This paper presents Atacama, the first open-source framework based on reconfigurable hardware for mixed-criticality communication in multi-segmented Ethernet networks. Atacama uses specialized modules for time-triggered communication of real-time data, which seamlessly integrate with a standard infrastructure using regular best-effort traffic. Atacama enables low and highly predictable communication latency on multi-segmented 1Gbps networks, easy optimization of devices for specific application scenarios, and rapid prototyping of new protocol characteristics. Researchers can use the open-source design to verify our results and build upon the framework, which aims to accelerate the development, validation, and adoption of Ethernet-based solutions in real-time applications.