The millis()-based programming idiom you are using is intended to handle delays without ever blocking the program in a delay() call. Then you are completely defeating its purpose by calling delay(90000). After this 1.5 min delay, of course one minute will have elapsed, and the test currentMillis - previousMillis > 60000 will always be true.

If you want to embrace the non-blocking paradigm millis() allows, you should completely ban delay() from your vocabulary (as well as other blocking loops).

Edit: Answering the question in the comment “Can you suggest as to how can I modify the code for 1.5 minutes delay”. As suggested by JRobert in another comment, you should follow the “Blink Without Delay” Arduino example. There is only one caveat: this Arduino example keeps the LED on for one second and off for another second, while you want to toggle your system between two states with different durations.

The solution is to remember what state you are currently on, and write separate code paths for handling each state. As an example, the code below toggles the on-board LED, keeping it on for 100 ms and off for 900 ms. This is achieved by remembering the current state (in the boolean variable is_on below) and conditioning the code execution on the current state (e.g. if (is_on && ...)).

/*
 * Toggle the on-board LED on for 100 ms every second.
 */

const unsigned long time_on  = 100;  // on time  = 100 ms
const unsigned long time_off = 900;  // off time = 900 ms
unsigned long previousMillis;   // previous time the LED was toggled
bool is_on;                     // is the LED currently on?

void setup() {
    pinMode(LED_BUILTIN, OUTPUT);
    digitalWrite(LED_BUILTIN, LOW);
    is_on = false;
}

void loop() {
    unsigned long currentMillis = millis();

    // If the LED has been on long enough, turn it off.
    if (is_on && currentMillis - previousMillis >= time_on) {
        digitalWrite(LED_BUILTIN, LOW);
        is_on = false;
        previousMillis = currentMillis;
    }

    // If the LED has been off long enough, turn it on.
    if (!is_on && currentMillis - previousMillis >= time_off) {
        digitalWrite(LED_BUILTIN, HIGH);
        is_on = true;
        previousMillis = currentMillis;
    }
}

Your problem is that the program gets stuck in the for loop.

• You need to also create direction variable so the program that knows which if statement to execute.
• You need to create some other logic that will increase the i variable without stopping the whole program.

The code:

//Initialize the i variable globaly:
int i = 0;
bool direction = 0;


//Your function:
void with_millis() {
  unsigned long currentTime = millis();

  if ((currentTime - previousTime >= eventInterval) && direction == true) {
    digitalWrite(MOTOR_IN1, HIGH);
    digitalWrite(MOTOR_IN2, LOW);
    i++;
    if (i <= 255) {
      ledcWrite(0, i);
      previousTime = currentTime;
    } elif (i > 255) {
      i = 0;
      direction = false;
    }
  }

  if ((currentTime - previousTime >= eventInterval) && direction == false) {
    digitalWrite(MOTOR_IN1, LOW);
    digitalWrite(MOTOR_IN2, HIGH);
    i++;
    if (i <= 255) {
      ledcWrite(0, i);
      previousTime = currentTime;
    } elif (i > 255) {
      i = 0;
      direction = true;
    }
  }
}