ATmega328P Pinout (DIP-28)

ATmega328P is the 8-bit AVR microcontroller behind the Arduino Uno and Nano. In the DIP-28 package it suits custom boards and breadboards – add a 16 MHz crystal and two capacitors and you have a “breadboard Arduino”.

The ATmega8, ATmega88 and ATmega168 share the same pin layout.

Specifications

8-bit AVR up to 20 MHz (16 MHz on Arduino), 8 MHz internal oscillator
32 kB flash, 2 kB SRAM, 1 kB EEPROM
23 GPIO, 6 channels of 10-bit ADC (8 on TQFP and QFN packages)
UART, SPI, I2C (TWI), 3 timers, 6× PWM
1.8–5.5 V supply (3.8 V and up at 16 MHz, 4.5 V and up at 20 MHz)

It is programmed over ISP (MOSI, MISO, SCK and RESET) – with a USBasp programmer or another Arduino running the ArduinoISP sketch. Once a bootloader is flashed, a USB-serial converter on RX and TX is all you need.

Pinout and pin table

Hover over a pin in the diagram or a table row; use the filter to highlight a bus. You can download the pinout as PNG or SVG.

Pin names:

Download:PNG SVG

License: CC BY 4.0 · How to credit

Pin table – ATmega328P (DIP-28)
Pin	Name	Arduino	Direction	UART	I2C	SPI	PWM	ADC	Note
1	PC6		input/output						Reset (active low) – usable as GPIO only after programming the RSTDISBL fuse, but then ISP programming no longer works
2	PD0	D0	input/output	RX					UART RX
3	PD1	D1	input/output	TX					UART TX
4	PD2	D2	input/output						External interrupt INT0
5	PD3	D3	input/output				PWM		External interrupt INT1; PWM from Timer 2 (OC2B)
6	PD4	D4	input/output						Counter T0 input
7	VCC		input						Power 1.8–5.5 V (16 MHz from 3.8 V, 20 MHz from 4.5 V)
8	GND		–
9	PB6		input/output						Crystal (XTAL1) – not free with an external crystal
10	PB7		input/output						Crystal (XTAL2) – not free with an external crystal
11	PD5	D5	input/output				PWM		PWM from timer 0 (OC0B); counter input T1
12	PD6	D6	input/output				PWM		PWM from timer 0 (OC0A); analog comparator AIN0
13	PD7	D7	input/output						Analog comparator AIN1
14	PB0	D8	input/output						Timer 1 input capture (ICP1); clock output CLKO
15	PB1	D9	input/output				PWM		PWM from timer 1 (OC1A)
16	PB2	D10	input/output			SS	PWM		PWM from timer 1 (OC1B); SPI SS
17	PB3	D11	input/output			MOSI	PWM		PWM from timer 2 (OC2A); SPI MOSI (ISP)
18	PB4	D12	input/output			MISO			SPI MISO (ISP)
19	PB5	D13	input/output			SCK			SPI SCK (ISP); v Arduinu D13 s LED
20	AVCC		input						Power for the A/D converter and pins PC0–PC3 (PC4–PC6 are powered by VCC) – connect to VCC, ideally through an LC filter (10 µH + 100 nF).
21	AREF		input						A/D converter reference
22	GND		–
23	PC0	A0	input/output					ADC0
24	PC1	A1	input/output					ADC1
25	PC2	A2	input/output					ADC2
26	PC3	A3	input/output					ADC3
27	PC4	A4	input/output		SDA			ADC4	I2C SDA
28	PC5	A5	input/output		SCL			ADC5	I2C SCL

What to watch out for

Pin 1 (PC6) is RESET and needs a pull-up (10 kΩ). It can only be used as GPIO after programming the RSTDISBL fuse – but then the chip can no longer be programmed over ISP.
With an external crystal, PB6 and PB7 are taken.
Connect AVCC to VCC even if you don’t use the ADC, and add a 100 nF decoupling capacitor to every supply pin.
From the factory the chip runs at 1 MHz (the internal 8 MHz divided by 8) – without setting the fuses, Arduino timing will be off.
Double-check fuse settings – choosing the wrong clock source “bricks” the chip.