Zephyr Native Simulation Speeds CoffeeCaller Development

Zephyr native simulation is allowing the CoffeeCaller team to build and test much of its embedded application on an ordinary host computer before repeatedly flashing the target board. The workflow sits behind the open-hardware CoffeeCaller nRF52840 board, a coffee-break coordinator that has grown into a development platform with buttons, indicator LEDs, WS2812 RGB LEDs, a buzzer, sensors, and exposed headers.

Zephyr Native Simulation Without the Target Board

In this Elektor video clip, hardware developer Andreas Kurz explains how software developer Stefan Kraus structured the application so it can be compiled for Zephyr’s native_sim board and executed on the development PC. Instead of waiting for the physical board after every change, the team can exercise application logic rapidly on the host and move to the nRF52840 hardware periodically for confirmation.

Zephyr’s native simulator does not emulate the nRF52840 itself. It builds the Zephyr kernel, libraries, and application as a normal Linux executable. That distinction matters: It is best suited to software kept cleanly separated from hardware-specific behavior, which is precisely what the CoffeeCaller architecture tries to achieve.

Mock Drivers Keep the Application Testable

A host PC does not conveniently have a strip of WS2812 LEDs attached to it. Kraus therefore wrote a mock LED-strip driver that implements the same API expected by the application. Tests can press a virtual button, request yellow LEDs, and check that the bytes sent through the interface appear in the correct order. The mock cannot prove signal timing or electrical behavior, but it can catch state-machine, API, and data-ordering mistakes before the target board enters the loop.

The public CoffeeCaller software repository builds for both native_sim and the custom board and includes multiple levels of tests. That makes the project more than a demonstration of an RTOS feature: It is a useful example of designing firmware around replaceable subsystems rather than letting every application component reach directly into physical peripherals.

Agile Hardware Means Reducing the Wait

The same separation supports the team’s broader goal of making hardware development less of a stop-start affair. Hardware revisions can take months, while software teams still need to make progress. With simulated targets, mock drivers, modular subsystems, and pin headers for adding peripherals, software work can continue while the next PCB revision is being designed or manufactured.

Zephyr Safety Architect Tobias Kästner also contributed ideas around agile hardware workflows, helping turn what began as a small office gadget into a shared development playground. The practical lesson is not that real hardware is optional. It is that engineers should reserve hardware time for the faults only hardware can reveal, rather than using the board as an expensive substitute for ordinary software tests. Zephyr native simulation gives CoffeeCaller a faster feedback loop, and the project shows what that looks like when the architecture is planned for it from the start.