ATmega8 — 8-Bit AVR Microcontroller with 8KB Flash (DIP-28)

Overview

The ATmega8 (ATmega8A / ATmega8L) is the historic 8-bit AVR RISC microcontroller manufactured by Microchip Technology (originally Atmel) in a standard 28-pin DIP (DIP-28) package. As the direct predecessor to the ATmega168 and ATmega328P, it powered the very first generation of official Arduino boards (Arduino Serial, Arduino USB, and Arduino NG) and remains the standard microcontroller inside the ubiquitous USBasp AVR In-System Programmer dongles.

Operating from $2.7\text{V}$ to $5.5\text{V}$ DC (ATmega8A) at clock speeds up to $16\text{ MHz}$ , the ATmega8 features $8\text{ KB}$ Flash, $1\text{ KB}$ SRAM, $512\text{ Bytes}$ EEPROM, a 6-channel 10-bit ADC, three PWM channels, and hardware serial interfaces (USART, SPI, I2C). Because it shares the identical 28-pin DIP socket footprint as the ATmega328P, it is widely used for budget-conscious AVR builds, legacy board repairs, and low-cost automated testers.

Quick reference


CPU Core	8-bit AVR RISC Architecture
Package	28-pin DIP (DIP-28) / 32-pin TQFP / 32-pin QFN
Max Clock Frequency	$16\text{ MHz}$ ( $8\text{ MHz}$ on older ATmega8L)
Flash Memory	$8\text{ KB}$ (In-system self-programmable)
SRAM	$1024\text{ Bytes}$ ( $1\text{ KB}$ )
EEPROM	$512\text{ Bytes}$
Supply Voltage Range	$2.7\text{ V}$ to $5.5\text{ V}$ DC (ATmega8A)
GPIO Count	23 Programmable I/O lines
Analog Peripherals	6-channel (DIP) / 8-channel (TQFP) 10-bit ADC
Hardware Serial	1x USART, 1x Master/Slave SPI, 1x Byte-oriented 2-wire I2C (TWI)

Pinout (DIP-28 Package)

text

                            ┌───┴───┐
       (Active-Low) RESET 1│ 1   28│ PC5 (ADC5 / SCL / PCINT13)
             (RXD) PD0 2│       │27 PC4 (ADC4 / SDA / PCINT12)
             (TXD) PD1 3│       │26 PC3 (ADC3 / PCINT11)
            (INT0) PD2 4│       │25 PC2 (ADC2 / PCINT10)
            (INT1) PD3 5│ATmega8│24 PC1 (ADC1 / PCINT9)
              (T0) PD4 6│ DIP-28│23 PC0 (ADC0 / PCINT8)
                   VCC 7│       │22 GND (AGND)
                   GND 8│       │21 AREF
                 XTAL1 9│       │20 AVCC
                XTAL2 10│       │19 PB5 (SCK)
              (T1) PD5 11│       │18 PB4 (MISO)
            (AIN0) PD6 12│       │17 PB3 (MOSI / OC2)
            (AIN1) PD7 13│       │16 PB2 (SS / OC1B)
            (ICP1) PB0 14│       │15 PB1 (OC1A)
                            └───────┘

Comparison: ATmega8 vs ATmega328P

Parameter	ATmega8 / ATmega8A	ATmega328P
Pinout / Package	Identical DIP-28	Identical DIP-28
Flash Memory	$8\text{ KB}$	$32\text{ KB}$
SRAM	$1\text{ KB}$ ( $1024\text{ B}$ )	$2\text{ KB}$ ( $2048\text{ B}$ )
EEPROM	$512\text{ Bytes}$	$1024\text{ Bytes}$
PWM Outputs	3 channels (Timer1 & Timer2)	6 channels (Timer0, Timer1, Timer2)
Pin Change Interrupts	Limited to INT0 / INT1	Pin Change Interrupts on all 23 GPIOs

Common mistakes

Assuming 6 PWM channels like the 328P: The ATmega8 only has 3 hardware PWM channels (PB1/OC1A, PB2/OC1B, and PB3/OC2). Timer0 on the ATmega8 lacks PWM mode.
Bootloader size with modern Arduino sketches: The classic ATmega8 bootloader takes $1\text{ KB}$ of Flash, leaving only $7\text{ KB}$ for user sketches. Use the optimized MiniCore / Optiboot bootloader (only $512\text{ Bytes}$ ) to maximize available sketch memory.
Fuses configuration for internal 8MHz RC vs external crystal: Brand-new factory ATmega8 chips ship configured for internal 1MHz clock. When soldering a 16MHz crystal on breadboards, set the low fuse byte (lfuse) accordingly.

Notes

Arduino Core Support: Fully supported in modern Arduino IDE via the open-source MiniCore package.