Универсальный асинхронный приемо-передатчик: список регистров

  Unused

  DLL

Divisor Latch Low
Lower 8 bits of a 16 bits, read/write, Divisor Latch Register contains the
baud rate divisor for the UART. This register may only be accessed when
the DLAB bit (LCR[7]) is set and the UART is not busy (USR[0] is zero).
The output baud rate is equal to the serial clock (sclk) frequency divided by
sixteen times the value of the baud rate divisor, as follows: baud rate =
(serial clock freq) / (16 * divisor).
Note that when the Divisor Latch Registers (DLL and DLH) are set to zero,
the baud clock is disabled and no serial communications occur. Also, once
the DLL is set, at least 8 clock cycles of the slowest UART clock should be
allowed to pass before transmitting or receiving data.

Команда U-Boot для чтения регистра

Bit fields structure

typedef union  uart_dll
{
  struct
  {
   unsigned dll : 8;
   unsigned unused0 : 24;
  } b;
   unsigned long w;
} UART_DLL