Why does timer1 not generate correct period on CTC mode with a resolution of 5us?

Question

I have defined my code to generate an interrupt over each 5μs on CTC mode using timer1 but the output shows a time of ~15μs.

Following the equation to set the counting number to achieve the desired time

(# timer counts + 1) = (target time) / (timer resolution)

I've got OCR1A = 79 for a desired time of 5μs using no prescaler and a source clock of 16MHz on Arduino Uno.

My code is as follows:

// avr-libc library includes
#include <avr/io.h>
#include <avr/interrupt.h>

#define LEDPIN 9

void setup()
{
    pinMode(LEDPIN, OUTPUT);
    digitalWrite(LEDPIN, HIGH);

    // initialize Timer1

    cli();          // disable global interrupts

    TCCR1A = 0;     // set entire TCCR1A register to 0
    TCCR1B = 0;     // same for TCCR1B

    // set compare match register to desired timer count:
    OCR1A = 79;

    // turn on CTC mode:
    TCCR1B |= (1 << WGM12);

    // Set CS10 bit for no prescaler:
    TCCR1B |= _BV(CS10);

    // enable timer compare interrupt:
    TIMSK1 |= (1 << OCIE1A);

    sei();          // enable global interrupts

}

void loop () {}

ISR(TIMER1_COMPA_vect)
{
  digitalWrite(LEDPIN, !digitalRead(LEDPIN));  
}

This is the output from my logic analyzer:

Is it a limitation of the Arduino or am I missing something?

My idea is to generate a PWM manually to control a servo motor using timer1 and precisely increase the period by 10μs during "on" time and get the desired angle.

After some experiments, I have discovered that my motor (MG996R) has a range from 0.6ms to 2.1ms during "on" time to move from 0 to 180 degrees.

I have tried the Arduino's Servo library but it goes far than 180 degrees (I don't know why). That's why I am building my own code for that purpose.

Answer 1

I got it working just analyzing time consumption of digitalWrite and digitalRead functions. They took around 13us to execute (each one), so this added up a few extra undesired microseconds to the final period.

Instead, I set up digital output using DDRD for configuring and PORTD for setting the desired output, which results in approximately 1us to set the output value (HIGH or LOW).

I changed the prescaler to 8 and the code below shows how to get a 100kHz wave output with 5us duration of "on" time using timer1 CTC mode.

void setup()
{
    DDRD = B00001000; // pin 3 as output
    PORTD &= ~_BV(PORTD3);   // set 3 as LOW

    // initialize Timer1

    cli();          // disable global interrupts

    TCCR1A = 0;     // set entire TCCR1A register to 0
    TCCR1B = 0;     // same for TCCR1B

    // set compare match register to desired timer count:
    OCR1A = 9;

    // turn on CTC (Clear Timer on Compare Match) mode:
    TCCR1B |= (1 << WGM12);

    // Set CS10 bit for clk/8 prescaler:
    TCCR1B |= _BV(CS11);

    // Output Compare A Match Interrupt Enable
    TIMSK1 |= (1 << OCIE1A);

    sei();          // enable global interrupts

}

ISR(TIMER1_COMPA_vect)
{
  PORTD = ~PORTD;
}