ATmega328P8-bit AVR microcontroller

Program the logic. Connect the physical world.

Manufacturer-referenced

Microchip Technology · ATMEGA328P-PU

SPDIP-28DS40002061B · 2020

ATmega328P is an 8-bit AVR microcontroller with 32 KB of programmable Flash, 2 KB of SRAM and 1 KB of EEPROM. Its CPU executes firmware while timers, serial interfaces and a 10-bit ADC connect that program to external signals. A bare IC needs power, clock configuration and programmed firmware; it is not a complete Arduino board. The selected ATMEGA328P-PU and ATMEGA328P-AU devices use 28-lead SPDIP and 32-lead TQFP packages. Their physical pin numbers differ. TQFP adds ADC6 and ADC7 as analog-only inputs plus extra power contacts; every supply and ground contact must be connected. Operation spans 1.8–5.5 V, with a supply-dependent clock limit: up to 4 MHz at 1.8 V, 10 MHz at 2.7 V and 20 MHz at 4.5 V or above. The selected industrial parts cover −40 to +85°C. Factory clock settings divide the internal 8 MHz RC oscillator by eight.

Real View

ATMEGA328P-PU black package with silver leads and readable markings.

Pinout

ATMEGA328P-PUSPDIP-28Top view

PCB Footprint

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3D View

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ResetDigital I/O & peripheralsPower & returnsClock contactsADC & reference

ATMEGA328P-PUSPDIP-28Top view

Click a lead or use the arrow keys. Top view

PIN 1 · Reset

PC6 / RESET

Reset / optional GPIO

Function

Active LOW reset; PC6 is GPIO only when the reset-disable fuse is programmed.

How to connect

Use a reset pullup, typically 10 kΩ.

Electrical note

Disabling reset removes normal ISP access; do not change this fuse casually.

At a glance

8-bit AVR microcontroller

A Harvard AVR core fetches instructions from Flash and works with SRAM. Peripheral registers configure the timers, GPIO, ADC and serial interfaces.

32 KBProgram Flash

2 KBSRAM

20 MHz · at 4.5–5.5 VMaximum clock

10 bits · 6 / 8 channelsADC resolution

Operating profile

ATMEGA328P-PU

Selected package: SPDIP-28ATMEGA328P-PU · 28 physical pins
Supply range: 1.8–5.5V · clock limit applies
Industrial ambient: −40–85°C

CPU and memory

Key parameters and their test conditions

Architecture: 8-bit AVR RISCConditions / notes: 32 general-purpose working registers
Instruction set: 131 instructionsConditions / notes: Most execute in a single clock cycle
Program Flash: 32 KBConditions / notes: Self-programmable; optional boot section
SRAM: 2 KBConditions / notes: Stack, runtime variables and buffers
EEPROM: 1 KBConditions / notes: Nonvolatile data
Write endurance: 10,000 Flash / 100,000 EEPROM cyclesConditions / notes: Manufacturer endurance specification

Package details

Follows the selected physical package

Manufacturer: Microchip Technology
Part number: ATMEGA328P-PU
Package: SPDIP-28
Connections: 28 pins
Mounting: Through-hole
Lead pitch: 2.54 mm · 7.62 mm rows
Package code: 28P3
Thermal reference: Board and package dependent; see manufacturer data

Connections by function

Select a pin to open its function, electrical notes and wiring guidance.

Reset1 pins

Digital I/O & peripherals14 pins

Power & returns4 pins

Clock contacts2 pins

ADC & reference7 pins

ATmega328P interactive datasheet with 28-pin SPDIP and 32-pin TQFP mappings, AVR memory and peripherals, a clock and Timer1 explorer, a standalone LED circuit and KiCad footprints. — Open the image for a closer look. The individual diagrams are also available in their own tabs.

Power and clock

Connect all VCC, AVCC and GND pins, decouple locally and select a clock source compatible with the supply voltage. Clock fuses and CKDIV8 determine the CPU clock.

Program the device

Use ISP through RESET, MOSI, MISO and SCK, or a compatible preinstalled bootloader. A blank chip does not understand Arduino serial uploads on its own.

Configure peripherals

Set DDR and peripheral registers. Timer1 can toggle OC1A in hardware without a software delay loop, while firmware handles other work.

The explorer calculates Timer1 hardware-toggle timing from the actual I/O clock. It does not execute uploaded firmware. In ProTech, the existing Arduino Uno model supplies AVR execution; this page documents the bare ATmega328P.

From clock cycles to a blinking LED.

Explore the Timer1 CTC output on PB1 / OC1A.

Hardware timing calculator

Supply voltage V Actual CPU / I/O clock MHz Timer prescaler Compare register · OCR1A

fOUT = fCPU ÷ [2 × prescaler × (OCR1A + 1)]. The output changes state once per compare match.

OC1A OUTPUT FREQUENCY—

——

TIME BETWEEN TOGGLES—One HIGH or LOW interval

SUPPLY-DEPENDENT CLOCK LIMIT—Manufacturer maximum-frequency curve

COMPARE REGISTER—OCR1A · unsigned 16-bit value

Output: PB1 / OC1A, physical pin 15. Set its DDR bit, CTC mode 4 and toggle-on-compare. Steady-state Timer1 CTC output calculation. The selected clock must already include CKDIV8 and the system prescaler. Firmware, fuse changes, startup delay and oscillator error are not emulated.

Functional block diagram, not a transistor-level die schematic.

Physical ATmega328P SPDIP28 with 5V on VCC7 AVCC20, grounds8and22, AREF21 bypass, RESET1 pullup, crystal between9and10 with load capacitors, LED and1k resistor fromPB1pin15, and ISP contacts. — Program firmware and configure the clock before expecting the LED to blink. Pin numbers are for SPDIP-28.

Pin reference

Select a pin to explore its function and connection.

28 pins

Signal	Type	Function
PC6 / RESET	Reset / optional GPIO	Active LOW reset; PC6 is GPIO only when the reset-disable fuse is programmed.
PD0 / RXD	GPIO / USART receive	PD0, RXD and pin-change interrupt PCINT16.
PD1 / TXD	GPIO / USART transmit	PD1, TXD and PCINT17.
PD2 / INT0	GPIO / interrupt	PD2, INT0 and PCINT18.
PD3 / OC2B	GPIO / timer / interrupt	PD3, OC2B, INT1 and PCINT19.
PD4 / T0	GPIO / clock	PD4, USART synchronous clock XCK, Timer0 clock T0 and PCINT20.
VCC	Supply	Digital power supply.
GND	Ground	Digital supply return.
PB6 / XTAL1	GPIO / oscillator input	PB6, XTAL1, TOSC1 and PCINT6.
PB7 / XTAL2	GPIO / oscillator output	PB7, XTAL2, TOSC2 and PCINT7.
PD5 / OC0B	GPIO / timer	PD5, OC0B, T1 and PCINT21.
PD6 / OC0A	GPIO / comparator	PD6, OC0A, AIN0 and PCINT22.
PD7 / AIN1	GPIO / comparator	PD7, AIN1 and PCINT23.
PB0 / ICP1	GPIO / capture	PB0, ICP1, CLKO and PCINT0.
PB1 / OC1A	GPIO / timer output	PB1, OC1A and PCINT1.
PB2 / SS	GPIO / SPI select	PB2, SS, OC1B and PCINT2.
PB3 / MOSI	GPIO / SPI / programming	PB3, MOSI, OC2A and PCINT3.
PB4 / MISO	GPIO / SPI / programming	PB4, MISO and PCINT4.
PB5 / SCK	GPIO / SPI / programming	PB5, SCK and PCINT5.
AVCC	Analog supply	Powers the ADC and specified Port C circuitry.
AREF	ADC reference	External reference input or decoupling node for a selected internal reference.
GND	Ground	Analog-side supply return.
PC0 / ADC0	GPIO / ADC	PC0, ADC0, PCINT8.
PC1 / ADC1	GPIO / ADC	PC1, ADC1, PCINT9.
PC2 / ADC2	GPIO / ADC	PC2, ADC2, PCINT10.
PC3 / ADC3	GPIO / ADC	PC3, ADC3, PCINT11.
PC4 / ADC4 / SDA	GPIO / ADC / TWI	PC4, ADC4, PCINT12 and TWI SDA.
PC5 / ADC5 / SCL	GPIO / ADC / TWI	PC5, ADC5, PCINT13 and TWI SCL.
GND	Ground	Additional TQFP ground contact.
VCC	Supply	Additional TQFP supply contact.
ADC6	Analog input only	Additional ADC channel in the 32-lead package.
ADC7	Analog input only	Additional ADC channel in the 32-lead package.

CPU and memory

ATmega328P-specific capacities

Manufacturer specifications 8 parameters

Parameter	Value	Conditions / notes
Architecture	8-bit AVR RISC	32 general-purpose working registers
Instruction set	131 instructions	Most execute in a single clock cycle
Program Flash	32 KB	Self-programmable; optional boot section
SRAM	2 KB	Stack, runtime variables and buffers
EEPROM	1 KB	Nonvolatile data
Write endurance	10,000 Flash / 100,000 EEPROM cycles	Manufacturer endurance specification
Retention	20 years at 85°C / 100 years at 25°C	Qualified reliability results
Multiplier	8 × 8 hardware	Two-cycle operation

Other members named on the shared datasheet have different memory capacities.

Clock, power and temperature

Selected PU and AU industrial orderables

Manufacturer specifications 8 parameters

Parameter	Value	Conditions / notes
Operating supply	1.8–5.5 V	VCC and AVCC requirements apply
Clock at 1.8 V	Up to 4 MHz	See maximum-frequency curve
Clock at 2.7 V	Up to 10 MHz	Frequency limit rises with supply
Clock at 4.5–5.5 V	Up to 20 MHz	16 MHz example uses 5 V
Factory clock	8 MHz internal RC / 8 = 1 MHz	CKDIV8 programmed by default
Industrial temperature	−40 to +85°C	Selected ATMEGA328P-PU / AU
Active current example	5.2 mA typical; 9 mA maximum	8 MHz, 5 V; datasheet measurement conditions
Power-down example	0.1 µA typical; 2 µA maximum	3 V, watchdog disabled; datasheet conditions

Current changes with clock, peripherals, loading and temperature. The clock explorer applies the datasheet maximum-frequency curve.

Timers and interfaces

Functions share physical GPIO contacts

Manufacturer specifications 9 parameters

Parameter	Value	Conditions / notes
Timer0 / Timer2	Two 8-bit timers	Prescalers and compare/PWM outputs
Timer1	One 16-bit timer	Compare outputs and input capture
PWM outputs	Six	OC0A/B, OC1A/B, OC2A/B
Serial interfaces	USART, SPI, two-wire interface	TWI is compatible with I²C
ADC	10 bits; 6 or 8 external channels	Six in SPDIP; eight in TQFP
Analog comparator	One	AIN0/AIN1 or selected multiplexed input
General-purpose I/O	Up to 23	Reset and oscillator choices reduce available pins
Sleep modes	Six	Idle, ADC Noise Reduction, Power-down, Power-save, Standby, Extended Standby
Supervision	Watchdog and brown-out detection	Configured by registers and fuses

SPI, PWM and other alternate functions are multiplexed with GPIO; they do not add separate pins.

ADC and analog connections

10-bit successive approximation

Manufacturer specifications 7 parameters

Parameter	Value	Conditions / notes
Nominal code range	0–1023	Ideal step = VREF / 1024
ADC clock for full resolution	50–200 kHz	Selected by a clock prescaler
Normal conversion	13 ADC clock cycles	First conversion after enabling takes 25 cycles
Reference choices	AREF, AVCC, internal nominal 1.1 V	Select with REFS bits; allow settling
AVCC connection	Required even if ADC is unused	Use filtering when analog accuracy matters
AREF bypass	Typically 100 nF to GND	Do not short an active internal reference to an external source
TQFP ADC6 / ADC7	Analog-only inputs	No digital driver or internal pullup

Reference tolerance, source impedance and noise affect absolute measurement accuracy.

I/O design and stress limits

Do not design to absolute maximum ratings

Manufacturer specifications 8 parameters

Parameter	Value	Conditions / notes
Input LOW / HIGH, general pins	≤ 0.3 × VCC / ≥ 0.6 × VCC	Reset and oscillator contacts have separate specifications
Internal pullup	20–50 kΩ	General GPIO, when enabled
Pin current absolute maximum	40 mA	Stress limit; not a recommended load
Supply/ground current absolute maximum	200 mA	Observe port-group current restrictions too
Output-voltage test at 5 V	20 mA load	Guaranteed voltage under specified test conditions
Output-voltage test at 3 V	10 mA load	Use the detailed DC table for margin
Typical LED example	1 kΩ series resistor at 5 V	A few mA; never connect an LED directly
Pin voltage stress range	−0.5 V to VCC + 0.5 V	RESET has a separate high-voltage programming limit

External loads must satisfy per-pin, port-group, total-current and voltage requirements.

Timer1 hardware toggle

CTC mode 4 · OCR1A is TOP

Manufacturer specifications 8 parameters

Parameter	Value	Conditions / notes
Output frequency	fCPU / (2 × N × (1 + OCR1A))	OC1A toggles at each compare match
Prescaler N	1, 8, 64, 256 or 1024	Internal timer clock options
OCR1A range	0–65535	Sixteen-bit unsigned compare value
Example clock	16 MHz	5 V supply; suitable external clock configured
Example divider / TOP	256 / 31249	One output cycle per second
Output pin	PB1 / OC1A	SPDIP15; TQFP13
Required configuration	DDR for PB1, COM1A0, WGM12, CS12	Mode4, toggle output, divide256 example
Live TOP changes	CTC TOP is not double buffered	Changing TOP below the running count can delay the next match

The calculator gives steady-state hardware timing; it does not emulate register-write transients.

Bring-up and programming

Standalone circuit checklist

Manufacturer specifications 7 parameters

Parameter	Value	Conditions / notes
ISP signals	RESET, MOSI, MISO, SCK, VCC reference, GND	Programmer target voltage must match the chip
Clock requirement	A valid selected clock must be present	Changing clock fuses can require an external clock for later ISP
Bootloader	Must be programmed separately if required	A bare IC is not automatically Arduino-ready
Fuse polarity	Programmed = 0; unprogrammed = 1	Read the specific fuse table before changing bits
Reset pulse	2.5 µs minimum	Electrical specification
Bypass capacitors	100 nF close to each supply group	Short connections to common ground
Crystal capacitors	Choose for crystal load and board capacitance	22 pF is illustrative, not universal

Keep the reset function available unless you have a deliberate alternate programming and recovery plan.

Embedded controllers

Read inputs, manage outputs and schedule control work using firmware and hardware timers.

Manufacturer application guide

Sensor instruments

Combine the ADC with digital sensors on SPI or TWI and communicate over USART.

Manufacturer application guide

Compact programmed products

Move a validated design from a development board to a standalone, correctly powered MCU.

Manufacturer application guide

Take it into ProTech

Find ATmega328P in the Components Library to place its schematic symbol and connect its pins.

Reference page · circuit simulation model not yet availableOpen ProTech