Zephyr RTOS — Introduction

What is Zephyr?

Zephyr is an open-source, real-time operating system (RTOS) hosted by the Linux Foundation, designed for resource-constrained embedded and IoT devices.

Property	Detail
License	Apache 2.0
Founded	2016 (Intel donated)
Target	Microcontrollers with as little as 4KB RAM
Kernel type	Monolithic with modules
Supported boards	700+ (STM32, nRF52/53, ESP32, RISC-V, ARM Cortex-M/R/A, x86)

Key Features

Preemptive multithreading — priority-based scheduler with time-slicing
Memory protection — optional MPU/MMU support
Device driver model — unified API across hardware vendors
Networking — built-in TCP/IP, Bluetooth, 802.15.4, CAN, USB
Power management — fine-grained sleep states
Devicetree — hardware description borrowed from Linux
ZBus — built-in publish/subscribe message bus (see 03 Zbus)

Build System — west + CMake

Zephyr uses CMake for building and west as its meta-tool (workspace manager + flash tool).

# Install west
pip install west

# Initialize a workspace
west init my_workspace
cd my_workspace
west update          # pulls Zephyr + all modules

# Build a sample for nRF52840 DK
west build -b nrf52840dk/nrf52840 samples/hello_world

# Flash to board
west flash

# Open serial monitor
west espressif monitor   # or: screen /dev/ttyACM0 115200

Project structure

my_workspace/
├── zephyr/               ← Zephyr kernel source (managed by west)
├── modules/              ← HAL drivers, nanopb, mbedTLS, etc.
├── bootloader/
└── my_app/               ← Your application
    ├── CMakeLists.txt
    ├── prj.conf          ← Kconfig — enable/disable features
    ├── app.overlay       ← Devicetree overlay — your board wiring
    └── src/
        └── main.c

prj.conf — Kconfig

This is how you turn features on/off — no #ifdefs in code:

# prj.conf
CONFIG_ZBUS=y              # enable ZBus
CONFIG_SPI=y               # enable SPI driver
CONFIG_I2C=y               # enable I2C driver
CONFIG_CAN=y               # enable CAN driver
CONFIG_NANOPB=y            # enable nanopb serialization
CONFIG_MAIN_STACK_SIZE=2048
CONFIG_THREAD_STACK_INFO=y # debug: show stack usage

app.overlay — Devicetree

Describes how YOUR hardware is wired — separate from board definition:

/* app.overlay */
&spi1 {
    status = "okay";
    pinctrl-0 = <&spi1_default>;
    cs-gpios = <&gpioa 4 GPIO_ACTIVE_LOW>;
};

&i2c1 {
    status = "okay";
    imu: icm42688@68 {
        compatible = "invensense,icm42688";
        reg = <0x68>;
    };
};

How Zephyr compares

	Zephyr	FreeRTOS	bare-metal
Ecosystem	Large, Linux Foundation backed	Large, AWS backed	None
Networking	Built-in	Add-on (FreeRTOS+TCP)	Fully manual
Security	Actively maintained CVEs, PSA certified	Variable	You own everything
Complexity	Higher — more to learn	Lower	Lowest
Message bus	ZBus (built-in)	None (use queues manually)	None
Devicetree	Yes (like Linux)	No	No
Best for	Complex IoT, multi-peripheral systems	Simple tasks, small MCUs	Ultra-low latency, tiny flash

Common Use Cases

IoT sensors — BLE devices, wearables, environmental monitors
Industrial — motor controllers, fieldbus gateways (CAN, RS-485)
Automotive — ECUs, CAN nodes, AUTOSAR alternatives
Robotics — sensor hubs, actuator controllers (our STM32 use case)
Security-critical — MCUs needing FIPS/PSA compliance

Good Starting Points

Official docs: https://docs.zephyrproject.org
Samples: zephyr/samples/ in the repo
samples/hello_world — basic threading
samples/drivers/spi/ — SPI
samples/subsys/zbus/ — ZBus
samples/bluetooth/beacon/ — BLE