Software Architecture

FreeRTOS-based firmware with concurrent multitasking, MQTT cloud connectivity, and over-the-air updates.

SDK: Simplicity SDK 2025.6.3 Connectivity: WiseConnect 3.5.2 RTOS: FreeRTOS Language: C Dashboard: Node-RED v4.1.8

FreeRTOS Task Architecture

Concurrent tasks with priority-based preemptive scheduling ensure deterministic real-time behavior for fish detection, IMU sensing, telemetry, and system reliability.

app_watchdog

Realtime Priority 2048 stack | 1000 ms

The system guardian task. Monitors all other tasks for stalls by tracking periodic check-ins. If any task fails to report within 5 seconds, the watchdog triggers a full system reset via the hardware WDT. A 15-second grace period at startup prevents false resets during initialization.

mqtt_connection_task

Above Normal Priority 8192 stack | 100 ms

The communication backbone. Manages the full Wi-Fi connection lifecycle and maintains a persistent MQTT session with the Azure-hosted broker. Handles subscription management and drains the publish queue every 100 ms, ensuring sensor data and fish detection events are delivered to the cloud with minimal latency. The 8 KB stack accommodates the TLS and MQTT library overhead.

ota_update_task

Normal Priority 4096 stack | 200 ms

Over-the-air firmware update handler. Listens on the CyberRod/Update MQTT topic for update commands. When triggered (payload "1"), downloads the new firmware image via HTTP OTAF v2 from the Azure VM and applies it to the Si917 SoC. The 200 ms polling interval keeps latency low while preserving CPU cycles for the time-critical fish detection task.

fish_detection_task

Normal Priority 2048 stack | 20 ms

The core sensing loop with the fastest period in the system. Continuously reads ultrasonic distance data over UART, classifies fish proximity into three states (none, near, very near), and publishes both the raw distance in millimeters and the detection classification to MQTT. The 20 ms period (50 Hz) provides responsive real-time fish detection without overwhelming the broker.

temperature_upload_task

Normal Priority 3072 stack | 300 ms

Environmental telemetry uploader. Reads temperature and relative humidity from the SHT4x sensor over I2C every 300ms and publishes both values to their respective MQTT topics. The period balances responsive telemetry with the gradual nature of environmental changes.

imu_task

Normal Priority 2048 stack | 100 ms

Motion and acceleration sensing task. Reads 3-axis acceleration and 3-axis angular velocity from the IMU sensor over the shared I2C0 bus. Computes acceleration magnitude and detects sudden rod jerks or vibrations that may indicate fish bite events. Publishes acceleration data to MQTT for real-time dashboard visualization and post-session analysis.

Communication Protocols

MQTT messaging, Wi-Fi connectivity, and HTTP-based over-the-air firmware updates.

MQTT Publish Topics

Sensor data and detection events are published to the Azure-hosted Mosquitto broker at 20.230.132.231:1883 with client ID si917-devboard-cyberrod.

MQTT Publish 
# Fish proximity classification
Topic:  CyberRod/Fish_detection
Format: integer  (0 = no fish, 1 = near, 2 = very near)

# Ultrasonic distance reading
Topic:  CyberRod/Distance
Format: string   (distance in mm)

# SHT4x temperature reading
Topic:  CyberRod/Temperature
Format: string   (degrees Celsius, 2 decimal places)

# SHT4x humidity reading
Topic:  CyberRod/Humidity
Format: string   (%RH, 2 decimal places)

# IMU acceleration data
Topic:  CyberRod/Acceleration
Format: string   (X,Y,Z acceleration values)

MQTT Subscribe & OTA Update

The device subscribes to a single command topic. Sending "1" triggers an HTTP OTAF v2 firmware download from the Azure VM.

MQTT Subscribe 
# OTA update trigger
Topic:  CyberRod/Update
Action: Send payload "1" to initiate

# OTA update process
Protocol:  HTTP OTAF v2
Source:    Azure Virtual Machine
File:      wifi_http_otaf_soc.rps
Target:    Si917 SoC flash

# Broker connection
Host:      20.230.132.231
Port:      1883
Client ID: si917-devboard-cyberrod

Safety & Reliability

Multi-layered fault detection ensures the device recovers autonomously from any software or hardware failure.

Hardware Watchdog Timer

Dedicated hardware WDT that cannot be disabled in software. The app_watchdog task services it periodically; if the firmware crashes entirely, the WDT times out and forces a hard system reset.

Task Stall Detection

The watchdog task tracks the last check-in time from every other task. If any task fails to report within 5 seconds, the system identifies the stalled task and triggers a reset. A 15-second grace window at boot accommodates slow initialization sequences.

Stack Overflow Hook

FreeRTOS vApplicationStackOverflowHook is enabled. If any task exceeds its allocated stack, the hook fires immediately and resets the system, preventing silent memory corruption that could cause unpredictable behavior in the field.

Malloc Failure Hook

The vApplicationMallocFailedHook callback catches heap exhaustion at runtime. If pvPortMalloc returns NULL, the hook triggers a system reset rather than allowing a NULL-pointer dereference to crash the device unpredictably.

HardFault Handler

A custom HardFault_Handler captures the fault at the ARM Cortex-M level. Whether caused by an invalid memory access, bus error, or usage fault, the handler initiates a clean system reset to restore operation without manual intervention.

Node-RED Dashboard

Real-time monitoring interface built with Node-RED v4.1.8 and FlowFuse Dashboard 2.0, featuring a custom nautical-themed design.

Live Sensor Gauges

Real-time gauges display ultrasonic distance, fish detection state, temperature, and humidity. Data streams from MQTT are displayed with sub-second latency, giving anglers immediate environmental awareness.

Historical Charts

Time-series line charts track temperature and humidity trends over configurable windows. Distance readings plot fish approach patterns, enabling post-session analysis of fishing conditions.

OTA Update Trigger

A dashboard button publishes to CyberRod/Update, triggering the OTA task on the device. Firmware updates can be deployed remotely without physical access to the fishing rod, critical for field deployments.

Nautical-Themed UI

Custom CSS gives the FlowFuse Dashboard a nautical aesthetic with ocean blues, signal-flag accents, and maritime typography. The theme reinforces the fishing context while maintaining clean data readability in outdoor lighting conditions.

Node-RED: v4.1.8 Dashboard: FlowFuse 2.0 Theme: Custom nautical CSS Broker: Mosquitto on Azure VM

Task Summary

Complete specification of all FreeRTOS tasks, their resource allocation, and scheduling parameters.

Task Stack Priority Period Function
app_watchdog 2048 Realtime 1000 ms Monitor all tasks, trigger WDT reset on stall
mqtt_connection_task 8192 Above Normal 100 ms Wi-Fi + MQTT broker session, publish queue
ota_update_task 4096 Normal 200 ms Listen for OTA trigger, download via HTTP OTAF v2
fish_detection_task 2048 Normal 20 ms Ultrasonic UART read, fish classification, publish
temperature_upload_task 3072 Normal 3000 ms SHT4x I2C read, publish temperature + humidity
imu_task 2048 Normal 100 ms IMU I2C read, acceleration & gyro, publish rod motion