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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
288 of 808
NXP Semiconductors
UM10360
Chapter 14: LPC17xx UART0/2/3
describes how to use TXEn bit in order to achieve software flow control.
4.14 UARTn FIFO Level register (U0FIFOLVL - 0x4000 C058, U2FIFOLVL -
0x4009 8058, U3FIFOLVL - 0x4009 C058, Read Only)
UnFIFOLVL register is a Read Only register that allows software to read the current FIFO
level status. Both the transmit and receive FIFO levels are present in this register.
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.
Table 265: UARTn Transmit Enable Register (U0TER - address 0x4000 C030,
U2TER - 0x4009 8030, U3TER - 0x4009 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
Table 266. UARTn FIFO Level register (UnFIFOLVL - address 0x4001 C0458, Read Only) bit
description
Bit
Symbol
Description
Reset
value
3:0
RXFIFILVL
Reflects the current level of the UART receiver FIFO.
0 = empty, 0xF = FIFO full.
0x00
7:4
-
Reserved. The value read from a reserved bit is not defined.
NA
11:8
TXFIFOLVL
Reflects the current level of the UART transmitter FIFO.
0 = empty, 0xF = FIFO full.
0x00
31:12
-
Reserved. The value read from a reserved bit is not defined.
NA