ItimerSet() sets the interval and callback function, or the hardware timer to use.

bool ItimerSet(unsigned long intervalMs, void (*callback)())
bool ItimerSet(uint8_t timerNumber, unsigned long intervalMs, void (*callback)())

volatile bool fastTick = false;
 
void onFastTimer() {
  fastTick = true;
}
 
void setup() {
  ItimerSet(3, 100, onFastTimer);
  ItimerStart(3);
}
 
void loop() {
  if (fastTick) {
    fastTick = false;
  }
}

• Timer interrupt callbacks run inside an ISR. For safety, change only flags in the callback and perform actual control processing in loop().
• Do not use slow or interrupt-dependent code such as delay(), Serial, I2C or SPI in callbacks.
• Normally, call ImodbusRTU() repeatedly in loop(). Call it from a timer interrupt with a sufficient interval only when long delay() calls or debug output slow down loop().
• Do not use the default ItimerSet() together with tone() using Timer2 or analogWrite() on Timer2 PWM pins.
• Explicitly using Timer1, Timer3, Timer4 or Timer5 may conflict with PWM, analog output or high-speed counter functions using that timer.
• Configure only one Itimer callback per hardware timer.
• Defining an ISR for the same timer directly in the sketch may conflict with Itimer operation.
• ItimerStopAll() stops only hardware interrupt timers. It does not reset PLC-style timer states such as Iton, Itof or Itmr.

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