This application relates to methods and apparatus for configuring a serial data interface, especially a data interface (300) for a transducer (324) such as a digital microphone or loudspeaker. The data interface is selectively operable in a first or second serial data mode of operation for input of data to or output data from the transducer device. The first and second serial data modes are different, e.g. correspond to different serial formats. The data interface has a controller (330) configured to determine a resistance value (331, 332) at a sense terminal (314) of the transducer device and to control the data interface in the first serial data mode if the resistance value is within a first resistance range and control the data interface in the second serial data mode if the resistance value is within a second, different, resistance range.