Flood Sensor — nRF9151 Firmware

Firmware for the Circuit Dojo Feather nRF9151 that wakes a SparkFun XM125 60GHz radar sensor, collects distance measurements, and transmits them over LTE-M to a Google Apps Script endpoint.

System Overview

nRF9151 Feather (master)
  │
  ├─ WAKE (P0.20) ──────────────────► XM125 WU pin
  ├─ INT  (P0.21) ◄──────────────────  XM125 MCU_INT pin
  └─ UART RX (P0.23) ◄───────────────  XM125 UART TX
                                        (921600 baud)

Measurement cycle (every 60 seconds):

1. nRF9151 asserts WAKE → XM125 wakes from hibernate
2. XM125 takes 7 measurements, sends each over UART
3. XM125 asserts INT → nRF9151 wakes from eDRX idle
4. nRF9151 reads 7 frames from UART ring buffer
5. nRF9151 deasserts WAKE → XM125 hibernates
6. nRF9151 timestamps frames (SNTP-anchored) and POSTs over LTE-M
7. nRF9151 returns to eDRX idle until next cycle

Hardware

• Cellular modem: Circuit Dojo Feather nRF9151
• Radar sensor: SparkFun XM125 (Acconeer A121, 60GHz)
• SIM: Hologram (LTE-M, APN: hologram)

Wiring

XM125 Pin	nRF9151 Pin	Direction	Purpose
UART TX	P0.23	XM125 → nRF	Measurement data
WU (cut jumper)	P0.20	nRF → XM125	Wake from hibernate
MCU_INT	P0.21	XM125 → nRF	Buffer ready signal
3V3	3V3	nRF → XM125	Power
GND	GND	—	Common ground

Note: The WU jumper on the XM125 SparkFun breakout must be cut before connecting WU to P0.20. Otherwise the pin is held high by the board's default pull-up and the XM125 will never hibernate.

Boot safety: P0.20 is driven low via a devicetree gpio-hog before main() runs, preventing the XM125 from waking due to a floating signal during nRF9151 startup. No external pull-down resistor is required.

The XM125 is powered from the nRF9151 Feather's 3V3 rail. The nRF9151 is powered via USB during development or via LiPo battery with solar charging in the field.

Prerequisites

• Circuit Dojo Zephyr Tools installed
• NCS v3.2.1 workspace initialized via west
• probe-rs installed for flashing
• ARM GNU Toolchain and CMake on PATH

Building

From the webhook_test folder, activate the environment:

# Windows
C:\Users\<user>\.zephyrtools\env\Scripts\Activate.ps1
$env:PATH = "C:\Users\<user>\.zephyrtools\cmake\cmake-3.22.0-windows-x86_64\bin;" + $env:PATH
$env:ZEPHYR_BASE = "C:\p\n\zephyr"

# macOS
source ~/.zephyrtools/env/bin/activate

Then build:

west build -b circuitdojo_feather_nrf9151/nrf9151/ns --pristine

Flashing

probe-rs download --chip nRF9151_xxAA --binary-format hex build/merged.hex

Press the RST button after flashing.

Configuration

All key parameters are in src/config.h:

Parameter	Default	Description
UPDATE_RATE_MS	60000	Measurement cycle interval (ms)
FRAMES_PER_BUFFER	7	Frames per LTE POST (must be multiple of 7)
MAX_PEAKS_PER_FRAME	3	Max peaks reported per frame
UART_RX_BUF_SIZE	512	UART ring buffer size (bytes)
UART_FRAME_TIMEOUT_MS	5000	Timeout waiting for XM125 INT (ms)
SNTP_SERVER	time.cloudflare.com	SNTP server for time sync

HTTP Endpoint

Edit src/config.h:

#define WEBHOOK_HOST  "script.google.com"
#define WEBHOOK_UUID  "macros/s/YOUR_SCRIPT_ID/exec"

TLS Certificate

The Google GTS root CA is in cert/google_gts.pem. Replace with the appropriate root CA if targeting a different endpoint.

Network Operator

LTE auto-selects by default. If auto-selection fails with the Hologram SIM, force T-Mobile by uncommenting in src/cellular.c:

nrf_modem_at_printf("AT+COPS=1,2,\"310260\",7");  // 310260 = T-Mobile US

Data Format

Each HTTP POST sends a JSON body with FRAMES_PER_BUFFER frames. Fields with no detected peaks omit distance and strength:

{
  "frames": [
    {
      "datetime": 1747123456789,
      "frame": 1,
      "num_peaks": 2,
      "d1": 1.234567,
      "s1": 45.2,
      "d2": 0.987654,
      "s2": 38.1
    },
    {
      "datetime": 1747123457000,
      "frame": 2,
      "num_peaks": 0
    }
  ]
}

Field	Description
datetime	Unix timestamp in milliseconds (SNTP-anchored)
frame	Frame number within buffer (1–7)
num_peaks	Number of detected peaks (0 = no detection)
d1–d3	Distance to peak in meters (up to 3 peaks)
s1–s3	Signal strength in dB (up to 3 peaks)

Google Sheet Columns

The Google Apps Script writes each frame as one row:

Timestamp | Datetime | Frame | Num Peaks | Distance 1 | Strength 1 | Distance 2 | Strength 2 | Distance 3 | Strength 3

Power

• Sleep mode: eDRX (20.48s cycle requested, network-granted)
• Solar: 2×2" panel recommended for continuous outdoor operation
• Update rate: 1 minute default — adjust UPDATE_RATE_MS in config.h

If UPDATE_RATE_MS is increased beyond 5 minutes, consider enabling PSM in cellular.c for better power efficiency.

XM125 Firmware

The XM125 runs a modified version of Acconeer's example_detector_distance_low_power_hibernate example from the A121 SDK. Key changes from the original:

• XM125 is slave — sleeps in hibernate until WAKE pin asserted by nRF9151
• Takes FRAMES_PER_BUFFER measurements per wake cycle
• Outputs one UART line per frame: distance,strength [distance,strength ...]\n or 0\n for no detection
• Asserts MCU_INT after last frame is sent
• Waits for WAKE deassert before returning to hibernate

See the xm125 repository for that firmware.

Serial Monitor

Connect at 115200 baud to view nRF9151 debug output (Zephyr logging).