The C8051F38x devices are the next generation of USB products and are pin and function compatible with the C8051F34x family. There are some changes to the family over the existing family. There is no longer a clock multiplier and the internal oscillator is calibrated to 48 MHz. This clock can be used to drive the USB clock directly. To maintain backward compatibility with existing firmware, the special function registers (SFR) that controlled the multiplier in the earlier family still exist, allowing existing firmware to be used without modification. The communications interfaces for the SMBus and the UART have been enhanced to add more flexibility and in some cases reduces code size and frees up other resources. The SMBus interfaces are also backwards compatible with existing firmware.