Why Zephyr fits modular hardware

We talk with Tobias Kästner, a physicist-turned-software-architect and technical consultant at Inovex, about his journey from painfully slow hardware-software integration cycles to achieving three-week hardware sprints. Tobias shares hard-won lessons from medical device development, where fuzzy requirements and constant feedback from life scientists forced his team to rethink traditional approaches.

The conversation centers on practical techniques: breaking monolithic PCB designs into modular "feature boards" connected via shields (think Arduino-style), using Git for hardware version control with SHA-1s printed on silkscreens, and leveraging tools like Zephyr RTOS to enable plug-and-play firmware that matches the modularity of the hardware. Tobias explains how relaxing constraints like board size and using automation to merge schematics allowed his team to iterate rapidly while maintaining a clear path to final form-factor designs. We discuss how this approach scaled to projects with 120+ people across multiple teams, and why the interplay between system architecture, organizational structure, and information flow matters more than most realize.

Key Topics

• [02:30] The painful reality of traditional hardware development: six-month wait for hardware, nine months of debugging
• [08:00] Breaking apart monolithic PCB designs into modular feature boards with shield connectors
• [12:45] Relaxing constraints: larger board areas, autorouting, and prioritizing testability over final form factor
• [18:20] Version control for hardware: putting schematics in Git and printing SHA-1s on silkscreens
• [22:00] Using automation to merge feature board schematics into final form-factor designs
• [26:15] Firmware architecture: NuttX, Zephyr, KConfig, and device trees for modular, plug-and-play software
• [35:40] Scaling agile hardware-software co-development to 120+ person projects across multiple teams
• [39:00] The interplay of system architecture, organizational architecture, and information architecture

Notable Quotes

"When the board arrived, not a single line of code had been written for it because no one had been able to touch it. It took us nine additional months to debug all the things out of it." — Tobias Kästner

"I've never seen any board working the first time. I've never seen any prototype without thin wires patching things out, but that's maybe a different story." — Tobias Kästner

"We cannot think these architectures as independent of one another. If we have limitations in two of these architectures, we will see these limitations in the third architecture as well." — Tobias Kästner

Resources Mentioned

• Inovex - Tobias's company offering engineering consulting services, trainings, and expertise in embedded systems, IoT, and full-stack development
• Zephyr RTOS - Open-source real-time operating system with KConfig, device tree support, and extensive driver library that Tobias recommends for modular firmware development
• NuttX RTOS - Apache Foundation RTOS with clean device driver model and KConfig support that Tobias used in earlier projects
• KiCad - Open-source PCB design software with emerging Python API support for schematic automation

Services and Contact

Through Inovex, Tobias provides trainings for both Zephyr and Yocto Linux, as well as consultancy and engineering support for embedded projects -- from 1-2 day workshops evaluating architectural state and cost/benefit analysis, to first prototypes, to full-fledged software development. With partners such as alpha-board (Berlin) and Blunk electronic (Erfurt), they also offer agile hardware services and help teams get started with the methods discussed in this episode.

• Email: tobias.kaestner@inovex.de
• Tobias Kästner on LinkedIn
• tobiaskaestner on the Zephyr Discord Channel

Links

Companies:

• Inovex -- Embedded Systems
• Blunk electronic
• alpha-board
• Navimatix

Talks and publications:

• Modular and Agile HW Development (2018 talk)
• Leveraging Zephyr's HW Abstraction for Agile Systems Engineering (2023 talk)
• Whitepaper: Agile in der Hardware -- by Gregor Gross, Christoph Schmiedinger, and Tobias Kästner
• Leveraging Zephyr for Functional Architecture Decomposition (2025 talk)

Books recommended by Tobias:

• Small Groups as Complex Systems -- Holly Arrow et al., SAGE Publications
• The Dao of Complexity -- Jean Boulton, DeGruyter

You can find Jeff at https://jeffgable.com.
You can find Luca at https://luca.engineer.

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