Zephyr RTOS vs SoftDevice

Firmware Over-The-Air (FOTA) updates enable remote bug fixes, security patches, and feature upgrades on IoT and embedded products. On Nordic Semiconductor devices, developers choose between two approaches: the legacy SoftDevice-based DFU and the modern Zephyr RTOS-based FOTA in the nRF Connect SDK. This blog compares both to help you choose the right strategy.

Understanding the Architectures

SoftDevice is Nordic’s proprietary BLE protocol stack running as a separate binary alongside the application. For FOTA, Nordic’s DFU bootloader supports BLE DFU but requires careful memory partitioning across the bootloader, SoftDevice, application, and update storage areas. Its proprietary package format requires Nordic’s tools to generate and is incompatible with other bootloaders or ecosystems.

Zephyr RTOS in the nRF Connect SDK is a unified, open-source RTOS in which the BLE controller and host run in the same environment. For FOTA, it uses MCUboot, an open-source secure bootloader supporting BLE, USB, UART, and custom transports.

Figure: SoftDevice vs Zephyr/MCUboot, separated vs unified firmware architecture

Update Process and Mechanisms

SoftDevice DFU supports three update types: application-only, SoftDevice-only, or combined. Firmware is transferred over BLE via Nordic’s proprietary DFU service and validated with CRC checks. A key limitation is that combined updates require two separate transfers and reboots. Each image is signature-verified before applying, or a rollback is triggered.

Zephyr/MCUboot delivers updates over BLE, USB, UART, or custom transports through a unified SMP interface. It supports test mode with rollback on failure and permanent upgrade options. Each image is signature-verified before applying, or a rollback is triggered.

Figure: SoftDevice DFU vs Zephyr/MCUboot update flow comparison

Security Considerations

SoftDevice provides basic security through CRC validation and optional signing, which is also sufficient.

Zephyr/MCUboot supports RSA-2048, ECDSA-P256, and ED25519 signatures, downgrade protection via security counters, and encrypted images for IP protection. On supported hardware, ARM TrustZone and Trusted Firmware-M (TF-M) can be integrated for additional security layers. Being open source, it also benefits from third-party security audits.

Figure: Security comparison

Ecosystem and Long-term Support

SoftDevice is a mature, proven solution, but Nordic has shifted focus to nRF Connect SDK, making it a legacy path with diminishing support over time.

Zephyr is actively growing, backed by the Linux Foundation, with faster bug fixes, more features, and stronger long-term support as more vendors adopt it.

Figure: Ecosystem trajectory, SoftDevice vs Zephyr/MCUboot long-term support

Making the Right Choice

Choose SoftDevice if:

• You’re maintaining an existing SoftDevice-based product
• Your team has extensive SoftDevice experience and limited time for new learning
• You need maximum stability with proven production deployment
• You are limited on MCU RAM and Flash Memory.
• Your project timeline is short and doesn’t accommodate learning new tools

Choose Zephyr/nRF Connect SDK if:

• You’re starting a new project
• You need flexibility in updating transports or memory management
• Security is a critical requirement
• You need support for newer Nordic chips or features like Zephyr.

Conclusion

SoftDevice remains a stable, proven choice for maintaining existing products, but is increasingly a legacy solution with proprietary constraints and a BLE-only update model.

For new designs, Zephyr RTOS with MCUboot is the superior FOTA solution, stronger security, multi-transport updates, and better long-term support. The learning curve is higher, but the long-term benefits clearly outweigh the upfront cost.

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Working on a Nordic-based product and need help with FOTA implementation? Feel free to Contact Us for firmware development, OTA architecture, and embedded systems services.