Firmware Engineer (ESP32 / STM32) (RARR Job 6295)

What You'll Build

• Firmware for ESP32-based edge gateway - Wi-Fi/4G connectivity, OTA update system, watchdog, low-power modes
• Modbus RTU master over RS485 - poll inverter registers (voltage, current, power, energy, temp, fault codes) every 60 seconds
• MQTT client - publish inverter data to EMQX broker with QoS 1, retain on local flash during network outages
• Local data buffering - store up to 24 hours of data in NVMe / SPIFFS / SD card during connectivity loss, auto-sync on reconnect
• Sensor integration - DS18B20 (panel temperature via 1-Wire), optional I²C sensors (irradiance, humidity)
• OTA (Over The Air) firmware updates - using ESP-IDF OTA partition or STM32 bootloader
• Device provisioning - Wi-Fi credential setup via BLE, web captive portal, or QR code
• STM32-based variant (if applicable) - low-power data logger with RS485 + GSM/4G modem

Required Skills

• C / C++ embedded programming - memory management, register-level access, structural firmware architectures
• ESP32 (ESP-IDF framework preferred) - Wi-Fi, BLE, SPIFFS/NVMe, GPIO, UART, I²C, SPI, timers
• Modbus RTU / ASCII - master implementation over UART/RS485, CRC validation, register map reading
• MQTT protocol - client implementation, QoS levels, last will, retained messages, reconnection logic
• UART / RS485 hardware interface - half-duplex, DE/RE control, baud rate configuration
• Debugging tools - JTAG, serial monitor, logic analyser, oscilloscope basics
• Git - firmware versioning, release tagging, change logs
• Basic understanding of power systems - inverter communication is safety-critical

Bonus Skills (Good to Have)

• FreeRTOS - task management, queues, semaphores on ESP32 or STM32
• STM32 HAL / LL drivers - UART, SPI, I²C, DMA, low-power modes (Stop, Standby)
• 4G LTE modems - SIM7600, Quectel EC25, AT command handling, PPP / ECM data mode
• TLS/SSL on embedded - MBEDTLS, certificate provisioning, MQTT over TLS
• Rust for Embedded - experience or strong interest in migrating safe systems to embedded Rust
• Python scripting - for device testing automation, register scan tools
• KiCad / EasyEDA - ability to read schematics (even if not designing them)

You'll Thrive Here If You

• Have built and shipped a real embedded project - not just dev-board demos
• Are methodical about debugging - you use a logic analyser before assuming a software bug
• Understand that firmware running next to a live solar inverter must be reliable, not clever
• Document your register maps, protocol notes, and wiring decisions - the hardware team needs it
• Are comfortable with ambiguity in hardware specs - inverter datasheets are often incomplete

What You Get

• Build production firmware for a device deployed at real solar sites - not lab prototypes
• Work alongside PCB hardware engineers - learn the full hardware-to-cloud stack
• Access to test equipment: oscilloscopes, logic analysers, RS485 sniffers
• Growth path into senior embedded or IoT architect role