for i = 1 to 1000
next

for (int i = 1; i <= 1000; i++) {
   // Your code here
}

hw_timer_t * timer = NULL;

void IRAM_ATTR onTimer() {
 Serial.println("Timer Interrupt Triggered");
}

void setup() {
 Serial.begin(115200);

 // Initialize timer with frequency of 1 Hz (1000000 microseconds)
 timer = timerBegin(1000000);
 timerAttachInterrupt(timer, &onTimer);
 
 // If timerWrite works without issues, let's use it
 timerWrite(timer, 0);
 timerAlarm(timer, 1000000, true, 0);
}

void loop() {
 // Your main code here
}