M5Stack Cardputer ADV Volume 3 — Pinout and Expansion Buses — M5Stack Cardputer ADV · Hacking

GPIO map, Grove HY2.0-4P top-edge port, 14-pin EXT bus underside, USB-OTG side, SPI + I²C bus sharing

Section	Topic
1	About this volume
2	GPIO map — full pin assignment table
3	Grove HY2.0-4P port (top edge)
· 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	14-pin EXT bus (underside)
· 4.1	Full pinout table
· 4.2	Signal-group organization
· 4.3	Power rails (5V_IN, 3V3, GND)
· 4.4	Mating connector — 2.54 mm header convention
5	USB-C OTG (side)
· 5.1	Device mode (default)
· 5.2	Host mode (OTG)
· 5.3	Build-flag selection
6	SPI bus sharing — display + microSD + EXT
7	I²C bus map — codec + keyboard + IMU + EXT + Grove
8	Bus-conflict reference
9	Practical wiring examples
10	Resources

1. About this volume

Vol 3 documents the expansion surfaces of the Cardputer ADV — 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. The information is the bridge between Vol 2 (what’s inside the box) and Vol 4 (what you can plug into the box).

Three expansion paths: Grove (top edge, HY2.0-4P, primary for quick sensors), EXT (underside, 14-pin 2.54 mm, primary for Cap-class daughter cards), USB-C OTG (side, for HID inspection + mass storage + serial console).

The bus-conflict reference in § 8 is the section most often referenced in practice: “can I plug X on Grove while Y is on EXT?” answers.

2. GPIO map — full pin assignment table

The ESP32-S3FN8 has 47 GPIOs routed externally on the Stamp-S3A; the Cardputer ADV uses essentially all of them. Authoritative reference: the Cardputer ADV pinout PDF in M5Stack docs.

GPIO	Function	Notes
0	BOOT button	Hold during USB plug-in to enter download mode
1	Grove TX (UART) / I²C SCL (secondary, re-tasked)	Grove pin 4
2	Grove RX (UART) / I²C SDA (secondary, re-tasked)	Grove pin 3
3	EXT RESET	Drives SX1262 RST on Cap LoRa-1262
4	EXT INT	SX1262 IRQ / DIO1 input
5	EXT CS	SX1262 NSS chip-select
6	EXT BUSY	SX1262 BUSY status input
7	(internal — power switch / battery routing)	—
8	I²C SDA (primary, shared bus)	Codec + keyboard scanner + IMU + EXT + Grove-when-I²C
9	I²C SCL (primary, shared bus)	Same
10	Battery ADC	Voltage divider ~0.49× V_BAT
11	Keyboard INT (from TCA8418)	Falling edge on event-FIFO entry
12	microSD CS	Active-low
13	EXT UART RX (from AT6668 TX)	UART2 RX in default firmware
14	SPI MOSI	Shared with display + microSD + EXT
15	EXT UART TX (to AT6668 RX)	UART2 TX in default firmware
16-17	(internal — display backlight control / power)	—
18-19	USB D-/D+	Native USB peripheral
20-32	(internal / display matrix / power management)	Various
33-37	LCD bus signals	LCD parallel-mode or SPI-bus auxiliary lines
38	RGB LED (WS2812-style on-board indicator)	Single addressable LED
39	SPI MISO	Shared with display (MISO unused there) + microSD + EXT
40	SPI SCK / microSD CLK	Shared
41-43	I²S bus (audio)	Bit clock + word select + data
44	IR TX LED drive	940 nm consumer-IR transmit
45-46	I²S audio (continued)	Master clock + data direction

Caveats when reading this table:

Multiple GPIOs are shared (SPI bus, I²C bus) — same pin, same signal, multiple peripherals on different CS / addresses.
Some assignments are firmware-conventional rather than hardware-fixed (e.g., G1/G2 as Grove UART is the default; firmware can re-task them to I²C).
The internal-only GPIOs (7, 16-17, 20-32) handle power management, level shifters, and display backlight — they’re not user-routable.

For Arduino sketches, the M5Cardputer library exposes named constants for the canonical assignments (M5.Lcd.cs, M5.Mic.bclk, etc.) so most application code doesn’t need this table. The table is for: writing bare-metal ESP-IDF code, debugging hardware-level issues, designing custom hardware that plugs into the Cardputer ADV.

3. Grove HY2.0-4P port (top edge)

3.1 Default function — PortC UART

The top-edge Grove HY2.0-4P connector is a standard M5Stack-convention “PortC” — a 4-pin Grove cable carrying GND + 5V + UART RX + UART TX.

Grove pin	Wire color	GPIO	Default function
1	Black	GND	Ground
2	Red	—	5V boost output (from USB or battery boost)
3	White	GPIO 2	UART2 RX (default)
4	Yellow	GPIO 1	UART2 TX (default)

This is the canonical wiring for serial sensors and actuators — GPS modules, RS232 / RS485 adapters, modems, etc. The 5V rail can power small peripherals directly (sensors typically draw < 50 mA so well within budget).

3.2 Re-tasking to I²C

Firmware can re-task G1/G2 to I²C by:

Switching the ESP32-S3 pin mux from UART2 to I²C peripheral.
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 (yellow=TX→SCL, white=RX→SDA — verify against your Grove cable)
    // ... use Wire1 instead of Wire for this bus
}

Many Cardputer firmwares (Bruce, RFID-PN532-i2c-CARDPUTER) do this for Grove I²C peripherals — PN532 NFC readers, environmental sensors, etc.

Pin-direction caveat: Grove cables are color-conventional but variants exist. Some breakout boards label pins differently. When wiring a new Grove peripheral, verify with a multimeter or trust the M5Stack-supplied cables.

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 instance in Arduino — Wire1 not Wire. Different ESP32-S3 I²C peripheral.

Implications:

An I²C device on Grove cannot share the same address as one on the primary bus.
An I²C device on Grove does not appear on the primary bus’s address scan. They’re physically isolated buses.
Pull-up resistors: if the sensor breakout doesn’t have built-in pull-ups, add 4.7 kΩ to 3.3 V on SDA and SCL. Most modern breakouts include them.
Speed: the secondary I²C bus can run at 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 5V sourced from USB-C input (when plugged in) or from a battery boost converter (when on battery). Budget: ~500 mA shared with the on-board peripherals — drawing too much on Grove can brownout the system.

Sensors typically draw < 50 mA; actuators (servos, relays, LEDs) can pull more. For high-current Grove peripherals, prefer USB-C-powered operation over battery-only.

For 3.3 V-only peripherals, use a small linear regulator (AMS1117-3.3 module breaks out a 3-pin step-down) between the Grove 5V and the sensor’s VCC. Or wire to the EXT bus’s 3V3 pin directly (Vol § 4.3).

4. 14-pin EXT bus (underside)

4.1 Full pinout table

The 14-pin EXT bus on the underside is a 2.54 mm 2-row header (7 × 2 = 14 positions). It’s the primary expansion path for Cap modules — tightly-coupled daughterboards that mate flush against the Cardputer ADV’s underside.

Approximate pinout (verify against the vendor’s Cardputer ADV pinout PDF — pin numbering convention varies by drawing):

Pin	Signal	GPIO	Function
1	3V3	—	3.3 V power rail (system rail)
2	5V_IN	—	5 V rail (from USB or battery boost)
3	EXT RESET	GPIO 3	Active-low reset to SX1262 (or similar EXT peripheral)
4	EXT INT	GPIO 4	Interrupt input (e.g., SX1262 DIO1)
5	EXT CS	GPIO 5	SPI chip-select for EXT peripheral
6	EXT BUSY	GPIO 6	Status input (e.g., SX1262 BUSY)
7	I²C SDA	GPIO 8	Shared primary I²C bus
8	I²C SCL	GPIO 9	Shared primary I²C bus
9	EXT UART TX	GPIO 15	UART TX to EXT (typically GNSS RX on AT6668)
10	MISO	GPIO 39	Shared SPI MISO
11	SCK	GPIO 40	Shared SPI SCK
12	MOSI	GPIO 14	Shared SPI MOSI
13	EXT UART RX	GPIO 13	UART RX from EXT (typically GNSS TX on AT6668)
14	GND	—	Ground

4.2 Signal-group organization

The EXT bus is organized by functional groups:

Power (1, 2, 14): 3V3 + 5V_IN + GND
Cap-control bundle (3, 4, 5, 6): RESET + INT + CS + BUSY — designed for the SX1262 LoRa modem’s exact pinout. Reusable for any SPI peripheral that needs reset + interrupt + chip-select + status, but most Caps follow the SX1262 convention.
I²C (7, 8): primary shared I²C bus. Caps can share peripherals here at non-conflicting addresses.
UART (9, 13): UART2 routed to the EXT. Default function on the Cap LoRa-1262 is GNSS UART (AT6668). Caps without a GNSS can ignore these or re-task them.
SPI (10, 11, 12): shared SPI bus. Combined with the Cap-control bundle (3-6), enables full SX1262-class peripherals.

This grouping reflects the Cap LoRa-1262 use case as the canonical EXT design. New Caps inherit the same organization to maintain compatibility — drop-in replacement is the goal.

4.3 Power rails (5V_IN, 3V3, GND)

Rail	Pin	Source	Budget	Typical use
5V_IN	2	USB-C input or battery 5V boost	~500 mA shared	Higher-current peripherals (SX1262 +22 dBm TX draws ~163 mA peak)
3V3	1	System rail post-buck	~200 mA budget on EXT	Low-current MCU-class chips, GNSS module
GND	14	Ground	—	—

Budget is shared with the on-board peripherals. Drawing too much on EXT can brown out the Cardputer ADV. The Cap LoRa-1262 is designed to fit within budget; custom Caps need to verify the math.

For peripherals that need both 3.3V and 5V (rare — most are one or the other), wire both.

4.4 Mating connector — 2.54 mm header convention

The Cardputer ADV side has a 14-pin female header (sockets, accepts standard 2.54 mm headers). The Cap side has 14-pin male pins.

For custom Caps, use any 2.54 mm 2-row male header (TE Connectivity, Samtec, generic AliExpress). Pin pitch must match exactly — 2.54 mm. M5Stack publishes the mechanical reference in the Cardputer-Adv Structure Files PDF.

The mating distance (Cap PCB → Cardputer ADV PCB) is set by the header heights. Standard 2.54 mm headers give ~8.5 mm spacing — typical for a daughter card with no tall components. If your custom Cap has tall components on the side facing the Cardputer ADV, use a longer header or add standoffs.

5. USB-C OTG (side)

5.1 Device mode (default)

In device mode, the Cardputer ADV is a USB peripheral connected to a host (laptop, desktop, phone via USB-C). What the host sees:

USB CDC serial (typical Arduino USB-CDC bridge — /dev/ttyACM0 on Linux, COM-N on Windows)
USB HID keyboard / mouse (when running BadCard / Bruce BadUSB modules)
USB Mass Storage (when configured — Cardputer becomes a USB flash drive)

Device mode is the default in all M5Launcher / Bruce / Marauder / NEMO firmwares. Switch happens at build time via -DARDUINO_USB_MODE=1.

5.2 Host mode (OTG)

In host mode (USB On-The-Go), the Cardputer ADV becomes the USB host and can enumerate USB peripherals plugged in via a USB-C-to-A adapter:

USB keyboards: physical Cherry MX or membrane keyboards — more comfortable than the 56-key chiclet for sustained typing.
USB mass-storage thumb drives: read / write files. Bruce supports this via file manager.
USB-to-Serial adapters (FTDI, CH340, CP210x, PL2303): the Cardputer becomes a portable serial console. Plug into a router / switch / industrial controller for debugging.
USB MIDI controllers: feed audio codec from MIDI input.
USB HID inspection (BadUSB Hunter, NEMO + M5Launcher): plug in any HID device, Cardputer reports its descriptors, warns if it’s a surreptitious USB Rubber Ducky-class device.

What doesn’t work in host mode (as of 2026-05-13):

USB Wi-Fi adapters — no community firmware ships a CDC-ECM driver suitable for the S3’s USB host stack.
USB Ethernet adapters — same.
Webcams — no UVC driver.
External GPUs — obviously not.

5.3 Build-flag selection

; platformio.ini snippet
build_flags =
    -DARDUINO_USB_MODE=1                          ; Device mode (the default for most firmwares)
    ; OR
    -DARDUINO_USB_MODE=0                          ; Host mode (OTG)
    -DARDUINO_USB_CDC_ON_BOOT=1                   ; USB-CDC available on boot (works in either mode)

Switching between modes requires a firmware re-flash. Some firmwares (M5Launcher’s BadUSB Hunter) provide a runtime-switchable mode by including both USB stacks compiled in — costlier in flash size but more user-friendly.

The SPI bus on the Cardputer ADV serves three peripherals:

Peripheral	CS pin	Notes
Display (ST7789V2)	(internal to Stamp-S3A)	Always present
microSD	GPIO 12	Present when card inserted
EXT bus (Cap LoRa-1262 SX1262)	GPIO 5	Present when Cap mated

Bus arbitration via chip-select discipline:

Each transaction asserts its CS active-low while transferring, deasserts at end.
ESP32-S3 SPI peripheral handles the CS toggling per spi_device_transmit() call.
M5GFX, SD.h, and RadioLib (the canonical drivers) cooperate correctly when used together.

Observed contention behaviors:

Long SD writes (multi-MB file dump) block display refresh visibly. The display shows a brief “freeze” for the duration of the write.
Concurrent display refresh + SD read for boot-splash image: works but startup is ~200 ms slower than display-only.
LoRa TX + SD write simultaneously: arbitrated, but throughput on each drops. Avoid by buffering SD writes in RAM and flushing between LoRa transmissions.

For high-throughput capture (e.g., probe-request flooding pcap), pre-allocate a 32-64 KB RAM buffer and flush to SD periodically rather than per-packet.

7. I²C bus map — codec + keyboard + IMU + EXT + Grove

The primary I²C bus (G8/G9 SDA/SCL) hosts a substantial set of devices:

Address (default)	Device	On board / external
0x18	ES8311 audio codec	On board
0x34	TCA8418 keyboard scanner	On board
0x68 (or 0x69)	BMI270 IMU	On board
0x43	PI4IOE5V6408 GPIO expander (Cap LoRa-1262 FM8625H antenna switch control)	On Cap LoRa-1262 when mated
varies	EXT-bus I²C devices	Any Cap with I²C peripherals
varies	Grove-I²C-mode peripherals	Only when Grove is re-tasked to I²C (G1/G2 secondary bus is separate)

Bus speed: 400 kHz nominal (Fast Mode). 100 kHz also supported for slower peripherals.

Address scan: M5Cardputer library exposes Wire.beginTransmission(addr); err = Wire.endTransmission(); pattern — sweep 0x03-0x77 to discover devices. Bruce and Marauder include built-in I²C scanner utilities.

Address-conflict avoidance: M5Stack-shipped peripherals are chosen to avoid clashing. Third-party I²C peripherals at conflicting addresses must be address-changed (most have DIP switches or solder bridges for ±1 addr) or routed to the secondary Grove I²C bus instead.

8. Bus-conflict reference

Combination	Compatible?	Notes
Cap LoRa-1262 (EXT) + Grove UART sensor	✓	Different buses
Cap LoRa-1262 (EXT) + Grove I²C sensor (G1/G2 secondary)	✓	Different I²C buses
Cap LoRa-1262 (EXT) + Grove I²C sensor at 0x43 (same as PI4IOE on Cap)	✓	Different I²C buses — no conflict
Cap LoRa-1262 (EXT) + USB OTG host	✓	Independent paths
Cap LoRa-1262 (EXT) + microSD	✓	Same SPI bus, different CS lines
CC1101 on EXT (no LoRa Cap) + Grove sensor	✓	CC1101 takes EXT; Grove independent
Two Caps stacked on EXT	✗	Only one Cap mates at a time (mechanical)
Grove UART + I²C on G1/G2 secondary bus	✗	Conflicting pin function — must pick one or the other
Cap LoRa-1262 + Grove I²C device at the same address as a Cap’s I²C device	✓	Different I²C buses
Multiple I²C sensors on primary bus (G8/G9)	✓	Provided no address collisions
USB OTG + microSD + display SPI	✓	Independent paths

The main constraint: only one Cap on EXT at a time (mechanical, not electrical). For multi-functional builds: pick the most-needed Cap, or use Grove-side modules for the additional capability. Or design a stacking Cap with pass-through pins (community-discussed but no shipping product yet).

9. Practical wiring examples

Wiring a generic I²C sensor (BME280) to Grove:

Sensor side                           Cardputer ADV
─────────                            ─────────
VCC (3.3V)  ─── (AMS1117-3.3) ─── 5V (Grove pin 2)
GND         ────────────────────── GND (Grove pin 1)
SDA         ────────────────────── G2 (Grove pin 3)
SCL         ────────────────────── G1 (Grove pin 4)

Add 4.7 kΩ pull-ups SDA→3.3V and SCL→3.3V if BME280 breakout lacks them.

Arduino code:
    Wire1.begin(2, 1);    // SDA=2, SCL=1 on Wire1
    sensor.begin(&Wire1, 0x76);    // BME280 default address

Wiring a CC1101 sub-GHz module directly to EXT (with Cap LoRa-1262 removed):

CC1101 module     EXT bus pin (Cardputer ADV underside)
─────────────     ──────────────────────
VCC (3.3V)        EXT pin 1 (3V3)
GND               EXT pin 14 (GND)
CSN               EXT pin 5 (CS, GPIO 5)
SCK               EXT pin 11 (SCK, GPIO 40)
MOSI              EXT pin 12 (MOSI, GPIO 14)
MISO              EXT pin 10 (MISO, GPIO 39)
GDO0              EXT pin 4 (INT, GPIO 4)
GDO2              (leave open or wire to a spare GPIO)

Bruce firmware Settings → SubGHz Pinout → CC1101 on EXT → maps the pins
above to CC1101 driver. Then Bruce's SubGHz menu (replay / jam / capture)
works as if a CC1101 Grove Unit were attached.

Wiring a PN532 NFC reader to Grove I²C:

PN532 side                           Cardputer ADV
─────────                            ─────────
VCC (5V or 3.3V)  ─────────────── 5V (Grove pin 2)
GND               ─────────────── GND (Grove pin 1)
SDA               ─────────────── G2 (Grove pin 3)
SCL               ─────────────── G1 (Grove pin 4)

PN532 board DIP switches: I²C mode (0:ON, 1:OFF) — module has SPI/I²C/HSU
modes; need to set I²C.

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

10. Resources

Cardputer ADV pinout PDF: https://docs.m5stack.com/en/core/Cardputer
ESP32-S3 GPIO matrix: ESP32-S3 datasheet § 5
HY2.0-4P connector: JST documentation
Bus Pirate 6 (sister deep dive on wired-protocol exploration): ../../../Bus Pirate 6/03-outputs/BusPirate6_Complete.html

Forward references

Cap modules in detail: Vol 4 § 2
Grove Units catalog: Vol 4 § 3
Cap LoRa-1262 SX1262 bring-up via these pins: Vol 5 § 3
USB-OTG BadUSB recipes: Vol 9 § 2.6 + Vol 9 § 7

This is Volume 3 of a twelve-volume series. Next: Vol 4 catalogs the module ecosystem — Cap modules, Grove Units, the HAT-doesn’t-fit warning, M5MonsterC5 for 5 GHz, and DIY hardware patterns.

M5Stack Cardputer ADV Volume 3 — Pinout and Expansion Buses

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