FreeRTOS, Zephyr, VxWorks

Mars Rover Curiosity has been running VxWorks since 2012 without a single reboot. Falcon 9 runs VxWorks from launch through first-stage landing. AirPods run FreeRTOS: IMU, audio, and BLE coexisting in 10 KB RAM. Smart locks and Matter devices are migrating to Zephyr. Every choice is a trade-off between license cost, memory, certification, and ecosystem. A VxWorks license costs from USD 12,000 per device. FreeRTOS: MIT, free. Zephyr: Apache 2.0, free. Price is not the deciding factor - Boeing 787 runs VxWorks because the certification package, not the license fee, is what matters.

• **NASA Curiosity Rover**: VxWorks on a RAD750 radiation-hardened processor - operating since 2012, software updates arrive via uplink with 20+ minute signal delay
• **AirPods Pro**: FreeRTOS on Apple H1/H2 chip - drives Active Noise Cancellation, Transparency mode, and Spatial Audio simultaneously within a ~10 KB RAM footprint
• **Nordic nRF5340 Matter devices**: Zephyr as the de-facto standard - BLE + Thread + Matter out of the box, PSA Certified Level 2 via TF-M
• **Siemens SIMATIC PLC**: VxWorks or Wind River Linux depending on safety level - IEC 62443 Industrial Security certification

FreeRTOS: the IoT standard

**FreeRTOS** is a monolithic kernel with preemptive scheduling and a minimum footprint of ~10 KB RAM. Created by Richard Barry in 2003, acquired by Amazon in 2017. Ships in over 40 billion devices. Architecture: single binary, all components statically linked. Memory management: five allocation strategies (heap_1 through heap_5). heap_4 is the most common - first-fit with block coalescing. No built-in POSIX, no networking stack in the kernel. The strength of FreeRTOS lies in API simplicity and a massive ecosystem of ports across 35+ architectures from AVR to RISC-V.

**AWS FreeRTOS (FreeRTOS LTS)**: Amazon added MQTT, TLS 1.3, OTA updates, and an HTTP client on top of the kernel. The kernel remains MIT licensed; AWS components carry MIT or the Amazon License. An ESP32 running FreeRTOS with AWS IoT Core is the canonical IoT sensor architecture for under USD 2 in BOM cost.

**heap_4 vs heap_5**: heap_4 uses a single memory pool; heap_5 allows multiple non-contiguous regions - useful when SRAM1 and SRAM2 are not adjacent in the address space (e.g., STM32H7). When heap fragmentation becomes a problem, switch to static allocation via xTaskCreateStatic() to eliminate runtime malloc entirely.

Zephyr: Linux Foundation RTOS

**Zephyr** is a Linux Foundation project (2015), Apache 2.0 licensed. Target audience: microcontrollers with < 256 KB RAM where Linux is too heavy and FreeRTOS lacks built-in connectivity. Key differentiators from FreeRTOS: devicetree for hardware description (borrowed from Linux), Kconfig for build configuration, a built-in networking stack (BLE, WiFi, LoRa, Thread, Ethernet), a POSIX API subset, and TF-M (Trusted Firmware-M) for PSA Certified security. Toolchain: west + CMake. Zephyr includes a native Bluetooth Host Stack - not just an HCI wrapper. Supports 500+ boards out of the box.

**ML on the Edge with Zephyr**: Arm Cortex-M55 and Ethos-U55 NPU are supported in Zephyr. TensorFlow Lite Micro runs as a Zephyr thread. Typical pipeline: IMU data via Zephyr sensor API -> inference thread -> BLE notify. Zephyr is the de-facto standard for ML at the Edge on ARM Cortex-M in 2024-2025.

**West workspace setup**: Zephyr requires the west meta-tool and a manifest file for dependency management. `west update` downloads dozens of repositories. Initial workspace initialization takes 15-30 minutes on a slow connection. Building Zephyr without west is not supported.

VxWorks: aerospace standard

**VxWorks** by Wind River Systems, 1987. Not open-source. License: from USD 12,000 per device for a DO-178C Level A certification package. The cost is justified: DO-178C certification alone runs into millions; VxWorks ships a ready-made certification kit (DAL A/B/C for aviation, ASIL-D for automotive ISO 26262, IEC 62443 for industrial security). Architecture: Wind microkernel with a POSIX-compatible API layer on top. Deterministic interrupt latency: < 1 us on modern silicon. Certified hypervisor support for mixed-criticality systems.

**Mixed-criticality with VxWorks Hypervisor**: the Boeing 787 runs a certifiable RTOS guest (flight controls) and a Linux guest (cabin entertainment) on the same multicore CPU. The hypervisor enforces spatial isolation (memory) and temporal isolation (CPU time). The DO-178C Level A certificate covers only the certifiable guest - Linux does not need certification.

**Why Curiosity and Falcon 9 run VxWorks**: not because it is the fastest, but because of heritage. JPL has used VxWorks since Spirit/Opportunity in 2004. Each mission involves decades of development and verification. Switching RTOS means restarting certification. Technical lock-in in aerospace is not a bug - it is deliberate risk management.

Decision matrix: choosing an RTOS

RTOS selection is a systems question, not a technical one: what needs certification, how much memory is available, which ecosystem is required. **Footprint**: FreeRTOS 6-10 KB RAM (kernel only), Zephyr 8-30 KB (depends on enabled subsystems), VxWorks 200 KB+. **Certification**: FreeRTOS - none official, Zephyr - PSA Certified Level 1-3 via TF-M, VxWorks - DO-178C / ISO 26262 / IEC 62443. **Ecosystem**: FreeRTOS leads in port count and examples, Zephyr leads in modern protocols (Matter, Thread, Bluetooth LE Audio), VxWorks leads in aerospace heritage. ML at the edge: Zephyr + TFLite Micro for Cortex-M55 with NPU - the practical path in 2024-2025.

**Zephyr is not a FreeRTOS replacement - it replaces Linux**: Zephyr targets devices with < 256 KB RAM where Linux was previously too heavy and FreeRTOS lacked a networking stack. If a project already runs FreeRTOS and does not need a BLE Host Stack or Matter, migrating to Zephyr provides no benefit and costs months of engineering work.

Key ideas

• **FreeRTOS**: 6-10 KB RAM, MIT license, 40B+ deployments. Standard for IoT sensors and MCUs without a connectivity stack. Simple API, 35+ architectures
• **Zephyr**: Linux Foundation, Apache 2.0, devicetree + Kconfig, native BLE Host Stack and Matter. De-facto standard for modern IoT with BLE/ML at the Edge on Cortex-M55
• **VxWorks**: DO-178C/ISO 26262/IEC 62443 certification, < 1 us interrupt latency, hypervisor for mixed-criticality. Aerospace, automotive safety, industrial. From USD 12K/device
• **Decision matrix**: memory + certification + ecosystem determine the choice. Zephyr is not a FreeRTOS replacement - it replaces Linux on resource-constrained MCUs. Each RTOS has a distinct niche

Related topics

RTOS choice intersects with embedded architecture, OS primitives, and parallelism models:

• RTOS Architecture (rts-04) — Scheduler, IPC, MPU - the mechanisms inside each of the three RTOS
• Embedded Systems (emb-05) — RTOS choice determines the HAL and driver model for the embedded project
• OS Synchronization (os-05-sync) — RTOS mutex vs POSIX mutex: same concept, different real-time guarantees
• Parallel Computing (par-05) — Multi-core RTOS scheduling and AMP/SMP modes - parallelism in a real-time context

Вопросы для размышления

• A team is developing an autonomous underwater vehicle (AUV): Cortex-M7, 512 KB RAM, real-time motor control (< 5 ms deadline), sonar processing, Ethernet telemetry. No safety certification is required. Which RTOS is the right fit, and what specifically makes FreeRTOS or Zephyr better than VxWorks in this scenario?
• VxWorks costs 100x more than FreeRTOS but is used in the Boeing 787. A DO-178C certification package costs millions separately. Why do aerospace companies not migrate to open-source RTOS even with massive budgets - what exactly does a VxWorks certification evidence package provide that open-source cannot?
• Zephyr devicetree describes hardware at compile time. FreeRTOS works with any BSP through conditional compilation. Which approach scales better when developing a product line across five different MCUs with similar peripherals?

Связанные уроки

• rts-01
• rts-02
• rts-03
• rts-04
• emb-05
• os-05-sync
• par-05
• os-01-intro