VBUS: Input. Recommended to tie to GND if
unused. However, leaving VBUS floating is also acceptable since
internally there is an 80 kΩ resistance to ground.
REFCLK: Input. If REFCLK is unused and 60
MHz clock is provided by MODEM (60 MHz should be connected to CLOCK pin in
this case), then tie REFCLK to GND.
CFG: Tie to GND if REFCLK is 19.2 MHz, or
tie to VDDIO if REFCLK is 26 MHz. Tie to either GND or
VDDIO (does not matter which) if REFCLK is not used (for
example, ULPI input clock configuration).