UM10462
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 5.5 — 21 December 2016
248 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
[1]
Values "0000", “0011”, “0101”, “0111”, “1000”, “1001”, “1010”, “1011”,”1101”,”1110”,”1111” are reserved.
[2]
For details see
Section 12.5.9 “USART Line Status Register (Read-Only)”
[3]
For details see
Section 12.5.1 “USART Receiver Buffer Register (when DLAB = 0, Read Only)”
[4]
For details see
Section 12.5.5 “USART Interrupt Identification Register (Read Only)”
“USART Transmitter Holding Register (when DLAB = 0, Write Only)”
The USART THRE interrupt (IIR[3:1] = 001) is a third level interrupt and is activated when
the USART THR FIFO is empty provided certain initialization conditions have been met.
These initialization conditions are intended to give the USART 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
USART 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).
The modem status interrupt (IIR3:1 = 000) is the lowest priority USART interrupt and is
activated whenever there is a state change on the CTS, DCD, or DSR or a trailing edge
on the RI pin. The source of the modem interrupt can be read in MSR3:0. Reading the
MSR clears the modem interrupt.
12.5.6 USART FIFO Control Register (Write Only)
The FCR controls the operation of the USART 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, DSR, RI, or DCD.
MSR Read
Table 236. USART Interrupt Handling
IIR[3:0]
value
[1]
Priority Interrupt
type
Interrupt source
Interrupt
reset