Both FreeRTOS and Zephyr are popular real-time operating systems (RTOS) for embedded systems, but they differ in architecture, features, and use cases. Below is a comprehensive comparison:

1. Overview

Feature	FreeRTOS	Zephyr
License	MIT (owned by AWS)	Apache 2.0 (Linux Foundation)
Primary Use	Small, resource-constrained devices	IoT, complex embedded Linux-like systems
Kernel Type	Microkernel (minimalist)	Monolithic (rich features)
Community	Large, industry-backed (AWS)	Growing, Linux Foundation-backed

2. Key Differences

A. Architecture

Aspect	FreeRTOS	Zephyr
Kernel Size	~5-10 KB (minimal)	~50-200 KB (larger due to features)
Modularity	Add-ons via libraries (e.g., TCP/IP)	Built-in modules (device tree, drivers)
Portability	Manual porting (CPU-specific)	Hardware-agnostic (uses Device Tree)

B. Scheduling & Multitasking

Feature	FreeRTOS	Zephyr
Scheduler	Preemptive, co-operative	Preemptive, priority-based
Max Priorities	56 (configurable)	256 (configurable)
Thread Model	Tasks (static/dynamic)	Threads + POSIX-like API

C. Memory Management

Feature	FreeRTOS	Zephyr
Heap Allocators	`heap_1` to `heap_5` (various schemes)	Single pool, slab allocator
Memory Protection	Limited (MPU optional)	MPU/MMU support (isolates threads)
Static Allocation	Supported (no dynamic allocation)	Encouraged (minimizes fragmentation)

D. Connectivity & Networking

Feature	FreeRTOS	Zephyr
Wi-Fi	Requires external stack (FreeRTOS+TCP)	Built-in (Wi-Fi, BLE, LoRaWAN)
Bluetooth	Limited (external libs)	Native support (BLE Mesh, HCI)
IP Stack	FreeRTOS+TCP (add-on)	Built-in (LwIP, BSD sockets)

E. Development & Debugging

Feature	FreeRTOS	Zephyr
Build System	Makefile (manual)	CMake + `west` (modern, modular)
Debugging	GDB, Tracealyzer	Built-in shell, logging, GDB
IDE Support	Eclipse, VS Code	VS Code, Segger Embedded Studio

F. Security

Feature	FreeRTOS	Zephyr
TLS/Encryption	AWS IoT Libs (add-on)	Built-in (mbedTLS, PSA Crypto)
Secure Boot	Limited (depends on hardware)	Supported (MCUboot integration)
Thread Isolation	No (unless MPU enabled)	Yes (memory domains)

3. Performance Comparison

Metric	FreeRTOS	Zephyr
Context Switch	~1-5 µs (Cortex-M)	~5-10 µs (due to richer features)
Interrupt Latency	~50-100 ns	~100-200 ns
RAM Usage	~2-5 KB (minimal task)	~5-15 KB (per thread)

4. Use Cases

FreeRTOS is Better For:

✔ Low-end MCUs (STM32F0, ESP32-C3)
✔ Quick prototyping (simple task scheduling)
✔ AWS IoT integration (FreeRTOS + AWS libraries)

Zephyr is Better For:

✔ Complex IoT devices (gateways, wearables)
✔ Multi-core/secure systems (Cortex-A/M hybrids)
✔ Linux-like development (POSIX API, device tree)

5. Code Examples

FreeRTOS Task Creation

void vTaskFunction(void *pvParameters) {
  while(1) {
    vTaskDelay(1000 / portTICK_PERIOD_MS);
  }
}

xTaskCreate(vTaskFunction, "Task1", 128, NULL, 1, NULL);

Zephyr Thread Creation

void thread_function(void *p1, void *p2, void *p3) {
  while(1) {
    k_sleep(K_MSEC(1000));
  }
}

K_THREAD_DEFINE(my_thread, 512, thread_function, NULL, NULL, NULL, 5, 0, 0);

6. Pros and Cons

FreeRTOS

✅ Pros:

Extremely lightweight
Easy to learn and port
Strong AWS ecosystem

❌ Cons:

Limited built-in features
No native networking

Zephyr