Choosing an RTOS for Your Device: Comparing FreeRTOS, Zephyr, ThreadX, and More

Getting Started: Why RTOS Selection Is a Game Changer

Selecting an RTOS (real-time operating system) is one of the most strategic decisions in embedded product development. It determines how reliably your device handles tasks, responds to events, and scales across platforms. With the growing diversity of connected devices—from low-power wearables to complex IoT gateways—the RTOS landscape in 2025 is more competitive than ever.

This article provides a side-by-side comparison of FreeRTOS, Zephyr, ThreadX, and Mbed OS to help you identify the most suitable RTOS for your product goals.

In this article, we compare four popular RTOS platforms—FreeRTOS, Zephyr, ThreadX (Azure RTOS), and Mbed OS—to help you make the best choice based on your device requirements.

Why RTOS Selection Matters

An RTOS provides predictable task scheduling and low-latency response for embedded applications such as wearables, medical devices, and industrial automation. The wrong choice can lead to performance bottlenecks, porting issues, or licensing conflicts.

Key evaluation criteria include:

Real-time performance
Licensing model
Community and commercial support
Ecosystem compatibility
Memory footprint and scalability

RTOS Comparison Table

Feature/RTOS	FreeRTOS	Zephyr RTOS	ThreadX (Azure RTOS)	Mbed OS
License	MIT	Apache 2.0	Proprietary/MIT	Apache 2.0
Vendor	AWS	Linux Foundation	Microsoft	Arm
Real-Time Capable	Yes	Yes	Yes	Limited
SMP Support	No	Yes	Yes	No
Memory Footprint	Very Low	Low	Moderate	High
Device Support	Excellent	Broad (600+ boards)	Moderate	Limited
Community	Large	Growing	Moderate	Shrinking
Commercial Support	Via Partners	Multiple Vendors	Microsoft	Deprecated in favor of CMSIS

Detailed Overview

FreeRTOS

FreeRTOS is one of the most widely adopted RTOSes globally, known for its simplicity and small footprint. Developed by Real Time Engineers Ltd. and maintained by AWS, it is MIT-licensed and free for commercial use.

Best for: Simple microcontroller-based applications with constrained resources.
Notable users: STMicroelectronics, Espressif (ESP32), Microchip.
Key strengths: Low memory usage, massive documentation, and cloud integration via Amazon FreeRTOS.

Zephyr RTOS

Zephyr is a scalable, open-source RTOS managed by the Linux Foundation. It supports multiple architectures including ARM, x86, RISC-V, and ARC.

Best for: IoT devices requiring advanced networking, modularity, and security.
Notable users: Intel, Nordic Semiconductor, NXP.
Key strengths: Built-in device drivers, Bluetooth stack, and security features like Trusted Execution.

ThreadX (Azure RTOS)

ThreadX, now part of Microsoft's Azure RTOS suite, is known for its deterministic performance and solid commercial-grade features. It’s available under a permissive license with optional commercial support.

Best for: Medical, consumer electronics, and devices requiring Microsoft Azure integration.
Notable users: NASA (Orion capsule), Hewlett Packard.
Key strengths: Low latency, priority inheritance, and trace analysis tools.

Mbed OS

Mbed OS was developed by Arm to support Cortex-M devices and IoT applications. While popular in academia and prototyping, its active development is slowing in favor of CMSIS and direct RTOS usage.

Best for: Educational or early-stage IoT projects.
Notable users: Arm-based academic labs and startups.
Key strengths: Cloud connectivity, TLS stack, and out-of-the-box support for Arm boards.

Upcoming Trends in RTOS Development

Security-first RTOSes: Increased focus on secure boot, runtime protection, and OTA updates.
AI/ML Integration: RTOSes are evolving to support lightweight ML inference at the edge.
Cloud-Native RTOS Stacks: More vendors are integrating RTOS with cloud services (e.g., AWS, Azure).
RISC-V Support: Zephyr and FreeRTOS are rapidly expanding their support for RISC-V.

Expert Insight

To gain a deeper understanding of RTOS selection, we spoke with Dr. Lena Weiss, Senior Embedded Systems Architect at EmbeddedNext, with over 15 years of experience in medical, automotive, and IoT applications:

"Choosing an RTOS today is not just about real-time guarantees or memory footprint. It's also about how well the RTOS integrates with your toolchain, cloud services, and long-term support strategy. For safety-critical environments, ThreadX and QNX often win due to certification support. For startups or agile development, Zephyr offers the fastest ramp-up thanks to its open ecosystem. FreeRTOS, while minimalistic, remains unbeatable in simplicity and community adoption."

Dr. Weiss emphasizes the importance of:

Aligning the RTOS with your product lifecycle — especially if regulatory certification is required.
Avoiding vendor lock-in by evaluating open-source vs proprietary ecosystems.
Testing performance early using vendor SDKs and profiling tools before full integration.

Final Thoughts: Making the Right RTOS Choice

Choosing the right RTOS is about balancing technical requirements with long-term vision. FreeRTOS continues to dominate in simple, resource-constrained environments. Zephyr is evolving into a full-featured, community-driven alternative with great RISC-V and networking support. ThreadX offers maturity, commercial stability, and deterministic performance, especially in regulated sectors. Mbed OS, while still used for education and prototyping, is seeing reduced momentum in production environments.

Use your project’s performance goals, certification needs, and ecosystem integrations as a checklist for RTOS evaluation. And remember — testing in context with real workloads is still the best way to make an informed choice.