Microchip (Atmel) · Microcontroller

ATmega328P (PDIP-28)

The chip inside the Uno and Nano, on its own in a 28-pin DIP. Useful when you want the Arduino toolchain on a bare board. The mapping between port pins and Arduino numbers is the part nobody remembers.

Cheat sheet (SVG)

Logic level: 1.8-5.5 V
Package: PDIP-28 / TQFP-32 / MLF-32

1.Pinout diagram

Highlight a signalhover to preview, click to keep

Pins 1-14

ATMEGA328P

Pins 15-28

Pin detail

Hover a pin for its mappings and gotchas. Click to keep it here.

Logic level: 1.8-5.5 V
Package: PDIP-28 / TQFP-32 / MLF-32

Fig. 1. ATmega328P (PDIP-28), PDIP-28. Hover or tap a signal to light up every pin that carries it.

2.Watch out

Arduino numbers map to ports as: D0-D7 = PD0-PD7, D8-D13 = PB0-PB5, A0-A5 = PC0-PC5. The chip pin order does not follow either sequence.
A factory chip runs on the 8 MHz internal oscillator with CKDIV8 set, so it actually ticks at 1 MHz until you change the fuses. Arduino boards use an external 16 MHz crystal and fuses to match.
VCC (7) and AVCC (20) must both be connected, with 100 nF at each. AVCC feeds the ADC and port C; leaving it floating gives you garbage analog readings and sometimes a chip that will not start.
PB6/PB7 (9/10) are the crystal pins. On the internal oscillator they free up as two extra I/O, but the Arduino core does not expose them.
RESET (1) needs a 10k pull-up to VCC. It is also the dW debug pin, and disabling it via fuse locks out ISP programming.
20 mA per pin recommended, 40 mA absolute max, 200 mA total through VCC/GND.
The ATmega328PB is a newer die with extra peripherals, but it is not a silent drop-in: some register names and vector numbers changed.
The speed grade tracks the supply: 20 MHz needs at least 4.5 V, 10 MHz needs 2.7 V, and 4 MHz works down to 1.8 V. Running a 16 MHz crystal on 3.3 V is out of spec, which is why 3.3 V Pro Minis ship at 8 MHz.

3.Minimum circuit to run

What has to go around the chip before it will run.

Fig. 2. Minimum circuit to run, drawn from Table 1. Dashed parts are optional. Crossing wires are not connected.

Table 1. External parts, and why each one is there.
Part	Between	Why
100 nF	VCC (7) to GND	Supply decoupling, as close to the pins as the layout allows.
100 nF	AVCC (20) to GND	The analog supply needs its own decoupling even if you never use the ADC.
Wire link	AVCC (20) to VCC	AVCC must be powered or the ADC and all of port C misbehave.
10 kOhm	PC6 (1) to VCC	There is no internal pull-up strong enough. A floating RESET gives random restarts.
16 MHz crystaloptional	PB6 (9) to PB7 (10)	Only if you want 16 MHz. The internal 8 MHz RC needs no parts at all.
22 pFoptional	PB6 (9) to GND	Crystal load capacitor. Omit it and the oscillator may not start.
22 pFoptional	PB7 (10) to GND	The other crystal load capacitor.
100 nFoptional	AREF (21) to GND	Quietens the ADC reference.

A factory chip runs on the internal 8 MHz RC with CKDIV8 set, so it ticks at 1 MHz until you change the fuses. Fitting the crystal is not enough on its own; the fuses have to select it.

4.Register configuration

Turning a pin on the diagram into a configured pin.

Drive a pin, and read one with a pull-up

avr-libc

#include <avr/io.h>

/* Three registers per port. DDR picks direction, PORT writes the
   output latch, PIN reads the physical pin. */

DDRB  |=  (1 << PB5);          /* PB5 = Arduino D13 = output */
PORTB |=  (1 << PB5);          /* high */
PORTB &= ~(1 << PB5);          /* low  */
PINB  |=  (1 << PB5);          /* writing PIN toggles the output */

DDRD  &= ~(1 << PD2);          /* PD2 = D2 = input */
PORTD |=  (1 << PD2);          /* PORT on an input enables the pull-up */

if (!(PIND & (1 << PD2))) {    /* switch to ground reads 0 when pressed */
}

On an input pin, PORT is the pull-up enable, not an output value. Forget it and a floating pin reads whatever the nearest mains cable is doing.

5.Full pin reference

Every pin, alias and mapping. This is the table that prints.

Table 2. Pin assignments.

Complete pin reference for ATmega328P (PDIP-28)
Pin	Name	Also called	Functions	Notes
1	PC6	RESET, dW	Active-low reset, debugWIRE	10k pull-up to VCC required.
2	PD0UART	D0, RXD	USART RXD, PCINT16
3	PD1UART	D1, TXD	USART TXD, PCINT17
4	PD2	D2, INT0	External interrupt 0, PCINT18
5	PD3PWM	D3, INT1, OC2B	Timer2 OC2B, External interrupt 1
6	PD4Clock	D4, T0, XCK	Timer0 external clock, USART clock
7	VCC1.8-5.5 V	-	Digital supply	100 nF at the pin.
8	GND	-	Ground
9	PB6Clock	XTAL1, TOSC1	Crystal in / external clock	Free as I/O only on the internal oscillator.
10	PB7Clock	XTAL2, TOSC2	Crystal out	Free as I/O only on the internal oscillator.
11	PD5PWM	D5, T1, OC0B	Timer0 OC0B, Timer1 external clock	Timer0 also drives millis(), do not reprogram its prescaler.
12	PD6PWM	D6, AIN0, OC0A	Timer0 OC0A, Analog comparator +
13	PD7	D7, AIN1	Analog comparator -
14	PB0Clock	D8, ICP1, CLKO	Timer1 input capture, Clock output
15	PB1PWM	D9, OC1A	Timer1 OC1A
16	PB2SPIPWM	D10, SS, OC1B	SPI SS, Timer1 OC1B
17	PB3SPIPWM	D11, MOSI, OC2A	SPI MOSI, Timer2 OC2A, ISP data in
18	PB4SPI	D12, MISO	SPI MISO, ISP data out
19	PB5SPIClock	D13, SCK	SPI SCK, ISP clock	The on-board LED pin on Arduino boards.
20	AVCC= VCC ±0.3 V	-	Analog / port C supply	Must be connected even if the ADC is unused. 100 nF, or an LC filter from VCC for quiet readings.
21	AREF	-	ADC reference	100 nF to ground. Only drive it externally after selecting the external reference.
22	GND	-	Analog ground
23	PC0ADC	A0, ADC0	ADC0
24	PC1ADC	A1, ADC1	ADC1
25	PC2ADC	A2, ADC2	ADC2
26	PC3ADC	A3, ADC3	ADC3
27	PC4ADCI²C	A4, ADC4, SDA	ADC4, I2C SDA
28	PC5ADCI²C	A5, ADC5, SCL	ADC5, I2C SCL

Looking for a substitute?

ATmega328P, 4 alternatives

Primary sources

Microchip ATmega328P product page ↗

Found a wrong pin? The whole board is one file, data/boards/atmega328p-pdip.json.

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