Arduino MKR Zero Pinout: GPIO, ADC, DAC, PWM, UART, SPI, I2C, I2S and microSD

The Arduino MKR Zero is a compact 32-bit board based on the SAMD21 Cortex-M0+ processor.

Its defining feature is the onboard:

connected to a dedicated SPI interface, making the board particularly useful for:

• audio playback;
• data logging;
• portable instruments;
• offline storage;
• media projects;
• battery-powered applications.

The core architecture is:

Quick MKR Zero Specifications

Feature	MKR Zero
Main MCU	SAMD21 Cortex-M0+
Clock	48 MHz
RTC clock	32.768 kHz
Logic voltage	3.3 V
Flash	256 KB
Bootloader	8 KB
SRAM	32 KB
EEPROM	No dedicated EEPROM
Digital I/O	22 usable digital-numbered pins including analogue aliases
PWM	12 documented PWM-capable pins
Analog inputs	7
ADC resolution	8/10/12-bit selectable
True DAC	1 × 10-bit on A0
UART	1 external hardware UART
SPI	1 external SPI bus
I2C	1 external I2C bus
I2S	Available
microSD	Onboard, dedicated SPI1
Native USB	Full-Speed USB device / embedded host capable
Li-Po battery	Single-cell 3.7 V, 700 mAh minimum recommended
GPIO current	7 mA maximum per pin

3.3 V Logic: Do Not Feed 5 V into GPIO

The most important electrical rule is:

Arduino explicitly warns that applying more than:

to an I/O pin can damage the board.

This is different from classic 5 V boards such as:

• UNO R3;
• Mega 2560;
• Leonardo;
• Micro;
• UNO R4.

GPIO Current Limit

The official MKR Zero pinout specifies:

with additional group-level source/sink current limits.

Use GPIO for logic signals, not power loads.

For:

• relays;
• motors;
• solenoids;
• high-current LEDs;

use a transistor, MOSFET or dedicated driver.

Digital Pin Mapping

The main board edge exposes:

with these primary SAMD21 mappings:

Arduino pin	SAMD21 pin	Main functions
D0	PA22	PWM
D1	PA23	PWM
D2	PA10	PWM / I2S SCK
D3	PA11	PWM / I2S frame sync
D4	PB10	PWM
D5	PB11	PWM
D6	PA20	PWM
D7	PA21	PWM
D8	PA16	PWM / SPI COPI
D9	PA17	SPI SCK
D10	PA19	PWM / SPI CIPO
D11	PA08	I2C SDA / I2S data
D12	PA09	PWM / I2C SCL / I2S MCK
D13	PB23	UART RX
D14	PB22	UART TX

Analog Pins A0-A6

The MKR Zero exposes seven analogue inputs:

Analog pin	Digital alias	SAMD21 pin	Main function
A0	D15	PA02	AIN0 / DAC0
A1	D16	PB02	AIN10
A2	D17	PB03	AIN11
A3	D18	PA04	AIN4 / PWM
A4	D19	PA05	AIN5 / PWM
A5	D20	PA06	AIN6
A6	D21	PA07	AIN7 / I2S data

Analog Pins Can Also Be Digital Pins

The Arduino core gives the analogue pins digital aliases:

That is why Arduino’s store specification can correctly list:

even though only D0-D14 are printed as the main digital row.

ADC Resolution

The SAMD21 ADC supports selectable:

resolution.

For maximum Arduino-level resolution:

Then:

returns values approximately from:

A0 Is a True DAC Output

A0 is not only an ADC input.

It is also connected to:

and provides a true:

according to Arduino’s board specification.

DAC Example

This is fundamentally different from ordinary PWM output.

PWM Pins

Arduino currently lists 12 PWM-capable pins:

The remaining header pins are not marked as default PWM outputs in the official board specification.

Hardware UART

The external hardware UART is:

Use:

for these pins.

USB Serial vs Serial1

Because the SAMD21 has native USB:

This lets a project communicate with a PC and an external serial device at the same time.

I2C Pins

The main I2C bus is:

Use:

I2C Example

External SPI Pins

The user-accessible SPI bus is:

Use:

The microSD Card Uses a Separate SPI Interface

This is one of the MKR Zero’s best design features.

The onboard microSD slot uses:

rather than the main user-facing SPI pins.

That means using the SD card does not normally consume:

from your external SPI bus.

Internal microSD Pin Mapping

The dedicated SD interface is wired internally as:

These are internal board signals rather than normal header pins.

Why Dedicated SPI1 Is Useful

A typical project can therefore use:

without sharing the same SPI wires.

This is especially useful for:

• audio players;
• data loggers;
• display projects;
• portable instruments.

SD Library

Arduino’s official MKR Zero documentation states that the SD library recognises the board’s dedicated SD interface automatically.

The important chip-select constant is:

rather than hard-coding an UNO-style SD chip-select number.

Basic microSD Example

Writing a File

microSD Card Uses 3.3 V Signalling

The onboard SD interface is already designed for the board’s 3.3 V domain.

No external SD level-shifter board is required when using the integrated socket.

I2S Makes MKR Zero Useful for Audio

The SAMD21 also exposes an:

digital-audio interface.

The official pinout identifies signals including:

This is one reason Arduino positions MKR Zero strongly for sound and music projects.

microSD + I2S Audio Architecture

A typical digital audio project can use:

The SD card handles storage while I2S moves digital audio data to an external audio device.

Native USB

The SAMD21 includes native:

with USB device and embedded-host capability.

The normal board connector is:

for:

• programming;
• power;
• USB serial;
• USB HID applications.

USB HID

Because the SAMD21 provides native USB, MKR Zero can be used for:

• keyboard HID;
• mouse HID;
• custom USB interfaces.

This makes it more flexible than classic ATmega328P boards for USB-device projects.

Li-Po Battery Connector

The board includes support for a:

with onboard charging.

Arduino currently recommends at least:

capacity.

Battery Voltage Monitoring

MKR Zero includes an internal battery-voltage measurement path.

The Arduino SAMD core defines:

for reading that internal battery monitor.

This can be useful for:

• battery gauges;
• low-voltage warnings;
• portable loggers;
• sleep/wake applications.

VIN Is Not a Classic UNO VIN

The board is designed around a:

from USB/VIN.

Do not assume you can feed:

into VIN simply because classic UNO boards accept those ranges.

MKR boards use a different power architecture.

3.3 V and 5 V Rails

Arduino currently specifies:

subject to the power source and board operating conditions.

Remember:

Built-In LED

The SAMD core defines:

This is not one of the normal D0-D14 board-edge numbers.

ATECC508 Secure Element

The official MKR Zero pinout also shows an:

secure element connected internally.

This can be used for secure key storage and cryptographic identity functions through appropriate libraries.

External Interrupts

Arduino’s current store specification lists interrupt-capable pins including:

The SAMD21 event/interrupt system is more flexible than classic AVR boards, but using:

remains the safest Arduino-level approach.

SWD Debugging

The bottom-side debug pads expose:

for advanced debugging and programming with a suitable SWD probe.

Common Mistake 1: Applying 5 V to GPIO

The SAMD21 I/O is:

Do not apply 5 V to digital or analogue inputs.

Common Mistake 2: Using UNO SD Wiring

The onboard microSD does not use the main:

SPI connection.

It uses its own internal:

bus.

Common Mistake 3: Hard-Coding the SD Chip Select

Use:

rather than assuming an UNO-style SS pin.

Common Mistake 4: Assuming D9 Is PWM

Arduino’s current MKR Zero specification does not list:

among its 12 default PWM pins.

D9 is the main SPI:

pin.

Common Mistake 5: Confusing Serial with the UART

On MKR Zero:

Common Mistake 6: Driving Too Much Current

The official pinout states:

so use drivers for larger loads.

Quick Digital Reference

Quick Analog Reference

Quick Communications Reference

Final Thoughts

The Arduino MKR Zero is essentially a compact SAMD21 board optimised for storage and audio-oriented projects.

Its strongest combination is:

The dedicated microSD bus is particularly useful because it leaves the normal external SPI pins free for another peripheral.

The most important pinout rules are:

For a wireless version of the same general MKR/SAMD21 concept, see our Arduino MKR WiFi 1010 pinout guide. For other native-USB choices, see our Arduino USB HID boards comparison.