UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 302 of 841
NXP Semiconductors
UM10360
Chapter 14: LPC176x/5x UART0/2/3
14.4.1 UARTn Receiver Buffer Register (U0RBR - 0x4000 C000, U2RBR -
0x4009 8000, U3RBR - 0x4009 C000 when DLAB = 0)
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.
14.4.2 UARTn Transmit Holding Register (U0THR - 0x4000 C000, U2THR -
0x4009 8000, U3THR - 0x4009 C000 when DLAB = 0)
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.
14.4.3 UARTn Divisor Latch LSB register (U0DLL - 0x4000 C000, U2DLL -
0x4009 8000, U3DLL - 0x4009 C000 when DLAB = 1) and UARTn
Divisor Latch MSB register (U0DLM - 0x4000 C004, U2DLL -
0x4009 8004, U3DLL - 0x4009 C004 when DLAB = 1)
The UARTn Divisor Latch is part of the UARTn Baud Rate Generator and holds the value
used, along with the Fractional Divider, to divide the APB clock (PCLK) 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. Details on how to
Table 271: UARTn Receiver Buffer Register (U0RBR - address 0x4000 C000, U2RBR - 0x4009 8000, U3RBR -
04009 C000 when DLAB = 0) 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
31:8 - Reserved, the value read from a reserved bit is not defined. NA
Table 272: UARTn Transmit Holding Register (U0THR - address 0x4000 C000, U2THR - 0x4009 8000, U3THR -
0x4009 C000 when DLAB = 0) 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
31:8 - Reserved, user software should not write ones to reserved bits. NA
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