sdk-nrf

nRF Connect SDK is Nordic Semiconductor's official kit for writing firmware for its wireless chips, the nRF52, nRF53, nRF54, nRF70 and nRF91 families inside trackers, wearables, smart locks and cellular sensors. It sits on Zephyr, a tiny real-time operating system for microcontrollers, and adds Bluetooth LE, Thread, Matter, Wi-Fi, LTE-M/NB-IoT and DECT NR+ support plus a secure bootloader. It costs nothing.

Setup is friendlier than most embedded toolchains. The nRF Connect for VS Code extension installs the SDK and a matching toolchain in one go; the command-line route uses nRF Util and SEGGER J-Link. Pin a stable release tag, don't track main.

There's no paid SDK tier. Nordic makes its money on chips and sells nRF Cloud separately for device management and over-the-air updates. Solo and small teams: free, buy a dev kit. Larger teams shipping products: still free, but assign someone to track SDK upgrades and Nordic's security advisories.

The catch: Nordic's own code isn't open source in the usual sense. Its license says it must only be used with a Nordic chip, and the binary libraries can't be reverse engineered. Want vendor freedom? Start from upstream Zephyr (Apache 2.0, still supports Nordic boards) or ESP-IDF on Espressif chips.