From the mobile phone side, the USB D− wire is used as either the USB D− signal, the UART Transmit data signal, the left stereo speaker audio channel, or the mono speaker audio channel, and the USB D+ wire is used as either the USB D+ signal, the UART Receive data signal, the right stereo speaker audio channel, or the mono microphone audio channel.
The carkit specification also finds optional support in ULPI specification for off chip USB Physical layer ICs.