ATtiny85 Button Debouncing

Interfacing a button to an ATtiny85 microcontroller is rather straight forward, however, it is often unappreciated that the majority of buttons rarely give a single signal per push of the button. This is due to the inherient noisy nature of the switch contacts.

This means that when the switch is pressed, instead of a single 5V pulse (for example) indicating a logic high, there can be perhaps a number of "noisy" spikes above the logic high voltage threshold, which the microcontroller will then interpret/process as a number of button presses, instead of a single event as intended. Such behaviour if not accounted for via hardware/software can lead to apparent "intermittent" faults and or erratic behaviour of the application.

This bouncing between logic states due to noisy switch contacts (instead of a clean immediate transition) before the signal settles into a steady state can be dealt with via hardware, and conveniently via a software approach, particularly amenable in a microcontroller solution (as external components can then be eliminated).

Software approaches to switch debouncing are demonstrated for an ATtiny85/Digispark using the Arduino IDE and available libraries.

Details on how to setup the Arduino IDE and environment to programme ATtiny85 microcontrollers are given in the ATtiny85 introduction.

The Digispark board has a surface mount ATtiny85 with headers for all pins. Additionally, the board provides a 5V regulator for external DC power input and PCB copper traces that enable insertion into standard USB connector which can then be used for DC power source. There is an onboard LED to indicate power-on and an additional LED/resistor connected to pin1 for user control.

For convenience for this simple test, the USB power supply is used.

Again for convenience for this simple test, the ATtiny85 onboard oscillator is used rather than an external crystal source.

ISP programming is used as detailed in the ATtiny85 introduction.

Connection between the ATtiny85/Digispark and USB to TTL converter cables is given in the Schematics Section (also see the Photographs Section for physical demonstation).

A toggle button is connected to pin B4 using a pull-up resistor. A pull-up resistor is required and not just simply connect the button between the ATtiny85/Digispark pin and positive or negative rail in order to ensure a defined voltage is present at both possible button states.

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  ATtiny85 Schematic

  ATtiny85 Schematic

Before connecting the Digispark to the USB port, check for any shorts (direct positive voltage to ground connection) as this could damage both the Digispark board and the USB/PC.

• Toggle Switch no Debouncing
• Toggle Switch with Software Debouncing
• Toggle Switch with Button Library

Toggle Switch no Debouncing

The extra code for this serial output of the counter to the Arduino serial monitor is given in code Snippet 2 and the results presented in the Video Section. How to use serial communications with the ATtiny85 is detailed in ATtiny85 serial communications.

The solution to the 'noisy' switch problem is to use 'debouncing' which can be done via hardware and or software. The software approach is advantageous in that additional hardware components are avoided, which in a situation using an ATtiny85, it is likely the small size of the ATtiny85 was attrative in keeping project PCB size small and inexpensive. See the next section for software debouncing code example.

When installing Digispark boards as part of the Arduino IDE (see ATtiny85 introduction) the SoftSerial.h library for serial communications is available (1).

Code Snippet 1: Toggle Switch no Debouncing

#define buttonPin 4    // the number of the pushbutton pin
#define ledPin 1       // the number of the LED pin

int buttonState = 0;   // variable for reading the pushbutton status
int lastButtonState = LOW;

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
}

void loop() { 
  // read the state of the pushbutton value:  
  buttonState = digitalRead(buttonPin);
  // if the pushbutton is pressed the buttonState is HIGH
  if (buttonState != lastButtonState) {
    if (buttonState == HIGH) {     
      digitalWrite(ledPin, HIGH); //turn LED on
    } else {    
      digitalWrite(ledPin, LOW);  //turn LED off
    }
    lastButtonState = buttonState;
  }
}
   				

Code Snippet 2: Toggle Switch no Debouncing/Serial Output

#include 
TinyDebugSerial mySerial = TinyDebugSerial();
                
#define buttonPin 4    // the number of the pushbutton pin
#define ledPin 1       // the number of the LED pin

int buttonState = 0;   // variable for reading the pushbutton status
int lastButtonState = LOW;
unsigned int counter=0;

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
  mySerial.begin(115200);
  delay(3000); //give a delay so can connect the port in the receiving software
  mySerial.println("hello");
}

void loop() { 
  // read the state of the pushbutton value:  
  buttonState = digitalRead(buttonPin);
  // if the pushbutton is pressed the buttonState is HIGH
  if (buttonState != lastButtonState) {
    if (buttonState == HIGH) {     
      digitalWrite(ledPin, HIGH); //turn LED on
      counter=counter+1;
    } else {    
      digitalWrite(ledPin, LOW);  //turn LED off
    }
    lastButtonState = buttonState;
  }
  if (counter==10) {
    mySerial.print("="); mySerial.println(counter);
    counter=0;
  }  
}                
                

Toggle Switch with Software Debouncing

#include 
TinyDebugSerial mySerial = TinyDebugSerial();
                
#define buttonPin 4    // the number of the pushbutton pin
#define ledPin 1       // the number of the LED pin

int buttonState = 0;   // variable for reading the pushbutton status
int lastButtonState = LOW;
unsigned long lastDebounceTime = 0;  // the last time the LED was toggled
unsigned long debounceDelay = 50;    // the debounce time
unsigned int counter=0;

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
  mySerial.begin(115200);
  delay(3000); //give a delay so can connect the port in the receiving software
  mySerial.println("hello");
}

void loop() { 
  // read the state of the pushbutton value:  
  int reading = digitalRead(buttonPin);
  if (reading != lastButtonState) {
    lastDebounceTime = millis();
  }
  if ((millis() - lastDebounceTime) > debounceDelay) {
    if (reading != buttonState) {
      buttonState = reading;
      if (buttonState == HIGH) {
        // turn LED on:
        digitalWrite(ledPin, HIGH);
        counter=counter+1;
      } else {
        // turn LED off:
        digitalWrite(ledPin, LOW);
      }
    }
  }
  if (counter==10) {
    mySerial.print("="); mySerial.println(counter);
    counter=0;
  }  
}                 
                

Toggle Switch with Button Library

The Button library for handling interfacing of switches is available (2).

Code Snippet 4: Toggle Switch with Button Library

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