LiquidCrystal library and timers don't seem to be compatible

Question

If I set timer2 to Compare Match mode, my LCD display stops working. If I use Overflow Interrupt it works fine.

Here's my test code. I have a loop() function which does its thing, and a timer interrupt which increments another variable, just to show it's working.

#include <LiquidCrystal.h>
#include <avr/interrupt.h>
#include <avr/io.h>

LiquidCrystal lcd( 8, 9, 4, 5, 6, 7 );   //Pins for the freetronics 16x2 LCD shield. LCD:

int i;
volatile int j;
void setup()
{
  i=0;
  j=0;
  //button adc input
  pinMode( A0, INPUT ); //ensure A0 is an input
  digitalWrite( A0, LOW ); //ensure pullup is off on A0
  //lcd backlight control
  pinMode( 10, OUTPUT ); //D10 is an output
  digitalWrite( 10, HIGH ); //backlight control pin D10 is high (on)
  //set up the LCD number of columns and rows:
  lcd.begin( 16, 2 );

  // setup timer interrupt
  cli(); //disable global interrupts
  TCCR1A = 0;   //set tccr1a and tccr1b to 0
  TCCR1B = 0;
  //set cs10 and cs12 bit so timer runs at clock iSpeed/ 1024
  TCCR1B |= (1 << CS10);
  TCCR1B |= (1 << CS12);

  // Compare Match Mode **Doesn't work**:
  //set compare match register
  OCR1A = 15624;
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // enable Timer1 compare interrupt
  TIMSK1 = (1 << OCIE1A);

  // Overflow mode **Works**
  // enable overflow interrupt
  // TIMSK1 = (1 << TOIE1);

  //enable global interrupt
  sei();
}
void loop()
{
  lcd.setCursor( 0, 0 ); //top left
  lcd.print(i);
  lcd.setCursor(0,1); //bottom left
  lcd.print(j);
  delay(500);
  lcd.clear();
  i++;
}

ISR(TIMER1_OVF_vect)
{
  j++;
}

If I use overflow interrupts it works fine, the loop() counter is displayed on the top line and the interrupt counter on the second, but by using overflow interrupts I'm limited to the whatever timing the various prescalers give me.

If I use compare match the LCD display just shows faint flickering squares on its first line, like it's doing something but too quickly for the LCD to display.

I looked at the source for liquidcrystal.cpp, and it doesn't seem to use any timer or interrupts, so I'm completely mystified.

Answer 1

Dang, it was a bit of a noobish error. I had the interrupt handler setup for the overflow interrupt, but when I swapped the code in for the Compare Match interrupt I didn't make a handler for it. So since there's an interrupt without a handler it seems it just resets, hence the flashy-flashy garbage I was seeing on the LCD.

Adding this fixed the problem:

ISR(TIMER1_COMPA_vect)
{
  j++;
}