ATmega328P — 8-Bit AVR Microcontroller with 32KB Flash

Overview

The ATmega328P is the single most iconic 8-bit AVR RISC microcontroller IC in open-source hardware history. Originally designed by Atmel (now Microchip Technology), it is the primary microcontroller chip powering the Arduino Uno R3, Arduino Nano, and Arduino Pro Mini development boards.

Featuring $32\text{ KB}$ of ISP Flash memory, $2\text{ KB}$ of SRAM, and $1\text{ KB}$ of EEPROM, the ATmega328P operates at clock speeds up to $20\text{ MHz}$ across a broad supply voltage range of $1.8\text{V}$ to $5.5\text{V}$ DC. Housed in a beginner-friendly 28-pin DIP package (DIP-28), it enables breadboard prototyping of standalone microcontroller circuits.

Quick reference


CPU Architecture	8-bit AVR RISC (131 powerful instructions, 32x8 working registers)
Max Clock Frequency	$20\text{ MHz}$ at $4.5\text{V} \dots 5.5\text{V}$ ( $16\text{ MHz}$ standard Arduino clock)
Memory Breakdown	$32\text{ KB}$ Flash, $2\text{ KB}$ SRAM, $1\text{ KB}$ EEPROM
Operating Voltage (`VCC`)	1.8 V to 5.5 V DC (5.0 V nominal)
Package	28-pin DIP (ATmega328P-PU) / 32-pin TQFP (ATmega328P-AU)
I/O & PWM	23 programmable GPIO lines, 6 PWM channels (8-bit)
Analog Peripherals	6-channel 10-bit ADC (8-channel on TQFP)
Communication Buses	1x USART (Hardware Serial), 1x SPI, 1x $I^2C$ (TWI)
Programming Interface	In-System Programming (ISP / ICSP) via SPI & UART Bootloader

Pinout (28-Pin DIP Package - ATmega328P-PU)

text

                       ┌───┴───┐
     (PC6/RESET) RESET ─┤ 1  28 ├─ PC5 (ADC5/SCL)
        (PD0/RXD) RXD ─┤ 2  27 ├─ PC4 (ADC4/SDA)
        (PD1/TXD) TXD ─┤ 3  26 ├─ PC3 (ADC3)
       (PD2/INT0) D2  ─┤ 4  25 ├─ PC2 (ADC2)
  (PD3/INT1/PWM) D3  ─┤ 5  24 ├─ PC1 (ADC1)
             (PD4) D4  ─┤ 6  23 ├─ PC0 (ADC0)
                   VCC ─┤ 7  22 ├─ GND
                   GND ─┤ 8  21 ├─ AREF
     (PB6/XTAL1) XTAL1 ─┤ 9  20 ├─ AVCC
     (PB7/XTAL2) XTAL2 ─┤ 10 19 ├─ PB5 (SCK/D13/LED)
       (PD5/PWM) D5  ─┤ 11 18 ├─ PB4 (MISO/D12)
       (PD6/PWM) D6  ─┤ 12 17 ├─ PB3 (MOSI/PWM/D11)
             (PD7) D7  ─┤ 13 16 ├─ PB2 (SS/PWM/D10)
             (PB0) D8  ─┤ 14 15 ├─ PB1 (PWM/D9)
                       └───────┘

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Supply Voltage	$V_{CC}$	1.8	5.0	5.5	V	DC supply
Active Supply Current	$I_{CC}$	—	1.5	9.0	mA	$V_{CC} = 5.0\text{V}, f_{CLK} = 16\text{MHz}$
Power-down Sleep Current	$I_{pd}$	—	0.1	1.0	µA	$V_{CC} = 3.0\text{V}, T_A = 25^\circ\text{C}$
GPIO Output Sink/Source	$I_{IO}$	-40	—	+40	mA	Absolute max per pin (20mA recommended)
System Clock Frequency	$f_{CLK}$	0	16.0	20.0	MHz	$V_{CC} = 4.5\text{V} \dots 5.5\text{V}$
Flash Endurance	$N_{END}$	10,000	—	—	Cycles	Write/Erase cycles

Standalone Breadboard Circuit (16MHz Crystal Setup)

text

        +5V DC Power Supply
           │
        [Pin 7: VCC] ──┬── [ 0.1µF Capacitor ] ── GND
                       │
        [Pin 1: RESET] ─── [ 10kΩ Resistor ] ─── +5V
                       │
   (Crystal 16MHz)     ├─── [Pin 9: XTAL1]  ─── [ 22pF Cap ] ─── GND
                       └─── [Pin 10: XTAL2] ─── [ 22pF Cap ] ─── GND

Wiring (USBasp / Arduino ISP Flashing Connections)

ICSP Programmer	ATmega328P DIP Pin	Arduino Pin Mapping
`VCC`	Pin 7 (`VCC`)	5V Power
`GND`	Pin 8 (`GND`)	GND
`RESET`	Pin 1 (`RESET`)	Reset Pin
`MOSI`	Pin 17 (`PB3`)	Digital D11
`MISO`	Pin 18 (`PB4`)	Digital D12
`SCK`	Pin 19 (`PB5`)	Digital D13

Example (Bare-Metal C Code for Port Bit Toggling)

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    // Set PB5 (Arduino Pin 13 LED) as output
    DDRB |= (1 << DDB5);

    while (1) {
        // Toggle PB5 high/low
        PORTB ^= (1 << PORTB5);
        _delay_ms(500);
    }
    return 0;
}

Common mistakes

Exceeding 40 mA GPIO current limits: Drawing $>40\text{ mA}$ from a single I/O pin or $>200\text{ mA}$ total across all pins burns out the output drivers.
Forgetting 22pF decoupling capacitors on crystal pins: Operating a standalone ATmega328P with a $16\text{ MHz}$ crystal without two $22\text{ pF}$ ceramic load capacitors prevents clock oscillation.

Notes

ATmega328P vs ATtiny85 vs ATmega2560: ATmega328P offers $32\text{ KB}$ Flash in 28-pin DIP; ATtiny85 offers $8\text{ KB}$ in 8-pin DIP; ATmega2560 offers $256\text{ KB}$ Flash in 100-pin TQFP.