An 8-bit AVR microcontroller with a full-speed USB device controller built in, in TQFP-44, the chip behind the Arduino Leonardo, Micro, and Pro Micro. Because the USB peripheral is on-chip (D+/D- go straight to the connector) it enumerates as a keyboard, mouse, MIDI device, or serial port with no FT232 or CH340 helper. It has 32 KB of flash, 2.5 KB SRAM, and 1 KB EEPROM, runs the AVR core at up to 16 MHz, and brings out 26 I/O, a 12-channel 10-bit ADC, four timers with PWM, and hardware SPI and I2C. Datasheet-verified 44-pin pinout.

Pinout

Pin	Name	Type	Function
1	PE6/INT6/AIN0	bidirectional	Port E bit 6 GPIO with internal pull-up. Alternate: INT6 external interrupt input, AIN0 analog comparator positive input. Tri-stated on reset.
2	UVCC	power_in	USB pads internal 3.3V regulator input supply voltage. Connect to VCC through a decoupling network per USB module requirements.
3	D-	bidirectional	USB 2.0 Full-Speed/Low-Speed negative data upstream port. Connect to the USB connector D- pin through a 22Ω series resistor.
4	D+	bidirectional	USB 2.0 Full-Speed/Low-Speed positive data upstream port. Connect to the USB connector D+ pin through a 22Ω series resistor.
5	UGND	power_in	USB pads ground reference. Tie to system GND.
6	UCAP	passive	USB pads internal 3.3V regulator output. Connect to an external 1µF ceramic bypass capacitor to GND.
7	VBUS	input	USB VBUS monitor input used by firmware to detect host connection/disconnection. Sensed via internal comparator; may be connected to the USB VBUS rail (nominal 5V) directly or through a divider.
8	PB0/SS/PCINT0	bidirectional	Port B bit 0 GPIO with internal pull-up. Alternate: SPI slave-select (SS), pin-change interrupt PCINT0.
9	PB1/SCLK/PCINT1	bidirectional	Port B bit 1 GPIO with internal pull-up. Alternate: SPI serial clock (SCLK/SCK), pin-change interrupt PCINT1.
10	PB2/PDI/MOSI/PCINT2	bidirectional	Port B bit 2 GPIO with internal pull-up. Alternate: SPI MOSI, ISP programming data input (PDI), pin-change interrupt PCINT2.
11	PB3/PDO/MISO/PCINT3	bidirectional	Port B bit 3 GPIO with internal pull-up. Alternate: SPI MISO, ISP programming data output (PDO), pin-change interrupt PCINT3.
12	PB7/OC0A/OC1C/RTS/PCINT7	bidirectional	Port B bit 7 GPIO with internal pull-up. Alternate: Timer/Counter 0 output compare A (OC0A), Timer/Counter 1 output compare C (OC1C), USART RTS flow-control, pin-change interrupt PCINT7.
13	RESET	input	Active-low external reset input. A low level longer than the minimum pulse length triggers a reset even without the clock running. Has an internal pull-up; drive with an open-drain source or add an external pull-up + decoupling cap.
14	VCC	power_in	Digital supply voltage (2.7-5.5V). Decouple to GND with a 100nF ceramic close to the pin.
15	GND	power_in	Digital ground reference.
16	XTAL2	output	Output from the inverting on-chip oscillator amplifier. Connect to the far side of the external crystal/resonator with the appropriate load capacitor. Leave open when driving XTAL1 from an external clock.
17	XTAL1	input	Input to the inverting on-chip oscillator amplifier and the internal clock generator. Connect to the near side of the external crystal/resonator or to an external CMOS clock source.
18	PD0/OC0B/SCL/INT0	bidirectional	Port D bit 0 GPIO with internal pull-up. Alternate: Timer/Counter 0 output compare B (OC0B), TWI (I²C) clock line SCL, external interrupt INT0.
19	PD1/SDA/INT1	bidirectional	Port D bit 1 GPIO with internal pull-up. Alternate: TWI (I²C) data line SDA, external interrupt INT1.
20	PD2/RXD1/INT2	bidirectional	Port D bit 2 GPIO with internal pull-up. Alternate: USART1 receive data (RXD1), external interrupt INT2.
21	PD3/TXD1/INT3	bidirectional	Port D bit 3 GPIO with internal pull-up. Alternate: USART1 transmit data (TXD1), external interrupt INT3.
22	PD5/XCK1/CTS	bidirectional	Port D bit 5 GPIO with internal pull-up. Alternate: USART1 synchronous clock (XCK1), USART CTS hardware flow-control.
23	GND	power_in	Digital ground reference (second GND pin).
24	AVCC	power_in	Supply voltage for Port F and the A/D converter. If the ADC is unused, tie directly to VCC; if used, connect to VCC through a low-pass (typ. 10Ω + 100nF).
25	PD4/ICP1/ADC8	bidirectional	Port D bit 4 GPIO with internal pull-up. Alternate: Timer/Counter 1 input capture (ICP1), ADC channel 8 analog input.
26	PD6/T1/OC4D/ADC9	bidirectional	Port D bit 6 GPIO with internal pull-up. Alternate: Timer/Counter 1 external clock (T1), Timer/Counter 4 output compare D inverted (OC4D̅), ADC channel 9 analog input.
27	PD7/T0/OC4D/ADC10	bidirectional	Port D bit 7 GPIO with internal pull-up. Alternate: Timer/Counter 0 external clock (T0), Timer/Counter 4 output compare D (OC4D), ADC channel 10 analog input.
28	PB4/PCINT4/ADC11	bidirectional	Port B bit 4 GPIO with internal pull-up. Alternate: pin-change interrupt PCINT4, ADC channel 11 analog input.
29	PB5/PCINT5/OC1A/OC4B/ADC12	bidirectional	Port B bit 5 GPIO with internal pull-up. Alternate: pin-change interrupt PCINT5, Timer/Counter 1 output compare A (OC1A), Timer/Counter 4 output compare B inverted (OC4B̅), ADC channel 12 analog input.
30	PB6/PCINT6/OC1B/OC4B/ADC13	bidirectional	Port B bit 6 GPIO with internal pull-up. Alternate: pin-change interrupt PCINT6, Timer/Counter 1 output compare B (OC1B), Timer/Counter 4 output compare B (OC4B), ADC channel 13 analog input.
31	PC6/OC3A/OC4A	bidirectional	Port C bit 6 GPIO with internal pull-up. Alternate: Timer/Counter 3 output compare A (OC3A), Timer/Counter 4 output compare A inverted (OC4A̅).
32	PC7/ICP3/CLK0/OC4A	bidirectional	Port C bit 7 GPIO with internal pull-up. Alternate: Timer/Counter 3 input capture (ICP3), system clock output (CLK0/CLKO), Timer/Counter 4 output compare A (OC4A).
33	PE2/HWB	bidirectional	Port E bit 2 GPIO with internal pull-up. Alternate: Hardware Bootloader (HWB) select input, latched at reset to force execution of the bootloader when pulled low. Often tied to a button + pull-up for USB DFU entry.
34	VCC	power_in	Digital supply voltage (2.7-5.5V), second VCC pin. Decouple locally with a 100nF ceramic to GND.
35	GND	power_in	Digital ground reference (third GND pin).
36	PF7/ADC7/TDI	bidirectional	Port F bit 7 GPIO with internal pull-up. Alternate: ADC channel 7 analog input, JTAG test data input (TDI). When JTAG is enabled the internal pull-up is forced active even in reset.
37	PF6/ADC6/TDO	bidirectional	Port F bit 6 GPIO with internal pull-up. Alternate: ADC channel 6 analog input, JTAG test data output (TDO).
38	PF5/ADC5/TMS	bidirectional	Port F bit 5 GPIO with internal pull-up. Alternate: ADC channel 5 analog input, JTAG test mode select (TMS). When JTAG is enabled the internal pull-up is forced active even in reset.
39	PF4/ADC4/TCK	bidirectional	Port F bit 4 GPIO with internal pull-up. Alternate: ADC channel 4 analog input, JTAG test clock (TCK). When JTAG is enabled the internal pull-up is forced active even in reset.
40	PF1/ADC1	bidirectional	Port F bit 1 GPIO with internal pull-up. Alternate: ADC channel 1 analog input.
41	PF0/ADC0	bidirectional	Port F bit 0 GPIO with internal pull-up. Alternate: ADC channel 0 analog input.
42	AREF	passive	Analog reference input for the A/D converter. Connect an external reference or decouple to GND with 100nF when using AVCC/internal 2.56V reference. Must not exceed AVCC.
43	GND	power_in	Analog ground reference; tie to GND star point near AVCC decoupling.
44	AVCC	power_in	Analog supply voltage for Port F and the A/D converter (second AVCC pin). If the ADC is unused, tie directly to VCC; if used, connect to VCC through a low-pass filter.

Key specifications

Parameter	Value
Core	8-bit AVR RISC
Flash	32 KB
SRAM	2.5 KB
EEPROM	1 KB
USB	full-speed device (on-chip, no external PHY)
Max clock	16 MHz
I/O	26
ADC	12-channel 10-bit
Supply voltage	2.7-5.5 V
Package	TQFP-44

Ordering variants

ATmega32U4-AU (TQFP-44)
ATmega32U4-MU (QFN-44)

CAD (KiCad official)

Symbol: MCU_Microchip_ATmega:ATmega32U4-A
Footprint: Package_QFP:TQFP-44_10x10mm_P0.8mm
3D model: TQFP-44 (KiCad packages3D)

Provenance

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

Datasheet: Microchip Technology datasheet (12 pp, sha256 8ead4c44bcf240ef, 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