Session 8: Timer Interrupt

Goal

• TIMER0: Kight pattern

• TIMER2: Shift pattern

• INT0: Control the speed

• INT1: Stop

Bonus

Connect LEDs and interrupts

Like session_7 we should connect 8 LEDs to PB. We connect two interrupts to PD2 and PD3. So the result would be something like below:

Progam a simple shift pattern on LED

First step is to program a simple shift pattern on LED that we learned
on session_7.

Add external intrrupt to it

Then we add an External Interupt to it which we also learned in session_7

Time Interrupt

Time Interrupt has a timer, when it reaches to a specific condition, an interrupt would occur.

Atmega32 has 3 main timers.

• Timer0: 8-bit

• Timer1: 16-bit

• Timer2: 8-bit

We will be working with Timer0.

How timer interrupt works

The Timer Interrupt has a counter that, each time a clock reaches it, it would increase. We can confing Timer Interrupt to send an Interrupt in this two conditions:

• When it overflows

• When it reaches to an specefic number

We can access all of them by some registers.

• Counter: TCNT

• Condition: OCR

if the mode is on overflow there would be an interrupt each time the counter exceeds 255. You can see in the image below that every time we reach 256, an interrupt would be sent.

If we put it on compare mode, and set OCR to 100, we would have an image like below:

As you can see the image above, still there would be an interrupt every 256 clock. We can set a register to prevent that from happening. If we reach to OCR then TCNT becomes 0 again. It would like the image below.

By knowing that we can calulate how many times an interrupt would be sent. For example, if our microcontroller works with 8.0Mhz frequency, which is 0.125us, and we set OCR to 100, we will be having an interrupt every 12.5us.

We can control when to send a clock to a Timer Interrupt as well. We call that pre-scaler. pre-scaler tells the microcontroller on how many clock of the microcontroller, timer should get a clock. It can be set only to this values: 1 8 64 256 1024. For example, if we set our pre-scaler to 8 and our microcontroller works with 8.0Mhz frequency, timer will work with 1.0Mhz frequency.

TIMSK

TIMSK stands for Timer/Counter Interrupt Mask Register. OCIE stands for Output Compare Match Interrupt Enable. TOIE stands for Timer/Counter0 Overflow Interrupt Enable.

bit	7	6	5	4	3	2	1	0
name	OCIE2	TOIE2	TICIE1	OCIE1A	OCIE1B	TOIE1	OCIE0	TOIE0
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

We need to compare the value of timer with OCR0 register so we should write a code like below:

TIMSK |= 1 << OCIE0

TCCR0

TCCR0 stands for Timer/Counter Clock Sources. To work with Timer0, we should initialize some indexes of this register. Table below shows all the available indexes.

bit	7	6	5	4	3	2	1	0
name	FOC0	WGM00	COM01	COM00	WGM01	CS02	CS01	CS00
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

WGM00	WGM01	Timer0 mode selection bit
0	0	Normal
0	1	CTC (Clear timer on Compare Match)
1	0	PWM, Phase correct
1	1	Fast PWM

We need to compare with the OCR0 register, so we set WMG01 index to 1.

TCCR0 |= 1 << WGM01

CS02	CS01	CS00	Description
0	0	0	No clock source (Timer/Counter stopped).
0	0	1	clkI/O/(No prescaling)
0	1	0	clkI/O/8 (From pre-scaler)
0	1	1	clkI/O/64 (From pre-scaler)
1	0	0	clkI/O/256 (From pre-scaler)
1	0	1	clkI/O/1024 (From pre-scaler)
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.

now we should our pre-scaler which controls how much frequency should reach to the timer0. For example if our chip is running with 8Mhz, if we put our pre-scaler value to 8, the timer0 would get 1Mhz of frequency. We can control pre-scaler as shown in the table above. For example, the code below puts the pre-scaler to 8.

TCCR0 |= 1 << CS01;

TCNT0

TCNT stands for Timer/Counter Register. Any time a clock reaches the timer, this increments. It has only 8bits so it can only reach to 255.

OCR0

OCR stands for Output Compare Register. The value of TCNT0 gets compare with OCR0. If TCNT0 reaches the value of OCR0, an interrupt would occur.

Timer Interrupt function

We can define a function for timer interrupt, like below:

• TIMER0_COMP

  • TIM0: Timer 0

  • COMP: Compare

• TIMER0_OVF

  • TIM0: Timer 0

  • OVF: Overflow

ISR(TIMER0_COMP_vect)
{
    // your code here
}

Change clock frequency on SimulIDE

You can change the clock frequency of ATmega32 on SimulIDE by taking the steps below:

• Right click on ATmega32

• Select properties

• In the frequency section you can change the frequency

In this session, we want it to have the frequency of 8Mhz.

Change clock frequency on PlatformIO

To change clock frequency on PlatformIO, you should add this like to your platformio.ini file:

board_build.f_cpu = 8000000L

Replace delay with Time Interrupt

Now we know when a Timer interrupt happens. So we can put the code that we wrote for the shifter on it. Then we can remove the delay.

Make the Shifter pattern to Kight pattern

We only need direction and some ifs to write that.

Setup another timer and put a Shifter pattern on it

Setup another timer for example TIMER2 and put a simple shifter on it. to show the result of both of them we can | the result of both in PORTB.