Хост-контроллер USB2.0: список регистров

  Unused

  FRAME_INDEX

The value in this register increment at the end of each time frame (e.g. micro-frame).
Bits[N:3] are used for the Frame List current index.
It means that each location of the frame list is accessed 8 times(frames or Micro-frames) before moving to the next index.
The following illustrates values of N based on the value of the Frame List Size field in the USBCMD register.
USB-CMD[Frame List Size]
00 Number Elements=1024 N=12
01 Number Elements=512 N=11
10 Number Elements=256 N=10
11 Reserved

Note
This register must be written as a DWord. Byte writes produce undefined results.

Команда U-Boot для чтения регистра

Bit fields structure

typedef union  e_frindex
{
  struct
  {
   unsigned frame_index : 14;
   unsigned unused0 : 18;
  } b;
   unsigned long w;
} E_FRINDEX