UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
990 of 1269
NXP Semiconductors
UM10503
Chapter 38: LPC43xx UART1
[1]
Values "0000", “0011”, “0101”, “0111”, “1000”, “1001”, “1010”, “1011”,”1101”,”1110”,”1111” are reserved.
[2]
For details see
Section 38.6.9 “UART1 Line Status Register”
[3]
For details see
Section 38.6.1 “UART1 Receiver Buffer Register (when DLAB = 0)”
[4]
For details see
Section 38.6.5 “UART1 Interrupt Identification Register”
Transmitter Holding Register (when DLAB = 0)”
The UART1 THRE interrupt (IIR[3:1] = 001) is a third level interrupt and is activated when
the UART1 THR FIFO is empty provided certain initialization conditions have been met.
These initialization conditions are intended to give the UART1 THR FIFO a chance to fill
up with data to eliminate many THRE interrupts from occurring at system start-up. The
initialization conditions implement a one character delay minus the stop bit whenever
THRE = 1 and there have not been at least two characters in the THR at one time since
the last THRE = 1 event. This delay is provided to give the CPU time to write data to THR
without a THRE interrupt to decode and service. A THRE interrupt is set immediately if the
UART1 THR FIFO has held two or more characters at one time and currently, the THR is
empty. The THRE interrupt is reset when a THR write occurs or a read of the IIR occurs
and the THRE is the highest interrupt (IIR[3:1] = 001).
It is the lowest priority interrupt and is activated whenever there is any state change on
modem inputs pins, DCD, DSR or CTS. In addition, a low to high transition on modem
input RI will generate a modem interrupt. The source of the modem interrupt can be
determined by examining MSR[3:0]. A MSR read will clear the modem interrupt.
38.6.6 UART1 FIFO Control Register
The write-only FCR controls the operation of the UART1 RX and TX FIFOs.
1100
Second Character
Time-out
indication
Minimum of one character in the RX FIFO and no
character input or removed during a time period depending
on how many characters are in FIFO and what the trigger
level is set at (3.5 to 4.5 character times).
The exact time will be:
[(word length)
7 - 2]
8 + [(trigger level - number of
characters)
8 + 1] RCLKs
RBR Read
0010
Third
THRE
THRE
IIR Read
(if source of
interrupt) or THR write
0000
Fourth
Modem
Status
CTS or DSR or RI or DCD
MSR Read
Table 855: UART1 Interrupt Handling
IIR[3:0]
value
[1]
Priority Interrupt
Type
Interrupt Source
Interrupt Reset
Table 856: UART1 FIFO Control Register (FCR - address 0x4008 2008) bit description
Bit
Symbol
Value Description
Reset value
0
FIFOEN
FIFO enable.
0
0
Must not be used in the application.
1
Active high enable for both UART1 Rx and TX FIFOs and FCR[7:1] access.
This bit must be set for proper UART1 operation. Any transition on this bit will
automatically clear the UART1 FIFOs.