Abstract: An integrated circuit includes circuitry to test input/output (I/O) devices. Test data is provided to a loopback circuit that drives data through the output buffer to the pad, and back onto the integrated circuit through the input buffer. Separate clock signals, with varying phase, are generated for input synchronous elements and output synchronous elements. The phase, and the relative time delay between the separate clocks, changes as an external clock is varied. The external clock is varied to verify the performance parameters of the I/O devices. Each I/O device includes a shift register that can be coupled to the other buffers in a chain, or can be configured to be in a loop.