System, methods and apparatus are described that support multimode operation of a data communication interface. An apparatus includes a physical layer interface coupled to a serial bus and configurable for a high-speed mode of communication and a low-speed mode of communication, and a rate detector configured to receive a clock signal from the serial bus, and to use a reference clock to determine a unit interval representative of a data rate of the serial bus. The apparatus may also include interval calculation logic configured to determine an interval related to timing of a data signal transmitted on the serial bus, the interval having a duration expressed as a number of cycles of the reference clock. The physical layer interface may be configured to use the interval to capture data in the data signal.