I2C backpack 7 segment stopwatch. start pause and stop

Hi, This is not MY sketch. My programming abilities are quite limited, typically I can cut/paste and write the small sections to achieve my goals and learn along the way. This one is really has me stumped. Currently when it powers up, it does a little line blinky thing and waits for D2 to be grounded. Once the button is activated the stopwatch runs, another press pauses, third press resets it to 0 and forth press starts running again.

I have tried lots of experimentation to add two more buttons and nothing is working. My goal is to have one button start the stopwatch. A second button will pause the stopwatch and a third button will reset to 0.

I have tried creating buttonPins in several configurations but nothing works. Can someone please give me a hint or two?!?!?

Thanks Bryan

#include <Wire.h>
#include "Adafruit_LEDBackpack.h"
#include "Adafruit_GFX.h"

/*--Declare Matrix--*/
Adafruit_7segment matrix = Adafruit_7segment();
#define INTERRUPT_PIN 2
#define DEBOUNCE_RATE 100

volatile int timerState = 0;
volatile unsigned long debounce = 0;
volatile unsigned long startTime;

void setup() {
  Serial.begin(9600);
  Serial.println("Starting timer sketch..");

  /*--setup the interrupt--*/
  pinMode(INTERRUPT_PIN, OUTPUT);
  digitalWrite(INTERRUPT_PIN, HIGH);
  attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN), updateTimerState, LOW);

  matrix.begin(0x70);
  matrix.clear();
  
  cycleZeros(250, 500);
  writeDashesToDisplay(true);
}

void cycleZeros(int wait, int pause) {
  for (int i = 4; i > -1; i--) {
    if (i == 2) {
      --i;
    }

    matrix.writeDigitNum(i, 0);
    matrix.writeDisplay();
    delay(wait);
  }
  matrix.clear();
  matrix.writeDisplay();
  delay(pause);
}

void writeDashesToDisplay(boolean isDrawingColon){
  
  matrix.clear();
  matrix.writeDigitRaw(0, B01000000);
  matrix.writeDigitRaw(1, B01000000);
  matrix.writeDigitRaw(3, B01000000);
  matrix.writeDigitRaw(4, B01000000);
  matrix.drawColon(isDrawingColon);
  matrix.writeDisplay();
}

void updateTimerState() {

  //debounce the interrupt button
  if (millis() - debounce > DEBOUNCE_RATE) {
    
    if (timerState == 0) {
      timerState = 1;
      /*--set the start time--*/
      startTime = millis();
      
    } else if (timerState == 1) {
      /*--pause the time--*/
      timerState = 2;

    } else if (timerState == 2) {
      /*--reset the time to zeros--*/
      timerState = 3;
      
    }else if (timerState == 3){
      /*--start the timer again--*/
      timerState = 1;
      startTime = millis();
    }

    /*--update the debounce--*/
    debounce = millis();

    Serial.print("Timer state updated! --> ");
    Serial.print(timerState);
    Serial.println("");
  }

}

void writeCurrentTime() {
  unsigned long elapsedTime, seconds, minutes, msValue;
  elapsedTime = millis() - startTime; 
  seconds = elapsedTime / 1000;

  matrix.drawColon(true);

  if (seconds < 10) {
    /*--pad seconds with zero--*/
    matrix.writeDigitNum(0, 0);
    matrix.writeDigitNum(1, seconds);

  } else if (seconds >= 10 && seconds < 60) {
    /*--if under 1 min. show seconds & sub-seconds--*/
    matrix.writeDigitNum(0, seconds / 10);
    matrix.writeDigitNum(1, seconds % 10);

  } else if (seconds >= 60) {
    /*--if equal or over 1 min., switch to min. & seconds--*/
    minutes = seconds / 60;

    if (minutes < 10) {
      /*--pad with zeros--*/
      matrix.writeDigitNum(0, 0);
      matrix.writeDigitNum(1, minutes);
    } else if (minutes > 99) {
      /*--if min. exceed 99, stop timer--*/
      timerState = 2;
      return;
    } else {
      matrix.writeDigitNum(0, minutes / 10);
      matrix.writeDigitNum(1, minutes % 10);
    }

    matrix.writeDigitNum(3, (seconds % 60) / 10);
    matrix.writeDigitNum(4, (seconds % 60) % 10);
  }

  //if less than 1 min, show milliseconds
  if (seconds < 60) {
    msValue = (elapsedTime % 1000) / 10;

    matrix.writeDigitNum(3, msValue / 10);
    matrix.writeDigitNum(4, msValue % 10);
  }

  matrix.writeDisplay();
}

void loop() {
  if(timerState == 1){
    writeCurrentTime();  
  }

  if(timerState == 3){
    writeDashesToDisplay(true);
  }
}

When in doubt, work your way through the relevant Arduino tutorials.

To understand how buttons are wired and work, one place to start is here.