Issue with Arduino Timing

Question

I am working with an Arduino on a project for which timing is very important. I use TimerOne to trigger timer interrupts and use micros() for delays (delayMicroseconds() was causing problems worse than the one explained below). The program is sending a manual PWM signal to an LED and it is very important that the signal is sent with an error that is less than 8 microseconds (ideally, the signal is sent at the same time in each period). My test code is shown below:

#include <TimerOne.h>
#include <SPI.h>

const int LED_PIN = 3;
const int CHIP_SELECT = 12;

const int PERIOD = 4000;
const double DUTY_CYCLE = .5;
const int HIGH_TIME = PERIOD * DUTY_CYCLE;
const int LOW_TIME = PERIOD - HIGH_TIME;
const int INITIAL_SIGNAL_DELAY = LOW_TIME / 2;
const int HIGH_TIME_TOTAL_DELAY = INITIAL_SIGNAL_DELAY + HIGH_TIME;    

const int RESISTOR_VALUE = 255;

boolean triggered = false;
boolean data = false;

unsigned long triggeredTime;
unsigned long s;
unsigned long e;

boolean found;
int i = 0;

void setup()
{
  s = micros();

  Timer1.initialize(PERIOD);
  Timer1.attachInterrupt(trigger);

  pinMode(LED_PIN, 3);
  pinMode(CHIP_SELECT, OUTPUT);

  SPI.begin();

  digitalWrite(CHIP_SELECT, LOW);
  SPI.transfer(B00010001);
  SPI.transfer(RESISTOR_VALUE);
  digitalWrite(CHIP_SELECT, HIGH);

  e = micros();

  Serial.begin(115200);
  Serial.print("s: ");
  Serial.println(s);
  Serial.print("e: ");
  Serial.println(e);
}

void loop()
{
  if(triggered)
  {
    while(micros() - triggeredTime < INITIAL_SIGNAL_DELAY)
    { }
    s = micros();
    digitalWrite(LED_PIN, data);
    while(micros() - triggeredTime < HIGH_TIME_TOTAL_DELAY)
    { }
    digitalWrite(LED_PIN, LOW);

    data = !data;
    triggered = false;

    e = micros();
    //micros();

    if(s % 100 > 28 || s % 100 < 12)
    {
     found = true; 
    }

    if(!found)
    {
      Serial.print("s: ");
      Serial.println(s);
    }
    else
    {
      Serial.print("ERROR: ");
      Serial.println(s);
    }
    //Serial.print("e: ");
    //Serial.println(e);
  }
}

void trigger()
{
  triggeredTime = micros();
  triggered = true;
}

(it should be noted that the first signal sent is always xx20, usually 5020).

So, with this code, I eventually get an error. I am not sure why, but this error occurs at the same point every single time:

.
.
.
s: 1141020
s: 1145020
s: 1149020
ERROR: 1153032
ERROR: 1157020
ERROR: 1161020
.
.
.

Now, the really weird part is if I remove the comments before micros() (the micros() right after e = micros()), there is no error (or at least there is not an error within the first 30 seconds). I was wondering if anybody could provide an explanation for why this happens. I have dedicated many hours trying to get the timing working properly and everything was working well until I encountered this error. Any help would be very much appreciated. Thank you!

Answer 1

the code flow doesn't make a lot of sense to me. seems to me that you can simplified quite a bit.

if(s % 100 > 28 || s % 100 < 12)

I would use a few brackets to make it clear to the compiler what you want it to do.