FreeRTOS, Azure RTOS, and Zephyr: A Real Comparison of the Most Used Real-Time Operating Systems in Embedded IoT

We analyze the strengths, weaknesses, and use cases of the three main RTOSes in the market: FreeRTOS, Azure RTOS, and Zephyr. Which one to choose in 2025 for your next embedded project?

In the current connected device landscape, real-time operating systems (RTOS) have become critical components for developing IoT, industrial, medical, or automotive solutions. Among the wide range available, three stand out due to their maturity, support, and community: FreeRTOS, Azure RTOS, and Zephyr.

While all enable deterministic task execution with low latency on microcontrollers and SoCs, each reflects different philosophies, licensing models, and architectures. Below, we break down their main technical and implementation differences.


Overview and Origins

FeatureFreeRTOSAzure RTOSZephyr
Initial developerRichard Barry (acquired by Amazon)Express Logic (acquired by Microsoft)The Linux Foundation (NXP, Intel, etc.)
Current maintenanceAmazon Web Services (AWS)MicrosoftLinux Foundation
License modelMITFree commercial use licenseApache 2.0
Main languageCCC
FocusGeneral / IoT / Low powerIndustrial IoT / Azure IoTEmbedded / Industrial / Edge / Modular

Main Technical Comparison

FeatureFreeRTOSAzure RTOSZephyr
RT kernelLightweight, preemptiveThreadX, deterministic and robustModular kernel, optional tickless
SMP support❌ No❌ No✅ Yes (limited to specific architectures)
SchedulingRound-robin and priorityPriority-basedConfigurable priorities + SMP
Minimum RAM size~2 KB~2–3 KB~8–20 KB depending on subsystem
Supported architecturesARM, RISC-V, MIPS, etc.ARM Cortex-M (official)ARM, RISC-V, x86, ARC, Xtensa, Nios II
FilesystemFAT + optional third-partyFileX (FAT, exFAT, wear leveling)LittleFS, FAT, NVS, Flash and SD support
Network supportFreeRTOS+TCP or lwIPNetX / NetX DuoZephyr Net stack (IPv4/IPv6, TLS, CoAP, MQTT)
TLS / CryptographymbedTLS or wolfSSLBuilt-in TLS with NetX SecurembedTLS, TinyCrypt, PSA Crypto
USB supportDevice / Host (optional)USBX (host/device/OTG)USB Host/Device/OTG, CDC, HID, MSC
RTOS + Shell / CLILimitedYes (ThreadX GUIX + optional shell)Yes (Zephyr Shell, modular, scriptable)

Cloud and DevOps Integration

FeatureFreeRTOSAzure RTOSZephyr
Cloud service integrationAWS IoT Core, GreengrassAzure IoT Hub, Azure RTOS StudioOptional, third-party modules
OTA updatesSupports AWS OTA AgentAzure Device Update (ADU)Zephyr DFU (MCUboot + OTA support)
Debugging / TracingFreeRTOS Tracealyzer, GDBTraceX, SEGGER RTTZephyr Logging, Tracing, SEGGER SystemView

Security and Certifications

FeatureFreeRTOSAzure RTOSZephyr
TrustZone (ARMv8-M) supportPartial, requires integration✅ Complete✅ Complete
Functional safety certificationIEC 61508, ISO 26262 (with partners)Certifiable (ThreadX used in DO-178C, IEC 62304)Partial (aimed to be certifiable)
Secure Boot / Root-of-TrustHardware dependentYes, with Azure Sphere / RT Secure BootMCUboot integrated

Ecosystem, Tools, and Community

FeatureFreeRTOSAzure RTOSZephyr
Development environment / IDEAny (VS Code, Eclipse)Visual Studio, IAR, KeilVSCode, Zephyr SDK, nRF Connect SDK
Community / SupportVery large (forum, GitHub)Professional support via MicrosoftActive, diverse, backed by major corporations
Supported partners / SoCEspressif, ST, NXP, Microchip…ST, NXP, Renesas, etc.Intel, Nordic, NXP, Laird, Adafruit, etc.
Simulator supportQEMU, Wokwi (limited)Partial simulationQEMU, Renode, Native POSIX port

Which to choose in 2025? Recommended use cases

  • FreeRTOS: Ideal for simple projects, low power consumption, and high portability. Excellent for developers already working with AWS or needing a lightweight RTOS for 32-bit MCUs.

  • Azure RTOS: Excellent choice for industrial, medical, or critical apps needing enterprise support, robustness, and direct connectivity with Azure IoT.

  • Zephyr: The most modular, flexible, and powerful option for those seeking a modern RTOS supporting multiple architectures, open tools, and expansion into edge computing. Especially useful for projects with security, vision, BLE, or embedded AI requirements.


Conclusion

FreeRTOS, Azure RTOS, and Zephyr complement each other rather than compete. Each has evolved to meet specific needs within embedded and IoT sectors. FreeRTOS is known for its simplicity and widespread adoption, Azure RTOS for its industrial integration and reliability, and Zephyr for its ambition and ability to scale from sensors to edge gateways.

Ultimately, the choice depends on the hardware ecosystem, certification requirements, cloud deployment models, and long-term firmware maintenance strategy.


Sources: Official documentation of FreeRTOS, Azure RTOS, and Zephyr, GitHub repositories, AWS, Microsoft, The Linux Foundation, technical analyses from Embedded.com, and developer experiences from communities like Stack Overflow and Reddit r/embedded.

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