Choosing the Right Arduino Timer: A Practical Guide for Precise Timing and PWM

When Timer Tweaks Go Wrong

Picture this: you’re building a 1 kHz motor control loop and you decide to “just” change the analogWrite frequency on pin 9. After uploading, millis() stops counting, tone() on pin 3 stops, and the board looks dead. The culprit? The three hardware timers that the Arduino core configures at boot. Understanding their roles and how to use them safely is the first step to reliable, high‑precision sketches.

Timer Overview on the ATmega328P

• Timer0 (8‑bit) – drives millis(), micros(), delay(), and analogWrite() on pins 5 and 6. Changing its prescaler or mode breaks core timing functions.
• Timer1 (16‑bit) – supports CTC, phase‑correct PWM on pins 9/10, input capture, and is used by the Servo library. With a 16 MHz clock and no prescaler, it offers 1 µs resolution.
• Timer2 (8‑bit) – runs asynchronously from a 32.768 kHz crystal (optional) for real‑time‑clock use; also drives analogWrite() on pins 3/11 and the tone() library.

All three timers are initialized in init() before setup() runs. If you need a timer for a custom task, you must first disable its interrupts, re‑configure the registers, and then re‑enable the interrupts.

Hands‑On: 1 kHz Square Wave on Timer1

Goal: generate a 1 kHz square wave on pin 13 (OC1A) using Timer1 in CTC mode, while leaving Timer0 (millis()) and Timer2 (tone()) untouched.

#include <Arduino.h>

// Pin to toggle
const uint8_t outputPin = 13;

void setup() {
  pinMode(outputPin, OUTPUT);

  // 1. Stop Timer1 and clear its interrupt flag
  TCCR1A = 0;          // Normal mode, no PWM
  TCCR1B = 0;          // Stop the timer
  TIMSK1 = 0;          // Disable all Timer1 interrupts

  // 2. Configure CTC mode, prescaler 8
  // OCR1A = 15999 gives 1 kHz: 16 MHz / 8 / 16000 = 1 kHz
  OCR1A = 15999;
  TCCR1B = (1 << WGM12) | (1 << CS11); // CTC, prescaler 8

  // 3. Enable compare‑match interrupt
  TIMSK1 = (1 << OCIE1A);
}

void loop() {
  // Normal code can run here; millis() works because Timer0 is untouched
}

// ISR for Timer1 Compare Match A
ISR(TIMER1_COMPA_vect) {
  // Toggle the output pin
  PORTB ^= (1 << outputPin); // Fast toggle using PORTB
}

To verify:

1. Upload the sketch to an Arduino Uno.
2. Open the Serial Monitor and print millis() every 100 ms to confirm it continues to increment.
3. Use an oscilloscope or logic analyzer on pin 13; you should see a clean 1 kHz square wave.

Because we never touched Timer0 or Timer2, millis() and tone() remain functional. The ISR is short (<10 µs) and avoids heavy operations, preventing timing overruns.

Common Pitfalls & How to Avoid Them

• Modifying Timer0 – Breaks core timing. If you need custom PWM on pins 5/6, use Timer1 or Timer2 instead.
• Using Servo Library – On the Uno, Servo claims Timer1, disabling PWM on pins 9/10. If you need both Servo and custom PWM on those pins, move to an Arduino Mega (Timer5) or use a dedicated PWM driver.
• Asynchronous Timer2 – Requires a 32.768 kHz crystal on pins TOSC1/TOSC2. Without it, the timer stops or runs at an unpredictable rate.
• Long ISRs – Keep ISR code minimal. Floating‑point math or Serial.print() inside an ISR can cause missed compare matches.
• Register Access While Running – Writing to TCNTn while the timer is running can cause glitches. Wrap such writes in noInterrupts() / interrupts() or stop the timer first.

When to Use Which Timer

Timer	Best Use	Library Conflicts
Timer0	Core timing, delay(), millis(), micros()	None – core functions depend on it
Timer1	High‑resolution PWM (9/10), CTC loops, input capture, Servo (Uno)	Servo library disables PWM on 9/10
Timer2	Async RTC, tone(), PWM (3/11)	None on Uno; on Mega, tone() uses Timer2, Servo uses Timer5

Actionable Checklist

1. Identify the task: timing loop, PWM, input capture, or tone.
2. Map the task to the appropriate timer from the table.
3. Check library documentation for timer claims (Servo, IRremote, etc.).
4. Disable the timer’s interrupts, re‑configure registers, then re‑enable interrupts.
5. Keep ISRs short; use PORTx toggles for speed.
6. Verify with millis() output and an oscilloscope or logic analyzer.

By following these steps, you can harness the full power of the ATmega328P timers without breaking the Arduino core or running into subtle conflicts.