Micro-controller units (MCUs) implement the de facto interface between the physical and digital worlds. As a consequence, they appear in a variety of sensing/actuation applications, from smart personal spaces to complex industrial control systems and safety-critical medical equipment. While many of these devices perform safety- and time-critical tasks, they often lack support for security features compatible with their importance to overall system functions. This lack of architectural support leaves them vulnerable to run-time attacks that can remotely alter their intended behavior, with potentially catastrophic consequences. In particular, we note that MCU software often includes untrusted third-party libraries (some of them closed-source) that are blindly used within MCU programs, without proper isolation from the rest of the system. In turn, a single vulnerability (or intentional backdoor) in one such third-party software can often compromise the entire MCU software state. In this paper, we tackle this problem by proposing, demonstrating security, and formally verifying the implementation of UCCA: an Untrusted Code Compartment Architecture. UCCA provides flexible hardware-enforced isolation of untrusted code sections (e.g., third-party software modules) in resource-constrained and time-critical MCUs. To demonstrate UCCA's practicality, we implement an open-source version of the design on a real resource-constrained MCU: the well-known TI MSP430. Our evaluation shows that UCCA incurs little overhead and is affordable even to lowest-end MCUs, requiring significantly less overhead and assumptions than prior related work.