The original 28-pin AVR, an 8-bit Microchip (Atmel) microcontroller that predates and shares its pinout with the ATmega328P. It has 8 KB of self-programming flash, 1 KB of SRAM, and 512 bytes of EEPROM, runs the AVR RISC core at up to 16 MHz, and brings out 23 GPIO across three ports, a 6-channel 10-bit ADC, three timers with PWM, and hardware USART, SPI, and I2C (TWI). A crystal on XTAL1/XTAL2 or the internal oscillator sets the clock, and it programs in-circuit over SPI (MOSI/MISO/SCK). Still common in legacy designs and where 8 KB is enough. Datasheet-verified 28-pin pinout.

Pinout

Pin	Name	Type	Function
1	PC6/RESET	bidirectional	Port C bit 6, or active-low external Reset input. When the RSTDISBL fuse is programmed, PC6 acts as a general-purpose I/O; when unprogrammed (default), a low level longer than the minimum pulse length forces a chip reset even without a running clock.
2	PD0/RXD	bidirectional	Port D bit 0. Alternate function is USART receive data input (RXD).
3	PD1/TXD	bidirectional	Port D bit 1. Alternate function is USART transmit data output (TXD).
4	PD2/INT0	bidirectional	Port D bit 2 with internal pull-up. Alternate function is external interrupt 0 (INT0).
5	PD3/INT1	bidirectional	Port D bit 3 with internal pull-up. Alternate function is external interrupt 1 (INT1).
6	PD4/XCK/T0	bidirectional	Port D bit 4. Alternate functions: USART external clock (XCK) and Timer/Counter0 external counter input (T0).
7	VCC	power_in	Digital supply voltage. Operating range 2.7-5.5 V. Decouple to GND with a 100 nF ceramic close to the pin.
8	GND	power_in	Digital ground reference.
9	PB6/XTAL1/TOSC1	bidirectional	Port B bit 6. Also serves as input to the inverting crystal oscillator amplifier (XTAL1) or Timer/Counter2 asynchronous oscillator input (TOSC1), depending on clock-source fuses and the AS2 bit in ASSR.
10	PB7/XTAL2/TOSC2	bidirectional	Port B bit 7. Also serves as output of the inverting crystal oscillator amplifier (XTAL2) or Timer/Counter2 asynchronous oscillator output (TOSC2).
11	PD5/T1	bidirectional	Port D bit 5. Alternate function is Timer/Counter1 external counter input (T1).
12	PD6/AIN0	bidirectional	Port D bit 6. Alternate function is Analog Comparator positive input (AIN0).
13	PD7/AIN1	bidirectional	Port D bit 7. Alternate function is Analog Comparator negative input (AIN1).
14	PB0/ICP1	bidirectional	Port B bit 0. Alternate function is Timer/Counter1 input capture pin (ICP1).
15	PB1/OC1A	bidirectional	Port B bit 1. Alternate function is Timer/Counter1 output compare match A (OC1A) - PWM output.
16	PB2/SS/OC1B	bidirectional	Port B bit 2. Alternate functions: SPI slave-select (SS, active low) and Timer/Counter1 output compare match B (OC1B).
17	PB3/MOSI/OC2	bidirectional	Port B bit 3. Alternate functions: SPI Master Out Slave In (MOSI) and Timer/Counter2 output compare match (OC2). Also serves as MOSI during SPI serial programming.
18	PB4/MISO	bidirectional	Port B bit 4. Alternate function is SPI Master In Slave Out (MISO). Also serves as MISO during SPI serial programming.
19	PB5/SCK	bidirectional	Port B bit 5. Alternate function is SPI serial clock (SCK). Also serves as SCK during SPI serial programming.
20	AVCC	power_in	Analog supply pin for the A/D converter, Port C (3:0), and (on TQFP/VQFN) ADC7:6. Must be externally connected to VCC even if the ADC is unused; if the ADC is used, tie to VCC through a low-pass filter (e.g. 10 uH + 100 nF).
21	AREF	passive	Analog reference input for the A/D converter. Bypass to AGND with a ~100 nF capacitor when using the external reference; leave for internal reference selection via ADMUX (REFS1:0).
22	GND	power_in	Analog/second ground reference. Tie to the same ground plane as pin 8.
23	PC0/ADC0	bidirectional	Port C bit 0. Alternate function is ADC channel 0 input (10-bit).
24	PC1/ADC1	bidirectional	Port C bit 1. Alternate function is ADC channel 1 input (10-bit).
25	PC2/ADC2	bidirectional	Port C bit 2. Alternate function is ADC channel 2 input (10-bit).
26	PC3/ADC3	bidirectional	Port C bit 3. Alternate function is ADC channel 3 input (10-bit).
27	PC4/ADC4/SDA	bidirectional	Port C bit 4. Alternate functions: ADC channel 4 input and Two-wire Serial Interface (I2C) data line SDA (open-drain when TWI enabled via TWCR).
28	PC5/ADC5/SCL	bidirectional	Port C bit 5. Alternate functions: ADC channel 5 input and Two-wire Serial Interface (I2C) clock line SCL (open-drain when TWI enabled via TWCR).

Key specifications

Parameter	Value
Core	8-bit AVR RISC
Flash	8 KB
SRAM	1 KB
EEPROM	512 B
Max clock	16 MHz
GPIO	23 (ports B/C/D)
ADC	6-channel 10-bit
Serial	USART + SPI + I2C (TWI)
Supply voltage	2.7-5.5 V
Package	PDIP-28

Ordering variants

ATmega8A-PU (PDIP-28)
ATmega8A-AU (TQFP-32)

CAD (KiCad official)

Symbol: MCU_Microchip_ATmega:ATmega8A-P
Footprint: Package_DIP:DIP-28_W7.62mm
3D model: DIP-28 (KiCad packages3D)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: Microchip Technology datasheet (14 pp, sha256 7219f52fb53d9a46, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF