# 2. Operating Modes

## 2.1. Normal Mode

![image](https://hackmd.io/_uploads/Byc5BRMtbl.png)

In **Normal Mode**, the timer acts as a **simple up-counter**. It starts from `0` and increments with every clock pulse (after passing through the prescaler) until it reaches its maximum value (`0xFF` for 8-bit, `0xFFFF` for 16-bit.

Once it hits the maximum, it "**rolls over**" or overflows back to `0`. Upon overflow, the **Timer Overflow Flag (TOVn)** is set, which can trigger an Interrupt Service Routine (ISR). This is perfect for tracking elapsed time by incrementing a software counter every time an overflow occurs.

**Formula for Overflow Frequency:**  
[![](https://learn.digilabdte.com/uploads/images/gallery/2026-03/scaled-1680-/image-1772552990355.png)](https://learn.digilabdte.com/uploads/images/gallery/2026-03/image-1772552990355.png)  
where N is the prescaler value.


## 2.2. CTC (Clear Timer on Compare) Mode

![image](https://hackmd.io/_uploads/B1IBLCGtWx.png)

CTC Mode is **far more precise** for generating specific frequencies. Instead of waiting for an overflow at the maximum possible value, the timer counts until it matches a value you pre-defined in the **Output Compare Register (OCRnx)**. As soon as the match occurs, the timer clears itself (**resets to `0`**).

You can configure the timer to "toggle" an output pin automatically when the match occurs, creating a perfect **square wave** without CPU intervention. For example, to generate a 1 kHz square wave using a 16 MHz clock and a 64 prescaler, you would calculate the required OCR value:  
[![](https://learn.digilabdte.com/uploads/images/gallery/2026-03/scaled-1680-/image-1772553040280.png)](https://learn.digilabdte.com/uploads/images/gallery/2026-03/image-1772553040280.png)

Using the values above:   
[![](https://learn.digilabdte.com/uploads/images/gallery/2026-03/scaled-1680-/image-1772553068165.png)](https://learn.digilabdte.com/uploads/images/gallery/2026-03/image-1772553068165.png)

## 2.3. PWM (Pulse Width Modulation) Mode

**PWM Mode** is used to **simulate an analog output** using digital signals. By rapidly switching a pin between HIGH and LOW, you can control the average power delivered to a component.
- **Fast PWM:** High frequency, suitable for power regulation and LED dimming. The timer counts from 0 to MAX, resetting to 0 immediately. The output pin changes state when the timer reaches the OCR value.
- **Phase Correct PWM:** Provides a symmetrical waveform by counting up to MAX and then counting back down to 0. This is preferred for motor control as it reduces electromagnetic noise.
**Duty Cycle:** Controlled by the value in the OCR register. A higher value means the signal stays "HIGH" longer within one period.