XIAO ESP32-C6 + DFRobot C4001 mmWave presence sensor

Live-data and calibration rig for a C4001 24 GHz radar on a Seeed XIAO ESP32-C6. The board hosts a web page showing presence, distance, velocity and signal energy in real time, with sliders for every sensor parameter.

Verified on this machine

Thing	Value
Chip	ESP32-C6FH4 (QFN32) rev v0.1, single core + LP core, 160 MHz
Flash	4 MB embedded (mfr 0x20, dev 0x4016)
Radios	WiFi 6, BT 5 LE, IEEE 802.15.4
MAC	e4:b3:23:b4:7b:c4
Port	/dev/ttyACM0 (native USB Serial/JTAG, VID 303a PID 1001)
Toolchain	PlatformIO 6.1.19, espressif32 2026.02.30, Arduino core 3.3.7
Build	succeeds, 57.6% flash / 11.9% RAM

Flashing was confirmed working: esptool connected, uploaded its stub loader, ran it, and read the flash chip ID back.

Arduino core version matters. ESP32-C6 needs Arduino-ESP32 3.x. The installed platform is the pioarduino fork (2026.02.30), which ships 3.3.7 and includes the XIAO_ESP32C6 variant. The old registry espressif32 @ 6.x has no C6 support — don't downgrade.

Pin diagram

The XIAO's pads are unlabelled on the top side. Hold the board component side up with the USB-C connector at the top — pin 1 of each column is nearest the USB connector.

                    ┌───┐ USB-C ┌───┐
              ┌─────┴───┴───────┴───┴─────┐
      D0  ────┤ o                       o ├──── VBUS  (5 V from USB)
     GPIO0    │                           │
      D1  ────┤ o                       o ├──── GND
     GPIO1    │                           │
      D2  ────┤ o     seeed studio      o ├──── 3V3   (3.3 V out)
     GPIO2    │      XIAO-ESP32-C6        │
      D3  ────┤ o                       o ├──── D10   GPIO18
     GPIO21   │                           │
      D4  ────┤ o                       o ├──── D9    GPIO20
     GPIO22   │                           │
      D5  ────┤ o                       o ├──── D8    GPIO19
     GPIO23   │                           │
      D6  ────┤ o    [u.FL]   [ant]     o ├──── D7    GPIO17
     GPIO16   └───────────────────────────┘

Full mapping, from Seeed's official pinout sheet cross-checked against the Arduino variant header:

Silkscreen	GPIO	Alt function
D0	0	ADC1 / A0, LP_GPIO0
D1	1	ADC1 / A1, LP_GPIO1
D2	2	ADC1 / A2, LP_GPIO2
D3	21	—
D4	22	I2C SDA
D5	23	I2C SCL
D6	16	U0 TXD
D7	17	U0 RXD
D8	19	SPI SCK
D9	20	SPI MISO
D10	18	SPI MOSI

Not on the headers but used by the firmware: GPIO15 user LED (active LOW), GPIO3 RF switch enable (LOW = on), GPIO14 antenna select (LOW = on-board ceramic, HIGH = external u.FL).

Wiring the C4001

Power the sensor from 3V3, not 5V. The C4001 accepts 3.3–5 V, but at 5 V its OUT pin can swing to 5 V and the C6's GPIOs are not 5 V tolerant. Running the sensor at 3.3 V makes every logic level match and removes the question entirely.

The firmware auto-detects the interface at boot, so wire whichever your board's mode switch is set to.

UART (works on every C4001 SKU):

C4001	XIAO	Note
VIN	3V3
GND	GND
TX	D9 (GPIO20)	sensor transmit → board receive
RX	D10 (GPIO18)	sensor receive ← board transmit
OUT	D2 (GPIO2)	optional

I2C (if your board has an I2C/UART switch set to I2C):

C4001	XIAO
VIN	3V3
GND	GND
SCL	D5 (GPIO23)
SDA	D4 (GPIO22)
OUT	D2 (GPIO2), optional

OUT is the sensor's own thresholded presence output. It is not required — the firmware reads everything over the data link — but wiring it lets you compare the sensor's internal decision against the raw numbers, which is genuinely useful while calibrating.

Don't use D6/D7 for the sensor UART. They are GPIO16/GPIO17, this chip's hard-wired U0TXD/U0RXD. UART0 already owns them, so pointing Serial1 at those pins deadlocks the UART driver and trips the interrupt watchdog — the board boot-loops before it ever reaches WiFi. Found this the hard way; the UART pins above route UART1 through the GPIO matrix to pins with no default peripheral function.

Heads up on TX/RX. DFRobot's own example table pairs the sensor's RX with the ESP32's RX, which is wrong. Their library also swaps the rxpin/txpin parameter names between the header and the .cpp. The table above is the electrically correct wiring: sensor TX → board RX, sensor RX → board TX.

Setup

cp include/secrets.h.example include/secrets.h
$EDITOR include/secrets.h        # your WiFi; the file is gitignored

Skipping this is fine — the board then goes straight to AP mode.

Build, flash, monitor

pio run                 # build
pio run -t upload       # flash /dev/ttyACM0
pio device monitor      # 115200 baud

If upload fails to auto-reset, hold B (boot), tap R (reset), release B, then upload.

Using it

On boot the firmware joins your WiFi; if that fails within 15 s it starts an access point:

• SSID xiao-mmwave · password mmwave1234
• Web UI at the IP printed on serial, or http://mmwave.local/

The page shows a presence indicator, live target count / range / speed / energy, a rolling 60-second chart, and a calibration panel that changes with the active sensor mode. Serial output at boot tells you which interface was detected and prints the full probe log if nothing was found.

The two sensor modes

Presence mode (eExitMode) reports occupancy only. Parameters: min/max/trigger range, trigger and keep sensitivity (0–9), trigger delay (×0.01 s), keep timeout (×0.5 s).

Speed mode (eSpeedMode) reports target count, distance, velocity and signal energy. Parameters: min/max range and a detection threshold (×0.1).

Calibrate in speed mode, because it shows you the numbers behind the decision. Then switch to presence mode for normal operation. The in-page guide walks through it. Settings apply immediately but are volatile — Save to sensor flash to persist them.

Layout

platformio.ini            board, framework, C4001 library from git
include/secrets.h.example template for WiFi creds (real one is gitignored)
src/main.cpp              detection, polling, WiFi, HTTP API
src/web_ui.h              the single-page UI, inlined into flash

HTTP API: GET /api/live, GET /api/config, POST /api/config, POST /api/action (start/stop/save/reset/factory).