Timer Input Capture Mode in AVR ATmega16/ATmega32

Introduction

Input capture function is used in many applications such as:

Pulse width measurement
Period measurement
Capturing the time of an event

In AVR ATmega32, Timer1 can be used as an input capture to detect and measure events happening outside the microcontroller.

Upon detection of a defined event i.e. rising edge or falling edge on ICP pin (PORTD.6), TCNT1(Timer / Counter register) value is loaded into the ICR1 (input capture) register and the ICF1 flag will get set.

Programming

To program, first let us see TCCR1B (Timer Counter Control Register B)

TCCR1B: Timer Counter Control Register B

Bit 7 – ICNC1: Input Capture Noise canceller

Setting this bit activates noise canceller. It causes a delay of 4 clock cycles as it considers a change only if it persists for at least 4 successive system clocks.

Bit 6 – ICES1: Input Capture Edge select

Select edge detection for input capture function.

0 = Capture on falling edge

1 = Capture on rising edge

Bit 4: 3 – WGM13 : WGM12: Timer1 Mode select

These bits are used for mode selection like Normal mode, PWM mode, CTC mode etc. here we will select normal mode, so set these bits to zero.

Bit 2: 0 – CS12: CS10:Timer1 Clock Select

Steps to Program

Initialize the TCCR1A and TCCR1B for proper timer mode (any mode other than 8, 10, 12, 14), to select the edge (Positive or Negative).
Monitor the ICF1 flag in TIFR register to see if edge is arrived. Upon the arrival of the edge, the TCNT1 value is loaded in to ICR1 register automatically by controller.

Note: Input capture pin, which is PORTD.6, has one more function i.e. output of analog comparator. We can use ACIC bit from ACSR register, to make this pin function as the ‘analog comparator output’ by setting it to logic HIGH. Otherwise this pin remains as ICP pin by default after power on or reset. So we don’t need to define this Register here.

Program

Assuming that the clock pulses are fed into the pin ICP1, following program will read TCNT1 value at every rising edge and place the result on PORTA and PORTB./*

#include “avr/io.h”
int main ( )
{
unsigned int t;
DDRA = 0xFF;
DDRB = 0xFF;
PORTD = 0xFF;
TCCR1A = 0;
TIFR = (1<<ICF1); /* clear input capture flag */
TCCR1B = 0x41; /* capture on rising edge */

while ((TIFR&(1<<ICF1)) == 0); /* monitor for capture*/
t = ICR1;
TIFR = (1<<ICF1); /* clear capture flag */

while ((TIFR&(1<>8;

while (1);
return 0;
}

Measuring Frequency and Duty Cycle

Program to measure the frequency and duty cycle and displaying it on the LCD16x2

#define F_CPU 8000000UL
#include
#include
#include
#include
#include “LCD_16x2_H_file.h”

int main ( )
{
unsigned int a,b,c,high,period;
char frequency[14],duty_cy[7];

LCD_Init();
PORTD = 0xFF; /* Turn ON pull-up resistor */

while(1)
{
TCCR1A = 0;
TCNT1=0;
TIFR = (1<<ICF1); /* Clear ICF (Input Capture flag) flag */

TCCR1B = 0x41; /* Rising edge, no prescaler */
while ((TIFR&(1<<ICF1)) == 0);
a = ICR1; /* Take value of capture register */
TIFR = (1<<ICF1); /* Clear ICF flag */

TCCR1B = 0x01; /* Falling edge, no prescaler */
while ((TIFR&(1<<ICF1)) == 0);
b = ICR1; /* Take value of capture register */
TIFR = (1<<ICF1); /* Clear ICF flag */

TCCR1B = 0x41; /* Rising edge, no prescaler */
while ((TIFR&(1<<ICF1)) == 0);
c = ICR1; /* Take value of capture register */
TIFR = (1<<ICF1); /* Clear ICF flag */

TCCR1B = 0; /* Stop the timer */

if(a<b && b<c) /* Check for valid condition,
to avoid timer overflow reading */
{
high=b-a;
period=c-a;

long freq= F_CPU/period;/* Calculate frequency */

/* Calculate duty cycle */
float duty_cycle =((float) high /(float)period)*100;
ltoa(freq,frequency,10);

itoa((int)duty_cycle,duty_cy,10);

LCD_Command(0x80);
LCD_String(“Freq: “);
LCD_String(frequency);
LCD_String(” Hz “);

LCD_Command(0xC0);
LCD_String(“Duty: “);
LCD_String(duty_cy);
LCD_String(” % “);

}

else
{
LCD_Clear();
LCD_String(“OUT OF RANGE!!”);
}
_delay_ms(50);
}

}