# 4. ADC In ATmega328p

### 4.1 ATmega328p ADC Specifications

The ATmega328p (used in the Arduino Uno) has a built-in ADC with the following specifications:

| Specification | Value |
| :--- | :--- |
| **Resolution** | 10-bit (produces values from 0 – 1023) |
| **Conversion Method** | Successive Approximation |
| **Input Channels** | 8 analog channels (A0 – A7), multiplexed |
| **Reference Voltage** | AVcc, Internal 2.56V, or external AREF pin |
| **Conversion Speed** | 50 kHz – 200 kHz (depending on prescaler) |
| **Result Registers** | ADCL (low-byte) + ADCH (high-byte) |

### 4.2 Successive Approximation Method

![image](https://hackmd.io/_uploads/rkugU2u3Zl.png)

The ATmega328p uses the Successive Approximation Register (SAR) method.
In this method, the ADC works by performing a binary search for the Vin value.

At each step:

- The SAR sets a trial bit
- The DAC generates a voltage (Vdac)
- A comparator compares Vin with Vdac
- The bit is either kept or changed based on the comparison result

This process is repeated N times (N = ADC resolution, which is 10-bit for the ATmega328p).

An example of SAR implementation can be seen in this image:

![image](https://hackmd.io/_uploads/H1LbL2O3-x.png)

1. **Started from the middle value (1000)**
   This is the representation of **½ of Vref** (MSB = 1).
2. **First comparison (Comparator)**
   - If **Vdac > Vin** → move down (red arrow)
   - If **Vdac < Vin** → move up (green arrow)
3. **Determining the next bit**
   Each step determines one additional bit.
   Example:
   - `1000 → 1100` (if Vin is larger)
   - `1000 → 0100` (if Vin is smaller)
4. **Repeat process (Binary Search)**
   The value range is continuously narrowed until all bits (**MSB → LSB**) are determined.
5. **Final result**
   The rightmost nodes show the **final binary code**.
   Example results: `1011`, `0110`, etc.