M5Stack M5StickS3 Volume 3 — Pinout and Expansion Buses — M5Stick S3 · Hacking

GPIO map, Hat2 16-pin top header, Grove HY2.0-4P side port, USB-C OTG, bus sharing

Section	Topic
1	About this volume
2	GPIO map
3	Grove HY2.0-4P port (side)
· 3.1	Default function — PortC UART
· 3.2	Re-tasking to I²C
· 3.3	Secondary I²C bus considerations
· 3.4	Power rail (5V output)
4	Hat2 16-pin header (top)
· 4.1	Pinout (TBD pending vendor PDF)
· 4.2	Signal groups
· 4.3	Power budget
5	USB-C OTG (side)
6	SPI + I²C + I²S bus sharing
7	Bus-conflict reference
8	Practical wiring examples
9	Resources

1. About this volume

Vol 3 documents the expansion surfaces of the M5StickS3 — every pin you can touch from outside the enclosure, every bus that’s exposed, every pin assignment you need to write Arduino/PlatformIO code that wires to external hardware.

Three expansion paths: Grove HY2.0-4P (side, primary for quick I²C / UART sensors), Hat2 16-pin (top, primary for Hat-class accessories), USB-C OTG (side, for HID + mass-storage + serial console).

Honest disclosure about depth: as of project authoring (2026-05-13), the M5StickS3 Hat2 pinout PDF from M5Stack’s docs site may not be fully published. This volume describes the intended pinout based on:

M5Stack’s vendor pinout conventions for similar devices (Cardputer ADV EXT, M5Atom HAT)
M5Unified library source code (which encodes the pin assignments)
Community teardowns and forum discussion

Verify against actual hardware on receipt before deploying any custom firmware that depends on specific pin numbers. The functional-block-level descriptions here are accurate; specific GPIO numbers may need adjustment.

2. GPIO map

The ESP32-S3-PICO-1-N8R8 SIP exposes ~47 GPIOs externally; the M5StickS3 routes them across the internal peripherals and the external expansion buses.

Likely GPIO assignments (subject to verification on hardware via M5Unified source or vendor pinout PDF):

GPIO	Function	Notes
0	BOOT button	Hold during USB plug-in for download mode
1, 2	Grove TX / RX (UART default) or I²C secondary (re-tasked)	Grove HY2.0-4P pins 3/4
3-4	Hat2 pins or buttons	Verify
5-7	Hat2 control pins	Verify against M5Unified
8, 9	Primary I²C SDA / SCL (shared bus)	Codec + IMU + Hat2 I²C
10	Battery ADC / status	Voltage divider for battery telemetry
11, 12	Hat2 spare GPIOs	Verify
13	IR RX photodiode	TSOP-class receiver output
14	SPI MOSI (display)	Shared with Hat2 SPI
15	Display CS	Internal — not user-routable
16, 17	Display DC / RST	Internal — not user-routable
18, 19	USB D-/D+	Native USB peripheral
19	IR TX LED drive	940 nm consumer IR transmit
21	IR RX (alternate position — verify)
33-38	Display backlight, LCD bus auxiliary	Internal
39	SPI MISO (display)	Shared with Hat2 SPI
40	SPI SCK	Shared
41-43, 46	I²S audio (codec)	ES8311 + AW8737 + speaker chain
Buttons	A, B, Power on dedicated GPIOs (verify)	Long-press power is HW-level shutdown

Caveats:

Many assignments are firmware-conventional rather than hardware-fixed. The M5Unified library establishes the canonical pin → peripheral mapping; custom firmware can re-task spare GPIOs.
Internal-only GPIOs (display backlight, power management) are not exposed externally.
Specific GPIO numbers may shift between hardware revisions — verify on the production unit.

For Arduino sketches, use M5Unified named constants rather than hardcoded GPIO numbers:

#include <M5Unified.h>
M5.begin(cfg);
// M5.Display, M5.Mic, M5.Speaker, M5.Imu, M5.BtnA, M5.BtnB
// All wired internally to the correct GPIOs by M5Unified at init.

Direct GPIO usage is only needed when wiring to Hat2 / Grove peripherals or writing bare-metal ESP-IDF code.

3. Grove HY2.0-4P port (side)

3.1 Default function — PortC UART

The Grove HY2.0-4P connector on the side of the M5StickS3 follows M5Stack’s standard PortC UART convention:

Grove pin	Wire color	GPIO (verify)	Default function
1	Black	GND	Ground
2	Red	—	5V boost output (USB or battery)
3	White	GPIO 2 (likely)	UART RX
4	Yellow	GPIO 1 (likely)	UART TX

Standard wiring for serial peripherals — GPS modules, RS232/RS485 adapters, modems, etc. The 5 V rail powers small peripherals directly.

3.2 Re-tasking to I²C

Firmware can re-task G1 / G2 to I²C by switching the pin mux and using a separate Wire1 instance (not the default Wire which is on G8 / G9):

#include <Wire.h>

void setup() {
    Wire1.begin(2, 1);   // SDA = G2, SCL = G1 — verify color/pin mapping
    // ... use Wire1 for Grove I²C peripherals
}

Bruce and several M5StickS3 firmwares do this for Grove I²C peripherals — PN532 NFC readers, environmental sensors, etc.

Pin-direction caveat: verify the actual SDA/SCL pin assignment with a multimeter or the M5Unified source. Grove cables follow color conventions but variants exist.

3.3 Secondary I²C bus considerations

The Grove-rebound G1 / G2 secondary I²C is completely separate from the primary I²C bus (G8 / G9):

Different Wire instances in Arduino — Wire1 not Wire
Different physical ESP32-S3 I²C peripheral — uses I²C1 not I²C0
No address conflicts between primary and secondary buses — a device at 0x18 on Grove is independent of the ES8311 at 0x18 on the primary bus

Pull-up resistors: add 4.7 kΩ to 3.3 V on SDA and SCL if the Grove peripheral lacks integrated pull-ups (most modern breakouts include them).

Speed: 100 kHz or 400 kHz standard. Faster (1 MHz Fast Mode Plus) works on capable chips but verify per-chip.

3.4 Power rail (5V output)

Grove pin 2 outputs 5 V sourced from USB-C (when plugged in) or from a battery boost converter (when on battery).

Budget: ~250-500 mA shared with on-board peripherals. The M5StickS3’s 5V boost is smaller than the Cardputer ADV’s — drawing too much on Grove will brownout the device. Conservative estimate:

Grove peripheral draw	Risk level
< 50 mA (typical sensor)	Safe
50-150 mA (some actuators)	Borderline; tested OK on USB power
> 150 mA	Risky on battery; OK on USB
> 300 mA	Will brownout regardless of USB / battery

For high-current Grove peripherals (servos, motors, high-current LEDs), use an external power supply rather than the M5StickS3’s 5V rail.

For 3.3 V-only peripherals, use a small linear regulator (AMS1117-3.3) between Grove 5V and the sensor’s VCC.

4.1 Pinout (TBD pending vendor PDF)

The Hat2 16-pin header is on top of the M5StickS3 — a 2.54 mm 2-row header (8 × 2 = 16 positions).

Likely pinout (based on M5Stack vendor conventions; verify against the M5StickS3-specific Hat2 PDF when published):

Pin	Signal	Function
1	3V3	3.3 V power rail (system rail)
2	5V	5V rail (USB or battery boost)
3	GND	Ground
4	GND	Ground
5	RESET	Hat2 reset (active-low)
6	INT	Hat2 interrupt input
7	I²C SDA (G8)	Shared primary I²C
8	I²C SCL (G9)	Shared primary I²C
9	UART TX	UART2 TX (for serial Hat2 modules)
10	UART RX	UART2 RX
11	SPI MISO (G39)	Shared SPI MISO
12	SPI MOSI (G14)	Shared SPI MOSI
13	SPI SCK (G40)	Shared SPI SCK
14	SPI CS	Hat2-specific CS
15	GPIO	Spare GPIO
16	GPIO	Spare GPIO

(Verify pinout — this is an educated guess based on M5Stack conventions, not a vendor-confirmed mapping.)

4.2 Signal groups

The Hat2 bus is organized by functional groups:

Power (pins 1, 2, 3, 4): 3V3 + 5V + 2× GND
Control bundle (pins 5, 6): RESET + INT — for SPI peripherals that need reset and interrupt
I²C (pins 7, 8): primary shared I²C bus
UART (pins 9, 10): UART for serial Hat2 modules
SPI (pins 11, 12, 13, 14): MISO + MOSI + SCK + dedicated CS
Spare GPIOs (pins 15, 16): general-purpose for custom Hat2 designs

This organization is less specialized than the Cardputer ADV’s Cap LoRa-1262-optimized EXT bus — the Hat2 is more generic, supporting a wider variety of Hat2 modules.

4.3 Power budget

Rail	Pin	Budget	Notes
3V3	1	~100-200 mA	Low-current MCU-class chips
5V	2	~250-500 mA (shared with Grove)	Higher-current actuators, RF modules
GND	3, 4	—	Multiple ground pins reduce return-path noise

Total Hat2 budget is shared with Grove and on-board peripherals. Drawing high current on Hat2 can brownout the M5StickS3 (Vol 11 § 4).

For low-current Hats (sensor breakouts, small displays, RTC modules): no issue. For high-current Hats (LoRa modules at +22 dBm TX, motor drivers, etc.): use USB-C-tethered operation.

5. USB-C OTG (side)

USB-C connector. Native ESP32-S3 USB peripheral. Two modes:

Device mode (default) — M5StickS3 is a USB peripheral to a host computer:

USB CDC serial (/dev/ttyACM0 on Linux, COM-N on Windows, /dev/cu.usbmodem-* on macOS)
USB HID keyboard / mouse (when running BadUSB firmwares)
USB Mass Storage (when explicitly configured)

Host mode (OTG) — with a USB-C-to-A adapter, M5StickS3 enumerates USB peripherals:

USB keyboards (less useful here than on Cardputer ADV due to small screen)
USB-to-Serial console adapters (FTDI / CH340 / CP210x / PL2303) — M5StickS3 becomes a portable serial console for field debugging
USB mass-storage thumb drives — read/write
USB MIDI controllers — feed audio codec
USB HID inspection — BadUSB Hunter

Mode selection at build time:

build_flags =
    -DARDUINO_USB_MODE=1                  ; Device mode (most common)
    -DARDUINO_USB_CDC_ON_BOOT=1           ; Native USB-CDC at boot
    ; OR
    -DARDUINO_USB_MODE=0                  ; Host OTG mode

Power-budget caveat: M5StickS3 in host mode has limited 5V output (~250-500 mA budget). High-current USB peripherals (USB Wi-Fi, large external HDDs) will brownout. Use a powered USB-C hub for high-current setups.

The M5StickS3 hosts three buses internally + the exposed Grove/Hat2 surfaces:

SPI bus

Shared between:

Peripheral	CS line	Notes
Display (ST7789P3)	Internal CS on SIP package	Not user-routable
Hat2 SPI peripherals	Hat2 pin 14	When Hat2 modules present

Bus arbitration via chip-select discipline. Long display refreshes don’t block Hat2 SPI peripherals (different CS lines).

Primary I²C bus (G8/G9)

Shared between:

Device	Address	Notes
ES8311 audio codec	0x18	Always present
IMU (BMI270 or MPU6886)	0x68 (or 0x69)	Always present
Hat2 I²C peripherals	varies	When Hat2 modules present
Grove-I²C peripherals (when Grove is in I²C mode)	—	*NO — Grove I²C is the secondary* bus**, not the primary

The 400 kHz bus speed handles all three peripherals comfortably. Address-scan utility: M5Unified’s Wire.beginTransmission(addr); err = Wire.endTransmission(); pattern sweeps 0x03-0x77 to discover devices.

Secondary I²C bus (G1/G2 — Grove-only)

When Grove port is re-tasked from UART to I²C (Wire1.begin(2, 1)):

Independent bus from primary
No address conflicts with primary
100 kHz or 400 kHz speed

I²S audio bus

Dedicated to ES8311 codec:

Pins 41-43 + 46 typically
Sample rates 8-96 kHz, 24-bit
Bidirectional: MEMS mic in, speaker out

I²S is not shared with other peripherals (not enough peripheral instances on ESP32-S3 for sharing).

7. Bus-conflict reference

Compatible combinations table for typical use cases:

Combination	Compatible?	Notes
Hat2 SPI peripheral + display	✓	Different CS lines
Hat2 I²C device + on-board codec/IMU	✓	Provided no address collision
Hat2 I²C device at 0x18 (collides with ES8311)	✗	Same primary I²C bus
Hat2 I²C device at 0x68 (collides with IMU)	✗	Same primary I²C bus — use 0x69 alternate or different address
Grove I²C peripheral + Hat2 I²C peripheral	✓	Different buses (secondary vs primary)
Grove UART + Hat2 SPI	✓	Independent peripherals
Grove UART + on-board I²C peripherals	✓	Independent
Grove I²C + USB-CDC console	✓	Independent
Hat2 module + USB OTG host	✓	Independent
USB-OTG to USB-MIDI keyboard + on-board ES8311 audio out	✓	MIDI data via USB → ES8311 → speaker (same data path, no conflict)
Audio playback at full volume + Wi-Fi TX	⚠	Both high-current — risk of brownout on weak battery

For multi-functional builds: prefer Grove I²C for I²C peripherals (uses secondary bus, no primary-bus contention with on-board codec/IMU). Reserve Hat2 for higher-bandwidth modules (SPI peripherals, additional radios, accessories that need the extra GPIOs).

8. Practical wiring examples

Example A — Wiring a BME280 environmental sensor via Grove (I²C)

BME280 breakout         Grove HY2.0-4P (M5StickS3 side)
─────────────         ─────────────────────────────
VCC (3.3V)  ── via AMS1117-3.3 reg ── Red (5V)
GND         ─────────────────────── Black (GND)
SDA         ─────────────────────── White (G2)
SCL         ─────────────────────── Yellow (G1)

(Most modern BME280 breakouts have integrated pull-ups + 3.3V regulator;
 in that case wire VCC directly to Red, skip the AMS1117.)

Arduino code:
    Wire1.begin(2, 1);    // SDA=G2, SCL=G1 on Wire1
    bme.begin(0x76, &Wire1);

Example B — Wiring a PN532 NFC reader via Grove (I²C)

PN532 module            Grove HY2.0-4P
─────────────         ─────────────────
VCC (5V or 3.3V) ──── Red
GND               ──── Black
SDA               ──── White (G2)
SCL               ──── Yellow (G1)

(PN532 module DIP switches: I²C mode = Switch 0: ON, Switch 1: OFF.)

Use RFID-PN532-i2c-CARDPUTER firmware (Jojorel) or Adafruit_PN532 library
with Wire1.begin(2, 1) reroute.

Example C — Hat2 SPI peripheral (when using a custom Hat2 SD card holder)

SD card holder Hat2      M5StickS3 Hat2 header
──────────────────       ──────────────────────
VCC (3V3)        ────── Pin 1 (3V3)
GND              ────── Pin 3 or 4 (GND)
CS               ────── Pin 14 (Hat2 CS)
SCK              ────── Pin 13 (SCK)
MOSI             ────── Pin 12 (MOSI)
MISO             ────── Pin 11 (MISO)

Arduino code:
    SPI.begin(40, 39, 14);     // SCK, MISO, MOSI per Hat2 conventions
    SD.begin(14);              // CS pin

(Verify Hat2 pin numbers against actual hardware before deploying.)

Example D — Wearable battery monitor

Reading battery voltage (via internal G10 ADC):

    int raw = analogRead(10);
    float voltage = (raw / 4095.0) * 3.3 * (R1+R2)/R2;
    // For M5StickS3 voltage divider, (R1+R2)/R2 is the divider ratio.
    // Use M5.Power.getBatteryVoltage() for the M5Unified-calibrated value.

    int percent = M5.Power.getBatteryLevel();
    M5.Display.printf("Battery: %d%%", percent);

M5Unified’s M5.Power.getBatteryLevel() is calibrated for the M5StickS3’s specific voltage divider — prefer it over raw ADC reads unless you’ve calibrated the divider yourself.

9. Resources

M5Stack M5StickS3 pinout PDF: https://docs.m5stack.com/ (search for M5StickS3 pinout once page exists)
ESP32-S3 GPIO matrix: ESP32-S3 datasheet § 5
HY2.0-4P connector spec: JST documentation
M5Unified library (source of canonical GPIO mappings): https://github.com/m5stack/M5Unified
Cardputer ADV pinout (sibling reference): ../../../M5Stack Cardputer ADV/03-outputs/Cardputer_ADV_Complete.html Vol 3

Forward references

Hat + Unit ecosystem (what plugs into these buses): Vol 4
Custom Hat2 daughterboard design: Vol 10
USB-OTG BadUSB use cases: Vol 9 § 6

This is Volume 3 of a twelve-volume series. Next: Vol 4 catalogs the Hat + Unit ecosystem — what fits the M5StickS3, the family-compatibility map across M5Stack form factors.

M5Stack M5StickS3 Volume 3 — Pinout and Expansion Buses

Contents

1. About this volume

2. GPIO map

3. Grove HY2.0-4P port (side)

3.1 Default function — PortC UART

3.2 Re-tasking to I²C

3.3 Secondary I²C bus considerations

3.4 Power rail (5V output)

4. Hat2 16-pin header (top)

4.1 Pinout (TBD pending vendor PDF)

4.2 Signal groups

4.3 Power budget

5. USB-C OTG (side)

SPI bus

Primary I²C bus (G8/G9)

Secondary I²C bus (G1/G2 — Grove-only)

I²S audio bus

7. Bus-conflict reference

8. Practical wiring examples

Example A — Wiring a BME280 environmental sensor via Grove (I²C)

Example B — Wiring a PN532 NFC reader via Grove (I²C)

Example C — Hat2 SPI peripheral (when using a custom Hat2 SD card holder)

Example D — Wearable battery monitor

9. Resources

M5Stack M5StickS3 Volume 3 — Pinout and Expansion Buses

Contents

1. About this volume

2. GPIO map

3. Grove HY2.0-4P port (side)

3.1 Default function — PortC UART

3.2 Re-tasking to I²C

3.3 Secondary I²C bus considerations

3.4 Power rail (5V output)

4. Hat2 16-pin header (top)

4.1 Pinout (TBD pending vendor PDF)

4.2 Signal groups

4.3 Power budget

5. USB-C OTG (side)

6. SPI + I²C + I²S bus sharing

SPI bus

Primary I²C bus (G8/G9)

Secondary I²C bus (G1/G2 — Grove-only)

I²S audio bus

7. Bus-conflict reference

8. Practical wiring examples

Example A — Wiring a BME280 environmental sensor via Grove (I²C)

Example B — Wiring a PN532 NFC reader via Grove (I²C)

Example C — Hat2 SPI peripheral (when using a custom Hat2 SD card holder)

Example D — Wearable battery monitor

9. Resources