Stuck before delay in sub program - c++

Question

I have the task to program two timers, where I display something on my LCD-Display. I have a matrix keyboard where I can type in some basic things like numbers and some letters with this code:
void keyboard_read()
{
digitalWrite(s1, LOW);
digitalWrite(s2, HIGH);
digitalWrite(s3, HIGH);
digitalWrite(s4, HIGH);
if(digitalRead(r1) == LOW){lcd.print("1"); delay(200);k++;feld[k]=1;}
if(digitalRead(r2) == LOW){lcd.print("4"); delay(200);k++;feld[k]=4;}
if(digitalRead(r3) == LOW){lcd.print("7"); delay(200);k++;feld[k]=7;}
if(digitalRead(r4) == LOW){lcd.print("A"); delay(200);k++;feld[k]='A';}
digitalWrite(s1, HIGH);
digitalWrite(s2, LOW);
digitalWrite(s3, HIGH);
digitalWrite(s4, HIGH);
if(digitalRead(r1) == LOW){lcd.print("2"); delay(200);k++;feld[k]=2;}
if(digitalRead(r2) == LOW){lcd.print("5"); delay(200);k++;feld[k]=5;}
if(digitalRead(r3) == LOW){lcd.print("8"); delay(200);k++;feld[k]=8;}
if(digitalRead(r4) == LOW){lcd.print("0"); delay(200);k++;feld[k]=0;}
digitalWrite(s1, HIGH);
digitalWrite(s2, HIGH);
digitalWrite(s3, LOW);
digitalWrite(s4, HIGH);
if(digitalRead(r1) == LOW){lcd.print("3"); delay(200);k++;feld[k]=3;}
if(digitalRead(r2) == LOW){lcd.print("6"); delay(200);k++;feld[k]=6;}
if(digitalRead(r3) == LOW){lcd.print("9"); delay(200);k++;feld[k]=9;}
if(digitalRead(r4) == LOW){lcd.print("B"); delay(200);k++;feld[k]='B';}
digitalWrite(s1, HIGH);
digitalWrite(s2, HIGH);
digitalWrite(s3, HIGH);
digitalWrite(s4, LOW);
if(digitalRead(r1) == LOW){lcd.print("F"); delay(200);k++;feld[k]='F';}
if(digitalRead(r2) == LOW){lcd.print("E"); delay(200);k++;feld[k]='E';}
if(digitalRead(r3) == LOW){lcd.print("D"); delay(200);k++;feld[k]='D';}
if(digitalRead(r4) == LOW){lcd.print("C"); delay(200);k++;feld[k]='C';}
}
When I type in the first Timer (Timer0) keyboard_read() works just fine, but when I go into the second Timer (Timer2) keyboard_read() stops at the delay command. When I tried to put keyboard_read() in the loop() it still stops at the delay command (There isn't this command in the loop at the moment).
Code for Timer0:
ISR(TIMER0_COMPA_vect) //Durchlaufendes Menü
{
cnt0++;
//Tastatureingabe_______________________________________________________________
keyboard_read();
if(feld[k]=='A') //Abfrage nach AutoStart
{
lcd.clear(); lcd.setCursor(0,0); lcd.print("AutoStart");
TCCR0B = 0x00; //Timer0 ausschalten
TCCR2B = 0x07; //Timer2 einschalten
}
else if(feld[k]=='E') //Abfrage nach Einstellungen
{
lcd.clear(); lcd.setCursor(0,0); lcd.print("Einstellungen");
TCCR0B = 0x00; //Timer0 ausschalten
//TCCR1B = 0x00; //Timer1 einschalten (not declared yet)
k=0;
}
else if((feld[k]!=NULL)) //Falsche Eingabe Abfrage
{
lcd.clear(); lcd.setCursor(0,0); lcd.print("Falsche Eingabe");
} //other things are not necessary
Code for Timer2:
ISR(TIMER2_COMPA_vect) //Ausgabe der Parameter
{
cnt2++;
loop();
//Tastatureingabe_________________________________________________________
keyboard_read();
if(feld[k]=='B') //Abfrage nach AutoStart
{
lcd.clear(); lcd.setCursor(0,0); lcd.print("Zuruek");
TCCR0B = 0x0D; //Timer0 einschalten
TCCR2B = 0x00; //Timer2 ausschalten
}
else if((feld[k]!=NULL))
{
lcd.clear(); lcd.setCursor(0,0); lcd.print("Falsche Eingabe");
}
Do I have to change something in the keyboard_read() sub program or the Timers to make it work?
Thanks in advance for any help.

Answer 1

You can read long article about interrupts: How do interrupts work on the Arduino Uno and similar boards? on Arduino SE by Nick Gammon
And in short:
Interrupt must be as short as possible. If you need delay, you are doing it wrong and you can handle it in loop later. Just set some flag (or you can use COMPA overflow flag directly without its interrupt, you just have to check this flag and clear it by writing logic 1 into it)
You can't use Arduinos delay as it needs Timer/Counter 0 overflow interrupt running. And all interrupts are blocked in the ISR handler automatically. So you are waiting for changing millis that never happen.
Also you can't use anything else that could relly on another interrupt. For example Serial.write/print works until the send buffer gets filled and then deadlock will appear.
Why are you calling loop()? It'll return back to the interrupted code right after the handler finishes the job.