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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
274 of 571
NXP Semiconductors
UM10316
Chapter 19: LPC29xx Universal Asynchronous Receiver/Transmitter
4.1 UARTn Receiver Buffer Register
The UnRBR is the top byte of the UARTn Rx FIFO. The top byte of the Rx FIFO contains
the oldest character received and can be read via the bus interface. The LSB (bit 0)
represents the “oldest” received data bit. If the character received is 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 UnRBR.
The UnRBR is always Read Only.
Since PE, FE and BI bits correspond to the byte sitting on the top of the RBR FIFO (i.e.
the one that will be read in the next read from the RBR), the right approach for fetching the
valid pair of received byte and its status bits is first to read the content of the U0LSR
register, and then to read a byte from the UnRBR.
4.2 UARTn Transmit Holding Register
The UnTHR is the top byte of the UARTn TX FIFO. The top byte is the newest character in
the TX FIFO and can be written via the bus interface. The LSB represents the first bit to
transmit.
The Divisor Latch Access Bit (DLAB) in UnLCR must be zero in order to access the
UnTHR. The UnTHR is always Write Only.
4.3 UARTn Divisor Latch LSB Register
The UARTn Divisor Latch is part of the UARTn Baud Rate Generator and holds the value
used to divide the APB clock (BASE_UART_CLK) in order to produce the baud rate clock,
which must be 16
×
the desired baud rate. The UnDLL and UnDLM registers together form
a 16 bit divisor where UnDLL contains the lower 8 bits of the divisor and UnDLM contains
the higher 8 bits of the divisor. A 0x0000 value is treated like a 0x0001 value as division by
zero is not allowed. The Divisor Latch Access Bit (DLAB) in UnLCR must be one in order
to access the UARTn Divisor Latches.
Table 229. UARTn Receiver Buffer Register (U0RBR - address 0xE004 5000,
U1RBR - 0xE004 6000 when DLAB = 0, Read Only) bit description
Bit
Symbol
Description
Reset Value
7:0
RBR
The UARTn Receiver Buffer Register contains the oldest
received byte in the UARTn Rx FIFO.
Undefined
Table 230. UARTn Transmit Holding Register (U0THR - address 0xE004 5000,
U1THR - 0xE004 6000 when DLAB = 0, Write Only) bit description
Bit
Symbol
Description
Reset Value
7:0
THR
Writing to the UARTn Transmit Holding Register causes the data
to be stored in the UARTn transmit FIFO. The byte will be sent
when it reaches the bottom of the FIFO and the transmitter is
available.
NA
Table 231. UARTn Divisor Latch LSB Register (U0DLL - address 0xE004 5000,
U1DLL - 0xE004 6000 when DLAB = 1) bit description
Bit
Symbol
Description
Reset Value
7:0
DLLSB
The UARTn Divisor Latch LSB Register, along with the UnDLM
register, determines the baud rate of the UARTn.
0x01