Forcing ISR to exit into a specific subroutine

Projects Programming
7

I tried to emulate your case in my own setup using MEGA and 4x4 keypad under Keypad.h Library. I can concurrently enter codes from the keypad and also can observe time out event. The 1-sec time-tick interrupt has been generated using register level instructions as I have no access to Timer1 Library. The codes may be helpful for you.

#include <Keypad.h>
#include<LiquidCrystal.h>
LiquidCrystal lcd(A0, A1, A2, A3, A4, A5);

const byte ROWS = 4; //four rows
const byte COLS = 4;//3; //three columns
char keys[ROWS][COLS] = {
  {'1', '2', '3', 'a'},
  {'4', '5', '6', 'b'},
  {'7', '8', '9', 'c'},
  {'*', '0', '#', 'd'}
};
byte rowPins[ROWS] = {9, 8, 7, 6};//connect to the row pin outs of the keypad
byte colPins[COLS] = {5, 4, 3, 2};//connect to the column pin outs of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

volatile int timer_s = 10; // start countdown with 10 seconds
int main_switch = 13;  // button to enter code
int main_switch_state = 0;  // state on the button
int curSorTracker=0;

void setup() 
{
  Serial.begin(9600);
  lcd.begin(20, 4);
  lcd.setCursor(0, 0);
  pinMode(main_switch, INPUT_PULLUP);

  TCCR1A = 0x00;
  TCCR1B = 0x00;
  TCNT1 = 0xC2F7;  //pre-set value for 1-sec time delay
  bitSet(TIMSK1, 0); //local interrupt enable bit
  sei();                    //global interrupt enable bit
  TCCR1B = 0x05;    //TC1 is running at clkTC1 = clkSYS/1024 = 15625 Hz

}

void loop() 
{
  main_switch_state = digitalRead(main_switch);
  if (main_switch_state == 0) 
  {
    char ch = keypad.getKey();
    if(ch !=0)
    {
      lcd.write(ch);
      curSorTracker++;
    }
  }
}

ISR(TIMER1_OVF_vect)
{
  TCNT1 = 0xC2F7;
  timer_s -= 1;  // decrease countdown by 1 second every time
  if (timer_s <= 0) 
  {
    Time_out();
  }
}

void Time_out() 
{
  lcd.setCursor(0, 1);
  lcd.print("Time Out!");
  while(1);
  // Print on lcd the time is over and spin in an infinite loop
}