I am new to microcontroller. The following code measures the period of a square wave. I have marked some lines which I haven't understood. The code is as follows:
#include <avr/io.h>
#include <avr/interrupt.h>
ISR(TIMER1_CAPT_vect)
{
int counter_value = ICR1; //16 bit value
PORTB = (counter_value >> 7); // What has been done here?
TCNT1 = 0; // why this line?
}
int main(void)
{
DDRB = 0xFF;
TCCR1A = 0x00;
TCCR1B = 0b11000010;
TIMSK = 0b00100000;
sei();
while(1);
cli();
}
What has actually been done in those lines?
ISR(TIMER1_CAPT_vect)
{
int counter_value = ICR1; //16 bit value
PORTB = (counter_value >> 7); // What has been done here?
PORTB is a set of 8 output lines. Presumably, they are connected by a bus to some device you haven't mentioned. Maybe even a set of LEDS to display a binary number.
The result from the counter is 16 bits. To get the most significant bits, shift the result to the right to discard the less significant bits. (This operation loses precision, but you only have 8 bits of output, not 16.) As to why the shift is only 7 instead of 8, or why the unsigned value of the counter is saved as a signed int first, I don't know. I suspect it is a mistake. I would have done PORTB = (ICR1 >> 8); instead.
TCNT1 = 0; // why this line?
Since we have recorded the time of the capture and sent it out PORTB, we now want to reset the timer for the next capture.
}