UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
155 of 362
NXP Semiconductors
UM10208
Chapter 14: LPC2800 UART
3.1 Receiver Buffer Register (RBR - 0x8010 1000 when DLAB=0, Read
Only)
The oldest received character in the Rx FIFO can be read from the RBR. The first
received data bit is in the LSB (bit 0). If the character received contains less than 8 bits,
the unused MSBs are padded with zeroes.
The Divisor Latch Access Bit (DLAB) in LCR must be zero in order to access the RBR.
The RBR is always Read Only.
Since the PE, FE and BI bits in the LSR correspond to the top byte of the Rx FIFO (i.e. the
one that will be read in the next read from the RBR), the right approach for fetching a
received byte and its status bits is first to read the LSR, and then read the byte from the
RBR.
3.2 Transmit Holding Register (THR - 0x8010 1000 when DLAB=0, Write
Only)
The THR is used to write data to the TX FIFO. Bit 0 is transmitted first.
The Divisor Latch Access Bit (DLAB) in the LCR must be zero in order to access the THR.
The THR is always Write Only.
3.3 Divisor Latch LSB Register (DLL - 0x8010 1000 when DLAB=1)
3.4 Divisor Latch MSB Register (DLM - 0x8010 1004 when DLAB=1)
The Divisor Latch is part of the Baud Rate Generator and holds the value used to divide
the UART baud rate clock (UART_CLK) in order to produce the baud rate clock, which
must be 16x the desired baud rate. The DLL and DLM registers together form a 16 bit
divisor where DLL contains the lower 8 bits of the divisor and DLM contains the higher 8
bits of the divisor. A zero value is treated like 0x0001, as division by zero is not allowed.
The Divisor Latch Access Bit (DLAB) in LCR must be one in order to access the Divisor
Latches.
Table 167. Receiver Buffer Register (RBR - 0x8010 1000 when DLAB=0, Read Only)
Bit
Symbol Description
Reset
Value
7:0
RBR
The Receiver Buffer Register contains the oldest received byte in the
Rx FIFO.
Undefined
31:8 -
Reserved. The value read from a reserved bit is not defined.
-
Table 168. Transmit Holding Register (THR - 0x8010 1000 when DLAB=0)
Bit
Symbol Description
Reset
Value
7:0
THR
Writing to the Transmit Holding Register causes the data to be stored in
the transmit FIFO. The byte is sent when it reaches the bottom of the
FIFO and the transmitter is available.
NA
31:8
-
Reserved, user software should not write ones to reserved bits.
-