Working Principle of AVR Timer

Take AVR mega16 as an example, it has three registers, timer0, timer1 and timer2, T0 and T2 are 8-bit timers, T1 is a 16-bit register, T2 is an asynchronous timer, and all three timers can be used to generate PWM.

Let's take timer T0 as an example to briefly introduce the operation of the timer. T0 has three registers that can be accessed by the CPU, TCCR0, TCNT0, and OCR0. Let's take a look at a timer initialization program generated by ICC.

CODE:

//TIMER0 initialize - prescale:8

// WGM: Normal

// desired value: 1KHz

// actual value: 1.000KHz (0.0%)

void timer0_init(void)

{

TCCR0 = 0x00; //stop

TCNT0 = 0x83; //set count

OCR0 = 0x7D; //set compare

TCCR0 = 0x02; //start timer

}

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TCCR0 is a control register used to control the working mode details of the timer;

TCNT0 is the T/C register, and its value is increased or decreased by one in each working cycle of the timer to realize the timing operation. The CPU can read and write TCNT0 at any time.

OCR0: Output compare register, it contains an 8-bit data, which is continuously compared with the counter value TCNT0. The match event can be used to generate an output compare interrupt, or to generate a waveform on the OC0 pin.

Here is the simplest mode. TCNT keeps adding one, reaches the maximum value 0xFF and then clears to enter the next count, in the above program.

TCCR0=0x00; turn off the clock source of T0 and the timer stops working.

TCNT0 = 0x83; Set the initial value of the T/C register and let the timer start timing or counting from TCNT0 at 0x83.

OCR0 = 0x7D; Set the value of the compare match register, which is not used in this program.

TCCR0 = 0x02; Select the clock source, which is the clock divided by 8. After setting, the timer starts working.

After initialization, the timer starts working. TCNT0 increases by one at each timer clock. When TCNT0 is equal to the value of OCR0, OCF0 in the T/C interrupt flag register - TIFR is set. If OCIE0 in TIMSK is 1 (that is, T0 compare match interrupt is allowed) and global interrupts are allowed, the compare match interrupt will run. In the interrupt program, TCNT0 and OCR0 can be operated to adjust the timer.

TCNT0 continues to increase by one. When it reaches 0xFF, TOV0 in the T/C interrupt flag register - TIFR is set. If TOIE0 in TIMSK is 1 (that is, T0 overflow interrupt is allowed) and global interrupts are allowed, the overflow interrupt will run. TCNT0 and 0CR0 can be operated in the interrupt program to adjust the timer.

The timer-related registers include SREG and TIMSK. Bit 1 of the former controls the global mid-segment enable, and bit 1 (OCIE0) and bit 0 (TOIE0) of the latter respectively control the compare match interrupt and overflow compare match interrupt enable.

In the actual process, the operation of timer-related registers is very flexible. The value of TCNT0 can be modified in the overflow interrupt, and the value of OCR0 can also be modified in the interrupt. In the following experiments, we will talk about how to use timer 1 to modify OCR1A to achieve 1S precise timing.

The master can only lead you to the door, but the practice depends on yourself. This is the basic principle of the timer. If you want to know more about the timer, please read the data manual.

Timing formula: Time=PRE*(MAX-TCNT0+1) /F_cpu unit S, where PRE is the frequency division number, which is 8 in this example, MAX is the maximum value 255, TCNT0 is the value at the time of initialization, which is 0x83 (decimal 131) in this example, and T_cpu is the system clock frequency, which is 1000000 in this example.

In this example, the timing time is: Time=8*(255-131+1)/1000000=0.001 S, which is 1ms, 1Khz. It can be seen that if the crystal oscillator is selected as 8M, the timing time becomes 0.000125S, that is, the larger the crystal oscillator, the shorter the timing time, and the larger the pre-scaling frequency, the longer the timing.

If you select 1ms during setup, you will get the following result, which is the same as 1Khz above.

CODE:

//TIMER0 initialize - prescale:8

// WGM: Normal

// desired value: 1mSec

// actual value: 1.000mSec (0.0%)

void timer0_init(void)

{

TCCR0 = 0x00; //stop

TCNT0 = 0x83; //set count

OCR0 = 0x7D; //set compare

TCCR0 = 0x02; //start timer

}

//ICC-AVR application builder : 2007-6-9 0:33:58
// Target : M16
// Crystal: 1.0000Mhz
// Purpose: Demonstrate the working principle of timer
// Author: Gu Xin
// AVR and virtual instrument [url]http://www.avrvi.com[/url]
#include
#include

void port_init(void)
{
PORTA = 0x00;
DDRA = 0x03; //PA0 PA1 output
PORTB = 0x00;
DDRB = 0xFF; //PB output
PORTC = 0x00; //m103 output only
DDRC = 0x00;
PORTD = 0x00;
DDRD = 0x00;
}

//TIMER0 initialize - prescale:8
// WGM: Normal
// desired value: 1KHz
// actual value: 1.000KHz (0.0%)
void timer0_init(void)
{
TCCR0 = 0x00; //stop
TCNT0 = 0x83; //set count
OCR0 = 0x7D; //set compare
TCCR0 = 0x02; //start timer
}

//Compare match interrupt
#pragma interrupt_handler timer0_comp_isr:20
void timer0_comp_isr(void)
{
//compare occured TCNT0=OCR0
if(OCR0==0x7D) //Adjust 0x7D
{
OCR0=0x7F;
}
else
{
OCR0=0x7D;
}
PORTA ^= 0x01; //PA0 inverted
}

//Overflow interrupt
#pragma interrupt_handler timer0_ovf_isr:10
void timer0_ovf_isr(void)
{
TCNT0 = 0x83; //reload counter value
PORTA ^= 0x01; //PA0 inverted
}

//call this routine to initialize all peripherals
void init_devices(void)
{
//stop errant interrupts until set up
CLI(); //disable all interrupts
port_init();
timer0_init();

MCUCR = 0x00;
GICR = 0x00;
TIMSK = 0x03; //timer interrupt sources enable timer zero match and overflow interrupt
SEI(); //re-enable interrupts
//all peripherals are now initialized
}

void main(void)
{
init_devices();
PORTA=0x00;
while(1)
{
PORTB = TCNT0; //TCNT0 can be read at any time
}
}