UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
953 of 1269
NXP Semiconductors
UM10503
Chapter 37: LPC43xx USART0_2_3
37.6.1 USART Receiver Buffer Register
The RBR is the top byte of the USART 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 RBR.
The RBR is always Read Only.
Since PE, FE and BI bits (see
) 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 LSR register, and then to read a byte from the RBR.
37.6.2 USART Transmitter Holding Register
The THR is the top byte of the USART 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 LCR must be zero in order to access the THR.
The THR is always Write Only.
37.6.3 USART Divisor Latch LSB and MSB Registers
The USART Divisor Latch is part of the USART Baud Rate Generator and holds the value
used, along with the Fractional Divider, to divide the USART_PCLK clock 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 0x0000 value is treated like a 0x0001
value as division by zero is not allowed.The Divisor Latch Access Bit (DLAB) in LCR must
be one in order to access the USART Divisor Latches. Details on how to select the right
value for DLL and DLM can be found in
Table 822. USART Receiver Buffer Registers when DLAB = 0, Read Only (RBR - addresses
0x4008 1000 (USART0), 0x400C 1000 (USART2), 0x400C 2000 (USART3)) bit
description
Bit
Symbol
Description
Reset value
7:0
RBR
Receiver buffer.
The USART Receiver Buffer Register contains the oldest
received byte in the USART RX FIFO.
undefined
31:8 -
Reserved
-
Table 823. USART Transmitter Holding Register when DLAB = 0, Write Only (THR -
addresses 0x4008 1000 (USART0), 0x400C 1000 (USART2), 0x400C 2000
(USART3)) bit description
Bit
Symbol
Description
Reset value
7:0
THR
Transmit Holding Register.
Writing to the USART Transmit Holding Register causes the data
to be stored in the USART transmit FIFO. The byte will be sent
when it reaches the bottom of the FIFO and the transmitter is
available.
NA
31:8 -
Reserved
-