This tutorial will teach you about the 8 and 16 bit timers on an ATmega168 microcontroller. Because the ATmega168 is very similar to the ATmega48, ATmega88 and ATmega328, the examples should also work on these. For other AVR microcontrollers the general principles will apply but the specifics may vary.

Timer interrupts on an atmega168 microcontroller

What are the 8 and 16 bit timers?

Consider a wrist watch. At regular intervals it ticks and the hand moves to the next number. At any point in time you can read the accumulated count (the time) and once the time reaches a certain threshold, an event occurs (the alarm rings). Timers on AVR microcontrollers are a little like this.

The ATmega168 has two 8-bit and one 16-bit timers. This means that there are 3 sets of counters, each with the ability to count at different rates. The two 8-bit counters can count to 255 whilst the 16 bit counter can count to 65,536.

When the counters reach certain thresholds we can trigger interrupts, which cause Interrupt Service Routines (ISRs) to be executed. Timers are very handy because they allow operations to run automatically, independently of the main processing thread.

8 bit timer registers

The ATmega168 has 2, 8-bit timers: Counter0 and Counter2. Each of these timers are controlled by the following registers.

Counter0 Counter2 Description
TCCR0A TCCR2A Timer/Counter Control Register A
TCCR0B TCCR2B Timer/Counter Control Register B
TCNT0 TCNT2 Timer/Counter Register
OCR0A OCR2A Output Compare Register A
OCR0B OCR2B Output Compare Register B
TIMSK0 TIMSK2 Timer/Counter Interrupt Mask Register
TIFR0 TIFR2 Timer/Counter Interrupt Flag Register

We'll focus on Counter0 and the most commonly used registers.

Each counter has 2 thresholds. for Counter0 these are stored in registers OCR0A and OCR0B. When these threshold are reached "something" happens. This "something" is defined in TCCR0A, TCCR0B and TIMSK0. TCCR0B also allows you to set the rate at which the timer is updated.

TCCR0A and TCCR0A are shown below.

bit 7 6 5 4 3 2 1 0
TCCR0A COM0A1 COM0A0 COM0B1 COM0B0 - - WGM01 WGM00
Read/Write R/W R/W R/W R/W R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
bit 7 6 5 4 3 2 1 0
TCCR0B FOC0A FOC0B - - WGM02 CS02 CS01 CS00
Read/Write W W R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0

WGM02, WGM01 and WGM00 control the counter's mode as shown in the table below.

Mode WGM02 WGM01 WGM00 Description
0 0 0 0 Normal
1 0 0 1 PWM, Phase Correct
2 0 1 0 Clear Timer on Compare (CTC)
3 0 1 1 Fast PWM
4 1 0 0 Reserved
5 1 0 1 PWM, Phase Correct
6 1 1 0 Reserved
7 1 1 1 Fast PWM

We'll focus on the normal and CTC modes. A detailed discussion of timer modes can be found in section 14.7 of the ATmega168 datasheet.

COM0A1, COM0A0, COM0B1 and COM0B0 control the behavior of the OC0A (PD6) and OC0B (PD5) pins. These pins can be controlled from the Output Compare Registers (OCR0A/OCR0B). The settings depend on the modes being used and is outside the scope of this tutorial. Section 14.9.1 of the ATmega168 datasheet describes these bits in detail.

An ATmega168 with default fuses runs at 8MHz with the system clock prescaler enabled. This means that the timer can be updated, up to 8,000,000 times per second. By setting CS02, CS01 and CS00 we can slow down this rate, as shown here.

CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped)
0 0 1 Clock(No prescaling)
0 1 0 Clock/8 (From prescaler)
0 1 1 Clock/64 (From prescaler)
1 0 0 Clock/256 (From prescaler)
1 0 1 Clock/1024 (From prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.

TIMSK0 controls which interrupts are enabled. Each counter has 3 interrupts, one for each Output Compare Register (threshold) and one for overflow. The full list of available interrupts can be found in the AVR libc <interrupt.h> Interrupts Documentation under the section labelled "Choosing the vector: Interrupt vector names".

bit 7 6 5 4 3 2 1 0
TIMSK0 - - - - - OCIE0B OCIE0A TOIE0
Read/Write R R R R R R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0

8 bit timer example

In this example we run run a sweep of the 8 red LEDs on the main routine. We will then blink the green LED on/off 4 times every second. The circuit diagram and source code is shown below.

Example circuit for atmega168 interrupts
  1. #include <avr/io.h>
  2. #include <util/delay.h>
  3. #include <avr/interrupt.h>  
  4.  
  5. #define green_led_on()  PORTC |= _BV(0)
  6. #define green_led_off()  PORTC &= ~_BV(0)
  7. #define green_led_is_on() bit_is_set(PORTC,0)
  8.  
  9. int main (void)
  10. {
  11.     DDRB  = 0b11111111;   // All outputs
  12.     DDRC  = 0b01111111;   // All outputs (Although we will just use PC0 )
  13.  
  14.     TIMSK0 = _BV(OCIE0A);  // Enable Interrupt TimerCounter0 Compare Match A (SIG_OUTPUT_COMPARE0A)
  15.     TCCR0A = _BV(WGM01);  // Mode = CTC
  16.     TCCR0B = _BV(CS02) | _BV(CS00);   // Clock/1024, 0.001024 seconds per tick
  17.     OCR0A = 244;          // 0.001024*244 ~= .25 SIG_OUTPUT_COMPARE0A will be triggered 4 times per second.
  18.    
  19.     sei();
  20.  
  21.     while(1)
  22.     {
  23.         sweep();
  24.     }
  25. }
  26.  
  27. void sweep()
  28. {
  29.     PORTB = 0b10000000;
  30.     for (int i=0;i<8;i++)
  31.     {
  32.         _delay_ms(100);
  33.         PORTB >>= 1;
  34.     }
  35. }
  36.  
  37. ISR(SIG_OUTPUT_COMPARE0A)
  38. {
  39.     if (green_led_is_on())
  40.         green_led_off();
  41.     else
  42.         green_led_on();
  43. }

Line 15 sets the CTC mode. This ensures that the counter is reset when it reaches OCR0A. After we have setup the timer registers, we call sei() to enable the global registers.

On line 37, you will see "ISR(SIG_OUTPUT_COMPARE0A)". This is the interrupt service routine and will get called each time the interrupt is triggered.

8 bit timer example 2 - Dual interrupts

In this example we will use TimerCounter0 Compare Match A and Match B interrupts. We we turn the LED on at Match B and off at Match A. This will produce the following waveform.

Timing diagram for timer interrupt example
  1. #include <avr/io.h>
  2. #include <util/delay.h>
  3. #include <avr/interrupt.h>  
  4.  
  5. #define green_led_on()  PORTC |= _BV(0)
  6. #define green_led_off()  PORTC &= ~_BV(0)
  7.  
  8. int main (void)
  9. {
  10.     DDRB  = 0b11111111;   // All outputs
  11.     DDRC  = 0b01111111;   // All outputs (Although we will just use PC0 )
  12.  
  13.     TIMSK0 = _BV(OCIE0A) | _BV(OCIE0B);  // Enable Interrupt TimerCounter0 Compare Match A & B (SIG_OUTPUT_COMPARE0A/SIG_OUTPUT_COMPARE0A)
  14.     TCCR0A = _BV(WGM01);               // Mode = CTC
  15.     TCCR0B = _BV(CS02) | _BV(CS00);    // Clock/1024, 0.001024 seconds per tick
  16.     OCR0A = 244;                       // 0.001024*244 ~= .25 SIG_OUTPUT_COMPARE0A will be triggered 4 times per second.
  17.     OCR0B = 220;                       // 0.001024*220 ~= .225 SIG_OUTPUT_COMPARE0B will be triggered 25ms before SIG_OUTPUT_COMPARE0A
  18.    
  19.     sei();
  20.  
  21.     while(1)
  22.     {
  23.         sweep();
  24.     }
  25. }
  26.  
  27. void sweep()
  28. {
  29.     PORTB = 0b10000000;
  30.     for (int i=0;i<8;i++)
  31.     {
  32.         _delay_ms(100);
  33.         PORTB >>= 1;
  34.     }
  35. }
  36.  
  37. ISR(SIG_OUTPUT_COMPARE0A)
  38. {
  39.     green_led_off();
  40. }
  41.  
  42. ISR(SIG_OUTPUT_COMPARE0B)
  43. {
  44.     green_led_on();
  45. }

16 bit timer

The ATmega168 has a single 16 bit timer, which is referred to as Counter1. It works like the 8 bit timer, except the counter has more bits in it. This intervals to be set with longer duration and greater precision. The registers used by this timer are:

Counter1 Description
TCCR1A Timer/Counter 1 Control Register A
TCCR1B Timer/Counter 1 Control Register B
TCCR1C Timer/Counter 1 Control Register C
TCNT1H Timer/Counter 1 High Register
TCNT1L Timer/Counter 1 Low Register
OCR1AH Output Compare Register 1 A High
OCR1AL Output Compare Register 1 A Low
OCR1BH Output Compare Register 1 B High
OCR1BL Output Compare Register 1 B Low
ICR1H Input Capture Register 1 High
ICR1L Input Capture Register 1 Low
TIMSK1 Timer/Counter Interrupt Mask Register
TIFR1 Timer/Counter Interrupt Flag Register

In some ways this list looks like the registers used by the 8 bit timers. We will now examine the important differences.

TCCR1A, TCCR1B and TCCR1C play a similar role to TCCR0A and TCCR0B. These are shown below.

bit 7 6 5 4 3 2 1 0
TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 - - WGM11 WGM10
Read/Write R/W R/W R/W R/W R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
bit 7 6 5 4 3 2 1 0
TCCR1B ICNC1 ICES1 - WGM13 WGM12 CS12 CS11 CS10
Read/Write R/W R/W R R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
bit 7 6 5 4 3 2 1 0
TCCR1C FOC1A FOC1B - - - - - -
Read/Write R/W R/W R R R R R R
Initial Value 0 0 0 0 0 0 0 0

Some of the differences include

  • 4 mode bits instead of 3 (ie more modes)
  • The Force Output Compare bits are in TCCR1C
  • Input Capture Noise Canceler bit in TCCR1B (outside the scope of this tutorial)
  • Input Capture Edge Select in TCCR1B (outside the scope of this tutorial)

TCNT1H and TCNT1L are similar to TCNT0, but being a 16 bit counter they are split across 2 registers. Similarly with OCR1AH, OCR1AL, OCR1BH and OCR1BL.

ICR1H and ICR1L don't have any equivalent in the 8 bit timers and allow you to capture the timer value on certain events.

16 Bit example

This example is similar to the previous one. Because we are using the 16 bit timer, we can increase the cycle time.

  1. #include <avr/io.h>
  2. #include <util/delay.h>
  3. #include <avr/interrupt.h>  
  4.  
  5. #define green_led_on()  PORTC |= _BV(0)
  6. #define green_led_off()  PORTC &= ~_BV(0)
  7.  
  8. int main (void)
  9. {
  10.     DDRB  = 0b11111111;   // All outputs
  11.     DDRC  = 0b01111111;   // All outputs (Although we will just use PC0 )
  12.  
  13.     TIMSK1 = _BV(OCIE1A) | _BV(OCIE1B); // Enable Interrupt Timer/Counter1, Output Compare A & B (SIG_OUTPUT_COMPARE1A/SIG_OUTPUT_COMPARE1B)
  14.     TCCR1B = _BV(CS12) | _BV(CS10) | _BV(WGM12);    // Clock/1024, 0.001024 seconds per tick, Mode=CTC
  15.     OCR1A = 1954;                       // 0.001024*1954 ~= 2 SIG_OUTPUT_COMPARE1A will be triggered every 2 seconds
  16.     OCR1B = 1929;                       // 0.001024*1929 ~= 1.975 SIG_OUTPUT_COMPARE1B will be triggered 25ms before SIG_OUTPUT_COMPARE1A
  17.    
  18.     sei();
  19.  
  20.     while(1)
  21.     {
  22.         sweep();
  23.     }
  24. }
  25.  
  26. void sweep()
  27. {
  28.     PORTB = 0b10000000;
  29.     for (int i=0;i<8;i++)
  30.     {
  31.         _delay_ms(100);
  32.         PORTB >>= 1;
  33.     }
  34. }
  35.  
  36. ISR(SIG_OUTPUT_COMPARE1A)
  37. {
  38.     green_led_off();
  39. }
  40.  
  41. ISR(SIG_OUTPUT_COMPARE1B)
  42. {
  43.     green_led_on();
  44. }

On line 15 and 16, where we set the value for the Output Compare Registers, we don't need to set value for the high and low registers. The AVR Libc
library
abstracts these as a single 16 bit value.

More Information

I hope you've enjoyed this tutorial. Timers are a complex subject and I have tried to keep them simple by just looking at a slice of what they can do. I encourage to read the documentation and experiment. I've listed 2 good resources below.

AVR libc <avr/interrupt.h> Interrupts Documentation
ATmega48/88/168/328 datasheet