UM11029
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© NXP Semiconductors N.V. 2017. All rights reserved.
User manual
Rev. 1.0 — 16 June 2017
288 of 515
NXP Semiconductors
UM11029
Chapter 17: LPC84x USART0/1/2/3/4
17.5 General description
The USART receiver block monitors the serial input line, Un_RXD, for valid input. The
receiver shift register assembles characters as they are received, after which they are
passed to the receiver buffer register to await access by the CPU or the DMA controller.
When RTS signal is configured as an RS-485 output enable, it is asserted at the
beginning of an transmitted character, and de-asserted either at the end of the character,
or after a one character delay (selected by software).
The USART transmitter block accepts data written by the CPU or DMA controllers and
buffers the data in the transmit holding register. When the transmitter is available, the
transmit shift register takes that data, formats it, and serializes it to the serial output,
Un_TXD.
The Baud Rate Generator block divides the incoming clock to create a 16x baud rate
clock in the standard asynchronous operating mode. The BRG clock input source is the
shared Fractional Rate Generator that runs from the common USART peripheral clock
U_PCLK).
In synchronous slave mode, data is transmitted and received using the serial clock
directly. In synchronous master mode, data is transmitted and received using the baud
rate clock without division.
Status information from the transmitter and receiver is saved and provided via the Stat
register. Many of the status flags are able to generate interrupts, as selected by software.
Remark:
The fractional value and the USART peripheral clock are shared between all
USARTs.
U3_SCLK I/O
external
to pin
any pin
PINASSIGN12
Serial clock input/output for USART3 in synchronous
mode.
U4_TXD
O
external
to pin
any pin
PINASSIGN12
Transmitter output for USART4. Serial transmit data.
U4_RXD
I
external
to pin
any pin
PINASSIGN12
Receiver input for USART4.
U4_SCLK I/O
external
to pin
any pin
PINASSIGN13
Serial clock input/output for USART4 in synchronous
mode.
Table 321. USART pin description
Function
I/O
Type
Connect
to
Use register
Reference
Description