# 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

```bash
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

```bash
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`).