setFeature(feature) method passing the various kFeatureSuppressXxx constants:

  • ButtonConfig::kFeatureSuppressAfterClick
    • suppresses the kEventReleased event after a Clicked event is detected
    • also suppresses the Released event from the first Clicked of a DoubleClicked, since kFeatureDoubleClick automatically enables kFeatureClick
  • ButtonConfig::kFeatureSuppressAfterDoubleClick
    • suppresses the kEventReleased event and the second Clicked event if a DoubleClicked event is detected
  • ButtonConfig::kFeatureSuppressAfterLongPress
    • suppresses the kEventReleased event if a LongPressed event is detected
    • (v1.8) automatically enables kEventLongReleased event as a substitute for the suppressed kEventReleased, see Distinguishing Pressed and Long Pressed subsection below for more details.
  • ButtonConfig::kFeatureSuppressAfterRepeatPress
    • suppresses the kEventReleased event after the last RepeatPressed event
  • ButtonConfig::kFeatureSuppressClickBeforeDoubleClick
    • The first kEventClicked event is postponed by getDoubleClickDelay() millis until the code can determine if a DoubleClick has occurred. If so, then the postponed kEventClicked message to the EventHandler is suppressed.
    • See Distinguishing Clicked and DoubleClicked subsection below for more info.
  • ButtonConfig::kFeatureSuppressAll
    • a convenience parameter that is the equivalent of suppressing all of the previous events

By default, no suppression is performed.

As an example, to suppress the kEventReleased after a kEventLongPressed (this is actually often the case), you would do this:

ButtonConfig* config = button.getButtonConfig();

The special convenient constant kFeatureSuppressAll is equivalent of using all suppression constants:

ButtonConfig* config = button.getButtonConfig();

All suppressions can be cleared by using:

ButtonConfig* config = button.getButtonConfig();

Note, however, that the isFeature(ButtonConfig::kFeatureSuppressAll) currently means "isAnyFeature() implemented?" not "areAllFeatures() implemented?" I don't expect isFeature() to be used often (or at all) for kFeatureSuppressAll.

You can clear all feature at once using:

ButtonConfig* config = button.getButtonConfig();

This is useful if you want to reuse a ButtonConfig instance and you want to reset its feature flags to its initial state.

Single Button Simplifications

Although the AceButton library is designed to shine for multiple buttons, you may want to use it to handle just one button. The library provides some features to make this simple case easy.

  1. The library automatically creates one instance of ButtonConfig called a "System ButtonConfig". This System ButtonConfig can be retrieved using the class static method ButtonConfig::getSystemButtonConfig().
  2. Every instance of AceButton is assigned an instance of the System ButtonConfig by default (which can be overridden manually).
  3. A convenience method allows the EventHandler for the System ButtonConfig to be set easily through AceButton itself, instead of having to get the System ButtonConfig first, then set the event handler. In other words, button.setEventHandler(handleEvent) is a synonym for button.getButtonConfig()->setEventHandler(handleEvent).

These simplifying features allow a single button to be configured and used like this:

AceButton button(BUTTON_PIN);

void setup() {

void loop() {

void handleEvent(AceButton* button, uint8_t eventType, uint8_t buttonState) {

To configure the System ButtonConfig, you may need to add something like this to the setup() section:


Multiple Buttons

When transitioning from a single button to multiple buttons, it's important to remember what's happening underneath the convenience methods. The single AceButton button is assigned to the System ButtonConfig that was created automatically. When an EventHandler is assigned to the button, it is actually assigned to the System ButtonConfig. All subsequent instances of AceButton will also be associated with this event handler, unless another ButtonConfig is explicitly assigned.

There are at least 2 ways you can configure multiple buttons.

Option 1: Multiple ButtonConfigs

#include <AceButton.h>
using namespace ace_button;

ButtonConfig config1;
AceButton button1(&config1);
ButtonConfig config2;
AceButton button2(&config2);

void button1Handler(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void button2Handler(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void setup() {
  pinMode(6, INPUT_PULLUP);
  pinMode(7, INPUT_PULLUP);

void loop() {

See the example sketch TwoButtonsUsingTwoButtonConfigs which uses 2 ButtonConfig instances to configure 2 AceButton instances.

Option 2: Multiple Button Discriminators

Another technique keeps the single system ButtonConfig and the single EventHandler, but use the AceButton::getPin() to discriminate between the multiple buttons:

#include <AceButton.h>
using namespace ace_button;

AceButton button1(6);
AceButton button2(7);

void button1Handler(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void button2Handler(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void buttonHandler(AceButton* button, uint8_t eventType, uint8_t buttonState) {
  switch (button->getPin()) {
    case 6:
      button1Handler(button, eventType, buttonState);
    case 7:
      button2Handler(button, eventType, buttonState);

void setup() {
  pinMode(6, INPUT_PULLUP);
  pinMode(7, INPUT_PULLUP);
  ButtonConfig* config = ButtonConfig::getSystemButtonConfig();

void loop() {

See the example code TwoButtonsUsingOneButtonConfig. which uses a single ButtonConfig instance to handle 2 AceButton instances.

Sometimes, it is more convenient to use the AceButton::getId() method to identify the button instead of the AceButton::getPin(). See ArrayButtons for an example.

Advanced Topics

Object-based Event Handler

The EventHandler is a typedef that is defined to be a function pointer. This is a simple, low-overhead design that produces the smallest memory footprint, and allows the event handler to be written with the smallest amount of boilerplate code. The user does not have to override a class.

In more complex applications involving larger number of AceButton and ButtonConfig objects, it is often useful for the EventHandler to be an object instead of a simple function pointer. This is especially true if the application uses Object Oriented Programming (OOP) techniques for modularity and encapsulation. Using an object as the event handler allows additional context information to be injected into the event handler.

To support OOP techniques, AceButton v1.6 adds:

  • IEventHandler interface class
    • contains a single pure virtual function handleEvent()
  • ButtonConfig::setIEventHandler() method
    • accepts a pointer to an instance of the IEventHandler interface.

The IEventHandler interface is simply this:

class IEventHandler {
    virtual void handleEvent(AceButton* button, uint8_t eventType,
        uint8_t buttonState) = 0;

At least one of ButtonConfig::setEventHandler() or ButtonConfig::setIEventHandler() must be called before events are actually dispatched. If both are called, the last one takes precedence.

See examples/SingleButtonUsingIEventHandler for an example.

Distinguishing Clicked and DoubleClicked

On a project using only a small number of buttons (due to physical limits or the limited availability of pins), it may be desirable to distinguish between a single Clicked event and a DoubleClicked event from a single button. This is a challenging problem to solve because fundamentally, a DoubleClicked event must always generate a Clicked event, because a Clicked event must happen before it can become a DoubleClicked event.

Notice that on a desktop computer (running Windows, MacOS or Linux), a double-click on a mouse always generates both a Clicked and a DoubleClicked. The first Click selects the given desktop object (e.g. an icon or a window), and the DoubleClick performs some action on the selected object (e.g. open the icon, or resize the window).

The AceButton Library provides 3 solutions which may work for some projects:

Method 1: The kFeatureSuppressClickBeforeDoubleClick flag causes the first Clicked event to be detected, but the posting of the event message (i.e. the call to the EventHandler) is postponed until the state of the DoubleClicked can be determined. If the DoubleClicked happens, then the first Clicked event message is suppressed. If DoubleClicked does not occur, the long delayed Clicked message is sent via the EventHandler.

There are two noticeable disadvantages of this method. First, the response time of all Clicked events is delayed by about 600 ms (kClickDelay + kDoubleClickDelay) whether or not the DoubleClicked event happens. Second, the user may not be able to accurately produce a Clicked event (due to the physical characteristics of the button, or the user's dexterity).

It may also be worth noting that only the Clicked event is postponed. The accompanying Released event of the Clicked event is not postponed. So a single click action (without a DoubleClick) produces the following sequence of events to the EventHandler:

  1. kEventPressed - at time 0ms
  2. kEventReleased - at time 200ms
  3. kEventClicked - at time 600ms (200ms + 400ms)

The ButtonConfig configuration looks like this:

ButtonConfig* buttonConfig = button.getButtonConfig();

See the example code at ClickVersusDoubleClickUsingSuppression.

Method 2: A viable alternative is to use the Released event instead of the Clicked event to distinguish it from the DoubleClicked. For this method to work, we need to suppress the Released event after both Clicked and DoubleClicked.

The advantage of using this method is that there is no response time lag in the handling of the Released event. To the user, there is almost no difference between triggering on the Released event, versus triggering on the Clicked event.

The disadvantage of this method is that the Clicked event must be be ignored (because of the spurious Clicked event generated by the DoubleClicked). If the user accidentally presses and releases the button too quickly, it generates a Clicked event, which will cause the program to do nothing.

The ButtonConfig configuration looks like this:

ButtonConfig* buttonConfig = button.getButtonConfig();

See the example code at ClickVersusDoubleClickUsingReleased.

Method 3: We could actually combine both Methods 1 and 2 so that either Released or a delayed Click is considered to be a "Click". This may be the best of both worlds.

The ButtonConfig configuration looks like this:

ButtonConfig* buttonConfig = button.getButtonConfig();

See the example code at examples/ClickVersusDoubleClickUsingBoth/.

Distinguishing Pressed and LongPressed

Sometimes it is useful to capture both a Pressed event and a LongPressed event from a single button. Since every button press always triggers a kEventPressed event, the only reasonable way to distinguish between Pressed and LongPressed is to use the kEventReleased as a substitute for the simple Pressed event. When we activate kFeatureLongPress, we then must activate the kFeatureSuppressAfterLongPress feature to suppress the kEventReleased event after the kEventLongPressed to avoid yet another overlap of events.

ButtonConfig* config = button.getButtonConfig();

This works most of the time, but I encountered an edge case. Occasionally we want to capture the Released event after the LongPressed event, even if kEventReleased must be suppressed as described above. To solve this edge case, in v1.8, I added a new event type kEventLongReleased which is triggered as a substitute for kEventReleased, only if kFeatureSuppressAfterLongPress is used to suppress kEventReleased.

See the example code at examples/PressVersusLongPress to see how all these come together.

Events After Reboot

A number of edge cases occur when the microcontroller is rebooted:

  • if the button is held down, should the Pressed event be triggered?
  • if the button is in its natural Released state, should the Released event happen?
  • if the button is Pressed down, and ButtonConfig is configured to support RepeatPress events, should the kEventRepeatPressed events be triggered initially?

I think most users would expect that in all these cases, the answer is no, the microcontroller should not trigger an event until the button undergoes a human-initiated change in state. The AceButton library implements this logic. (It might be useful to make this configurable using a ButtonConfig feature flag but that is not implemented.)

On the other hand, it is sometimes useful to perform some special action if a button is pressed while the device is rebooted. To support this use-case, call the AceButton::isPressedRaw() in the global setup() method (after the button is configured). It will directly call the digitalRead() method associated with the button pin and return true if the button is in the Pressed state.

Orphaned Clicks

When a Clicked event is generated, the AceButton class looks for a second Clicked event within a certain time delay (default 400 ms) to determine if the second Clicked event is actually a DoubleClicked event.

All internal timestamps in AceButton are stored as uint16_t (i.e. an unsigned integer of 16 bits) in millisecond units. A 16-bit unsigned counter rolls over after 65536 iterations. Therefore, if the second Clicked event happens between (65.636 seconds, 66.036 seconds) after the first Clicked event, a naive-logic would erroneously consider the (long-delayed) second click as a double-click.

The AceButton contains code that prevents this from happening.

Note that even if the AceButton class uses an unsigned long type (a 32-bit integer on the Arduino), the overflow problem would still occur after 2^32 milliseconds (i.e. 49.7 days). To be strictly correct, the AceButton class would still need logic to take care of orphaned Clicked events.

Binary Encoded Buttons

Instead of allocating one pin for each button, we can use Binary Encoding to support large number of buttons with only a few pins. The circuit can be implemented using a 74LS148 chip, or simple diodes like this:

8 To 3 Encoding

Three subclasses of ButtonConfig are provided to handle binary encoded buttons:

  • Encoded4To2ButtonConfig: 3 buttons with 2 pins
  • Encoded8To3ButtonConfig: 7 buttons with 3 pins
  • EncodedButtonConfig: M=2^N-1 buttons with N pins

See docs/binary_encoding/ for information on how to use these classes.

Resistor Ladder Buttons

It is possible to attach 1-8 (maybe more) buttons on a single analog pin through a resistor ladder, and use the analogRead() to read the different voltages generated by each button. An example circuit looks like this:

Parallel Resistor Ladder

The LadderButtonConfig class handles this configuration.

See docs/resistor_ladder/ for information on how to use this class.

Dynamic Allocation on the Heap

All classes in this library were originally designed to be created statically at startup time and never deleted during the lifetime of the application. Since they were never meant to be deleted through the pointer, I did not include the virtual destructor for polymorphic classes (i.e. ButtonConfig and its subclasses). The AceButton class is not polymorphic and does not need a virtual destructor.

Most 8-bit processors have limited flash and static memory (for example, 32 kB flash and 2 KB static for the Nano or UNO). Adding a virtual destructor causes 600 additional bytes of flash memory to be consumed. I suspect this is due to the virtual destructor pulling the malloc() and free() functions which are needed to implement the new and delete operators. For a library that consumes only about 1200 bytes on an 8-bit processor, this increase in flash memory size did not seem acceptable.

For 32-bit processors (e.g. ESP8266, ESP32) which have far more flash memory (e.g. 1 MB) and static memory (e.g. 80 kB), it seems reasonable to allow AceButton and ButtonConfig to be created and deleted from the heap. (See Issue #46 for the motivation.) Testing shows that the virtual destructor adds only about 60-120 bytes of flash memory for these microcontrollers, probably because the malloc() and free() functions are already pulled in by something else. The 60-120 bytes of additional consumption seems trivial compared to the range of ~256 kB to ~4 MB flash memory available on these 32-bit processors.

Therefore, I added a virtual destructor for the ButtonConfig class (v1.5) and enabled it for all architectures other than ARDUINO_ARCH_AVR (v1.6.1). This prevents 8-bit processors with limited memory from suffering the overhead of an extra 600 bytes of flash memory usage.

Even for 32-bit processors, I still recommend avoiding the creation and deletion of objects from the heap, to avoid the risk of heap fragmentation. If a variable number of buttons is needed, it might be possible to design the application so that all buttons which will ever be needed are predefined in a global pool. Even if some of the AceButton and ButtonConfig instances are unused, the overhead is probably smaller than the overhead of wasted space due to heap fragmentation.

Digital Write Fast

The digitalWriteFast libraries provide smaller and faster alternative versions the digitalWrite(), digitalRead(), and pinMode() functions. I have used 2 of the following libraries, but there probably others:

These libraries provide the following functions: digitalWriteFast(), digitalReadFast(), and pinModeFast() which are usually valid only AVR processors. These alternative functions depend on the pin number and value to be compile-time constants, bypassing the pin number lookup tables used by the standard versions. These fast versions can be 20-50X faster. More importantly in many situations, they can save 100-500 bytes of flash memory by not pulling in the pin number lookup tables.

I created 3 alternative versions of ButtonConfig which use the digitalWriteFast libraries:

(If ButtonConfigFast4.h is needed, it is easy to copy ButtonConfigFast3.h and create a 4-pin version.)

These classes use C++ templates on the pin numbers, so that they can be passed to the digitalReadFast() functions as compile-time constants. Because they depend on an external digitalWriteFast library, they are not included in the <AceButton.h> header file. They must be included explicitly, as shown below:

#include <Arduino.h>
#include <AceButton.h>
#include <digitalWriteFast.h>
#include <ace_button/fast/ButtonConfigFast2.h>

using namespace ace_button;

// Physical pin numbers attached to the buttons.
const uint8_t BUTTON1_PHYSICAL_PIN = 2;
const uint8_t BUTTON2_PHYSICAL_PIN = 3;

// Virtual pin numbers attached to the buttons.
const uint8_t BUTTON1_PIN = 0;
const uint8_t BUTTON2_PIN = 1;

AceButton button1(&buttonConfig, BUTTON1_PIN);
AceButton button2(&buttonConfig, BUTTON2_PIN);

void handleEvent(AceButton* button, uint8_t eventType, uint8_t buttonState) {

void setup() {

void loop() {
  // Should be called every 4-5ms or faster, for the default debouncing time
  // of ~20ms.

Each physical pin number given as template arguments to the ButtonConfigFast2<> class corresponds to a virtual pin number (starting with 0) assigned to the AceButton object. Within the event handler, everything is referenced by the virtual pin number, just like the EncodedButtonConfig and LadderButtonConfig classes.

Here are the example programs for each ButtonConfigFast{N} class:

The LadderButtonConfig class uses analogRead() which does not seem to directly benefit from digitalWriteFast libraries. However, if you use pinModeFast() instead of pinMode() in your global setup() function, you can save about 50 bytes of flash (I think).

The Encoded4To2ButtonConfig and Encoded8To3ButtonConfig classes would probably benefit from digitalWriteFast libraries, but I have not created the "fast" versions of these (i.e. Encoded4To2ButtonConfigFast and Encoded8To3ButtonConfigFast) because I have not needed them personally.

The general EncodedButtonConfig class is more difficult to convert into a "fast" version, because its constructor takes a pointer argument to an array of physical pins. These are not compile-time constants so we would not be able to use the digitalWriteFast libraries directly. I think the best we could do is create special Encoded16To4ButtonConfigFast and Encoded32To5ButtonConfigFast classes.

Heart Beat Event

Version 1.10 added the kEventHeartBeat event. By default it is disabled. It can be enabled using the kFeatureHeartBeat flag:

ButtonConfig* config = button.getButtonConfig();

When enabled, the AceButton object sends a kEventHeartBeat event at a periodic interval, with the number of milliseconds managed by the following methods on the ButtonConfig object:

  • void setHeartBeatInterval(uint16_t interval)
  • uint16_t getHeartBeatInterval() const

The default is 5000 milliseconds.

The primary purpose of the HeartBeat event is to allow the user-provided event handler (IEventHandler will likely be easiest for this purpose) to generate custom event types which are not provided by AceButton itself by default. When the button does not undergo any change in state explicitly initiated by the user (e.g. Released for a long time), the AceButton object will not trigger any events normally. By activating the kFeatureHeartBeat, the event handler can generate custom events such as "Pressed for 5 minutes", or "Released for 5 Minutes". See examples/HeartBeat for an example of an IEventHandler that implements this.

The kEventHeartBeat is triggered only by the progression of time, and is not affected by any internal state of the AceButton, such as the debouncing state, or the various logic for detecting Clicked, DoubleClicked, and so on. The HeartBeatInterval is intended to be relatively large, with the default set to 5000 milliseconds, to avoid the overhead of calling the event handler too often. Using a smaller interval may affect the detection logic of various other button events if the HeartBeat handler consumes too much CPU time.

The buttonState is passed to the event handler by the HeartBeat dispatcher, through the callback function or IEventHandler interface, for example:

typedef void (*EventHandler)(AceButton* button, uint8_t eventType,
    uint8_t buttonState);

This button state will be the last known, debounced and validated state. It will not be the current button state. This is because the HeartBeat detector operates independently of the debouncing logic, and it did not seem appropriate for the unvalidated buttonState to be passed to the event handler just because the timer for the HeartBeat triggered in the middle of the debouncing logic.

Resource Consumption

SizeOf Classes

Here are the sizes of the various classes on the 8-bit AVR microcontrollers (Arduino Uno, Nano, etc):

sizeof(AceButton): 17
sizeof(ButtonConfig): 20
sizeof(ButtonConfigFast1<>): 20
sizeof(ButtonConfigFast2<>): 20
sizeof(ButtonConfigFast3<>): 20
sizeof(Encoded4To2ButtonConfig): 23
sizeof(Encoded8To3ButtonConfig): 24
sizeof(EncodedButtonConfig): 27
sizeof(LadderButtonConfig): 28

For 32-bit microcontrollers:

sizeof(AceButton): 20
sizeof(ButtonConfig): 24
sizeof(Encoded4To2ButtonConfig): 28
sizeof(Encoded8To3ButtonConfig): 28
sizeof(EncodedButtonConfig): 36
sizeof(LadderButtonConfig): 36

(An early version of AceButton, with only half of the functionality, consumed 40 bytes. It got down to 11 bytes before additional functionality increased it to its current 17.)

Flash And Static Memory

MemoryBenchmark was used to determine the size of the library for various microcontrollers (Arduino Nano to ESP32). Here are 2 samples:

Arduino Nano

| functionality                   |  flash/  ram |       delta |
| Baseline                        |    462/   11 |     0/    0 |
| Baseline+pinMode+digitalRead    |    766/   11 |   304/    0 |
| ButtonConfig                    |   1970/   56 |  1508/   45 |
| ButtonConfigFast1               |   1686/   56 |  1224/   45 |
| ButtonConfigFast2               |   1586/   73 |  1124/   62 |
| ButtonConfigFast3               |   1628/   90 |  1166/   79 |
| Encoded4To2ButtonConfig         |   2098/   93 |  1636/   82 |
| Encoded8To3ButtonConfig         |   2318/  162 |  1856/  151 |
| EncodedButtonConfig             |   2362/  185 |  1900/  174 |
| LadderButtonConfig              |   2360/  198 |  1898/  187 |


| functionality                   |  flash/  ram |       delta |
| Baseline                        | 260105/27892 |     0/    0 |
| Baseline+pinMode+digitalRead    | 260201/27892 |    96/    0 |
| ButtonConfig                    | 261589/27944 |  1484/   52 |
| Encoded4To2ButtonConfig         | 261741/27984 |  1636/   92 |
| Encoded8To3ButtonConfig         | 261885/28064 |  1780/  172 |
| EncodedButtonConfig             | 262013/28104 |  1908/  212 |
| LadderButtonConfig              | 262057/28116 |  1952/  224 |

CPU Cycles

The profiling numbers for AceButton::check(), EncodedButtonConfig::checkButtons(), and LadderButtonConfig::checkButtons() can be found in examples/AutoBenchmark. Here are 2 samples, in units of microseconds.

Arduino Nano:

| Button Event              | min/avg/max | samples |
| idle                      |  12/ 16/ 24 |    1929 |
| press/release             |  12/ 17/ 28 |    1924 |
| click                     |  12/ 16/ 28 |    1925 |
| double_click              |  12/ 16/ 32 |    1922 |
| long_press/repeat_press   |  12/ 18/ 28 |    1923 |
| ButtonConfigFast1         |  12/ 16/ 24 |    1932 |
| ButtonConfigFast2         |  20/ 30/ 40 |    1905 |
| ButtonConfigFast3         |  32/ 44/ 52 |    1880 |
| Encoded4To2ButtonConfig   |  60/ 73/ 80 |    1831 |
| Encoded8To3ButtonConfig   | 168/196/204 |    1645 |
| EncodedButtonConfig       |  84/110/116 |    1769 |
| LadderButtonConfig        | 184/211/288 |    1625 |


| Button Event              | min/avg/max | samples |
| idle                      |   6/  8/ 62 |    1920 |
| press/release             |   6/  8/ 45 |    1921 |
| click                     |   6/  7/ 18 |    1921 |
| double_click              |   6/  7/ 12 |    1922 |
| long_press/repeat_press   |   6/  8/ 12 |    1920 |
| Encoded4To2ButtonConfig   |  22/ 27/ 46 |    1879 |
| Encoded8To3ButtonConfig   |  56/ 67/ 76 |    1810 |
| EncodedButtonConfig       |  43/ 54/ 70 |    1841 |
| LadderButtonConfig        |  81/ 93/212 |    1772 |

System Requirements


Tier 1: Fully Supported

These boards are tested on each release:

  • Arduino Nano (16 MHz ATmega328P)
  • SparkFun Pro Micro (16 MHz ATmega32U4)
  • Seeeduino XIAO M0 (SAMD21, 48 MHz ARM Cortex-M0+)
  • STM32 Blue Pill (STM32F103C8, 72 MHz ARM Cortex-M3)
  • Adafruit ItsyBitsy M4 (SAMD51, 120 MHz ARM Cortext-M4)
  • NodeMCU 1.0 (ESP-12E module, 80MHz ESP8266)
  • WeMos D1 Mini (ESP-12E module, 80 MHz ESP8266)
  • ESP32 Dev Module (ESP-WROOM-32 module, 240MHz dual core Tensilica LX6)

Tier 2: Should work

These boards should work but I don't test them as often:

  • ATtiny85 (8 MHz ATtiny85)
  • Arduino Pro Mini (16 MHz ATmega328P)
  • Mini Mega 2560 (Arduino Mega 2560 compatible, 16 MHz ATmega2560)
  • Teensy LC (48 MHz ARM Cortex-M0+)
  • Teensy 3.2 (96 MHz ARM Cortex-M4)

Tier 3: May work, but not supported

  • Any platform using the ArduinoCore-API (
    • For example, Nano Every, MKRZero, and Raspberry Pi Pico RP2040.
    • Most of my libraries do not work on platforms using the ArduinoCore-API.
    • However AceButton is simple enough that it may still work on these boards.

Tool Chain

This library was developed and tested using:

It should work with PlatformIO but I have not tested it.

The library works on Linux or MacOS (using both g++ and clang++ compilers) using the EpoxyDuino emulation layer.

Operating System

I use Ubuntu Linux 22.04 or its variants (i.e. Linux Mint) for most of my development.

Background Motivation

There are numerous "button" libraries out there for the Arduino. Why write another one? I wanted to add a button to an addressable strip LED controller, which was being refreshed at 120 Hz. I had a number of requirements:

  • the button needed to support a LongPress event, in addition to the simple Press and Release events
  • the button code must not interfere with the LED refresh code which was updating the LEDs at 120 Hz
  • well-tested, I didn't want to be hunting down random and obscure bugs

Since the LED refresh code needed to run while the button code was waiting for a "LongPress" delay, it seemed that the cleanest API for a button library would use an event handler callback mechanism. This reduced the number of candidate libraries to a handful. Of these, only a few of them supported a LongPress event. I did not find the remaining ones flexible enough for my button needs in the future. Finally, I knew that it was tricky to write correct code for debouncing and detecting various events (e.g. DoubleClick, LongPress, RepeatPress). I looked for a library that contained unit tests, and I found none.

I decided to write my own and use the opportunity to learn how to create and publish an Arduino library.


An Arduino UNO or Nano has 16 times more flash memory (32KB) than static memory (2KB), so the library is optimized to minimize the static memory usage. The AceButton library is not optimized to create a small program size (i.e. flash memory), or for small CPU cycles (i.e. high execution speed). I assumed that if you are seriously optimizing for program size or CPU cycles, you will probably want to write everything yourself from scratch.

That said, examples/MemoryBenchmark shows that the library consumes between 970-2180 bytes of flash memory, and AutoBenchmark shows that AceButton::check() takes between ~15 microseconds on a 16MHz ATmega328P chip and 2-3 microseconds on an ESP32. Hopefully that is small enough and fast enough for the vast majority of people.

With v1.9, I started using AceButton on an ATtiny85 which has only 8kB of flash and 0.5 kB of static RAM. Flash memory consumption became more important and I created the ButtonConfigFast1, ButtonConfigFast2, and ButtonConfigFast3 classes to decrease the flash memory consumption by using one of the <digitalWriteFast.h> 3rd party libraries. See Digital Write Fast for more info.

Bugs and Limitations

This is the first Arduino library that I ever created. It has grown organically over time, while maintaining backwards compatibility as much as possible. There are some early design decisions that could have been better with infinite insight. Here are some limitations and bugs.

  • The ButtonConfig class should have been named ButtonGroup.
    • This was not obvious until I had implemented the EncodedButtonConfig and the LadderButtonConfig classes.
    • But I cannot change the names without breaking backwards compatibility.
  • The Single Button Simplifications was probably a mistake.
    • It makes the API more complex and cluttered than it could be.
    • It causes an automatic creation of the SystemButtonConfig instance of ButtonConfig even when it is not needed by the application, unless special (non-obvious) precautions are taken.
    • On the other hand, it makes the simplest HelloButton program very simple.
  • Embedding multiple AceButton objects in an array or a struct is difficult if not impossible.
    • See Discussion#106 for some details.
    • The C++ rules for object initialization are so complicated, and have changed with different versions of C++ (C++11, C++14, C++17, C++20, C++23??), that I cannot understand them anymore.
    • Probably will never be fixed because I'm tired of the complexity of the C++ language.
  • AceButton does not provide built-in support for simultaneous buttons.
    • See Discussion#83, Discussion#94, and Discussion#96.
    • The examples/SimultaneousButtons program shows how it could be detected using a custom IEventHandler. However, it has not been extensively tested. I don't even remember writing it 2 years ago.
    • This remains an open problem because I don't use simultaneous buttons in my applications, and I have not spent much time thinking about how to handle all the combinations of events and their timing interactions that are possible with 2 buttons.
  • The EventHandler and IEventHandler send an AceButton* pointer into the arguments, instead of a const AceButton* pointer.
    • Too late to change that without breaking backwards compatibility.
  • All internal timing variables are uint16_t instead of uint32_t.
    • This means that various timing parameters (ClickDelay, LongPressDelay, etc) can be maximum of 65535 milliseconds.
    • Using 16-bit integers saves RAM, at least 8 bytes for each instance of AceButton, and 14 bytes for ButtonConfig and its subclasses. And probably saves flash memory on 8-bit processors because fewer machine instruction are needed to operate on 16-bit variables compared to 32-bit variables.
    • Client applications were assumed to use as many as 10-20 buttons. That's a savings of 80-160 bytes which makes a difference on 8-bit AVR processors with only 2 kB of RAM.
  • The library does not support the mirror-image version of kEventLongPressed for the Released state.
    • In other words, an event that is triggered when a button has been released for a long time (several seconds).
    • The current kEventLongReleased is a different type of event.
    • See Discussion#118 for info.
    • An alternative solution might be to use the kEventHeartBeat and a custom IEventHandler to generate a custom event. See examples/HeartBeat for a example.
  • The Event Supression features are complicated, hard to understand and remember.
    • I wrote the code and I cannot remember how they all work. I have to refer to this each time I want to use them.
    • These features grew organically. Maybe there is a better, more consistent way of implementing them, but it was not obvious during the development of these features.


I changed to the MIT License starting with version 1.1 because the MIT License is so simple to understand. I could not be sure that I understood what the Apache License 2.0 meant.

Feedback and Support

If you have any questions, comments, or feature requests for this library, please use the GitHub Discussions for this project. If you have a bug report, please file a ticket in GitHub Issues. Feature requests should go into Discussions first because they often have alternative solutions which are useful to remain visible, instead of disappearing from the default view of the Issue tracker after the ticket is closed.

Please refrain from emailing me directly unless the content is sensitive. The problem with email is that I cannot reference the email conversation when other people ask similar questions later.


Created by Brian T. Park (