OBUFT

OBUFT stands for Output Buffer, Unidirectional, Tri-State. It is a class of I/O primitive used in many Xilinx FPGAs to drive an external pin with a controllable tri-state capability. An OBUFT implements a unidirectional buffer from an internal logic signal to the FPGA’s I/O pin and includes a separate tri-state control input.

The core function is to present a stable logic level on the pin when enabled, and to release the pin to a high-impedance state when disabled. Typically, the primitive has at least two inputs: D, the data input, and T (or OE), the tri-state control. When the tri-state input allows, the pin reflects D; when the tri-state input disables, the pin becomes high impedance, allowing other devices to drive the line. This enables the implementation of tri-state buses and interfaces where multiple devices may share an external line.

OBUFTs are configured and instantiated through the FPGA design tool and are mapped to the appropriate I/O bank and voltage standard. In many designs, tri-state behavior is inferred by synthesis tools from top-level port directions and external connection requirements, while in other cases explicit OBUFT instantiation provides precise control over the IO behavior. OBUFTs differ from simple output buffers (OBUF), which drive a line without a tri-state enable. They are commonly used in external communication interfaces, memory buses, and any scenario requiring controlled multi-device sharing of a single pin.