NXP Semiconductors LPC24XX UM10237 - Manual (Page 441)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 441 of 792
NXP Semiconductors
UM10237
Chapter 16: LPC24XX UART0/2/3
Table 16–394 describes how to use TXEn bit in order to achieve software flow control.
5. Architecture
The architecture of the UARTs 0, 2 and 3 are shown below in the block diagram.
The APB interface provides a communications link between the CPU or host and the
UART.
The UARTn receiver block, UnRX, monitors the serial input line, RXDn, for valid input.
The UARTn RX Shift Register (UnRSR) accepts valid characters via RXDn. After a valid
character is assembled in the UnRSR, it is passed to the UARTn RX Buffer Register FIFO
to await access by the CPU or host via the generic host interface.
The UARTn transmitter block, UnTX, accepts data written by the CPU or host and buffers
the data in the UARTn TX Holding Register FIFO (UnTHR). The UARTn TX Shift Register
(UnTSR) reads the data stored in the UnTHR and assembles the data to transmit via the
serial output pin, TXDn.
The UARTn Baud Rate Generator block, UnBRG, generates the timing enables used by
the UARTn TX block. The UnBRG clock input source is the APB clock (PCLK). The main
clock is divided down per the divisor specified in the UnDLL and UnDLM registers. This
divided down clock is a 16x oversample clock, NBAUDOUT.
The interrupt interface contains registers UnIER and UnIIR. The interrupt interface
receives several one clock wide enables from the UnTX and UnRX blocks.
Status information from the UnTX and UnRX is stored in the UnLSR. Control information
for the UnTX and UnRX is stored in the UnLCR.
Table 394. UARTn Transmit Enable Register (U0TER - address 0xE000 C030,
U2TER - 0xE007 8030, U3TER - 0xE007 C030) bit description
Bit Symbol Description Reset
Value
6:0 - Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined. NA
7 TXEN When this bit is 1, as it is after a Reset, data written to the THR is output
on the TXD pin as soon as any preceding data has been sent. If this bit
is cleared to 0 while a character is being sent, the transmission of that
character is completed, but no further characters are sent until this bit is
set again. In other words, a 0 in this bit blocks the transfer of characters
from the THR or TX FIFO into the transmit shift register. Software
implementing software-handshaking can clear this bit when it receives
an XOFF character (DC3). Software can set this bit again when it
receives an XON (DC1) character. 1