This is a multi-function register. This register holds receives and transmit data and controls the least-signficant 8 bits of the baud rate divisor.
Module Instance Base Address Register Address
uart0 0xFFC02000 0xFFC02000
uart1 0xFFC03000 0xFFC03000

Offset: 0x0

Access: RW

Important: To prevent indeterminate system behavior, reserved areas of memory must not be accessed by software or hardware. Any area of the memory map that is not explicitly defined as a register space or accessible memory is considered reserved.
Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16


15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0



RW 0x0

rbr_thr_dll Fields

Bit Name Description Access Reset
7:0 value

Receive Buffer Register: This register contains the data byte received on the serial input port (uart_rxd). The data in this register is valid only if the Data Ready ( bit [0] in the Line Status Register(LSR)) is set to 1. If FIFOs are disabled(bit[0] of Register FCR is set to 0) the data in the RBR must be read before the next data arrives, otherwise it will be overwritten, resulting in an overrun error. If FIFOs are enabled(bit [0] of Register FCR is set to 1) this register accesses the head of the receive FIFO. If the receive FIFO is full, and this register is not read before the next data character arrives, then the data already in the FIFO will be preserved but any incoming data will be lost. An overrun error will also occur. Transmit Holding Register: This register contains data to be transmitted on the serial output port. Data should only be written to the THR when the THR Empty bit [5] of the LSR Register is set to 1. If FIFOs are disabled (bit [0] of Register FCR) is set to 0 and THRE is set to 1, writing a single character to the THR clears the THRE. Any additional writes to the THR before the THRE is set again causes the THR data to be overwritten. If FIFO's are enabled bit [0] of Register FCR is set to 1 and THRE is set up to 128 characters of data may be written to the THR before the FIFO is full. Any attempt to write data when the FIFO is full results in the write data being lost. Divisor Latch Low: This register makes up the lower 8-bits of a 16-bit, Read/write, Divisor Latch register that contains the baud rate divisor for the UART. This register may only be accessed when the DLAB bit [7] of the LCR Register is set to 1. The output baud rate is equal to the serial clock l4_sp_clk frequency divided by sixteen times the value of the baud rate divisor, as follows: baud rate = (serial clock freq) / (16 * divisor) Note that with the Divisor Latch Registers (DLL and DLH) set to zero, the baud clock is disabled and no serial communications will occur. Also, once the DLL is set, at least 8 l4_sp_clk clock cycles should be allowed to pass before transmitting or receiving data.

RW 0x0