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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
276 of 808
NXP Semiconductors
UM10360
Chapter 14: LPC17xx UART0/2/3
The UARTn RDA interrupt (UnIIR[3:1] = 010) shares the second level priority with the CTI
interrupt (UnIIR[3:1] = 110). The RDA is activated when the UARTn Rx FIFO reaches the
trigger level defined in UnFCR[7:6] and is reset when the UARTn Rx FIFO depth falls
below the trigger level. When the RDA interrupt goes active, the CPU can read a block of
data defined by the trigger level.
The CTI interrupt (UnIIR[3:1] = 110) is a second level interrupt and is set when the UARTn
Rx FIFO contains at least one character and no UARTn Rx FIFO activity has occurred in
3.5 to 4.5 character times. Any UARTn Rx FIFO activity (read or write of UARTn RSR) will
clear the interrupt. This interrupt is intended to flush the UARTn RBR after a message has
been received that is not a multiple of the trigger level size. For example, if a peripheral
wished to send a 105 character message and the trigger level was 10 characters, the
CPU would receive 10 RDA interrupts resulting in the transfer of 100 characters and 1 to 5
CTI interrupts (depending on the service routine) resulting in the transfer of the remaining
5 characters.
[1]
Values "0000", “0011”, “0101”, “0111”, “1000”, “1001”, “1010”, “1011”,”1101”,”1110”,”1111” are reserved.
[2]
For details see
Section 14–4.8 “UARTn Line Status Register (U0LSR - 0x4000 C014, U2LSR -
0x4009 8014, U3LSR - 0x4009 C014, Read Only)”
[3]
For details see
Section 14–14.4.1 “UARTn Receiver Buffer Register (U0RBR - 0x4000 C000, U2RBR -
0x4009 8000, U3RBR - 0x4009 C000 when DLAB = 0, Read Only)”
[4]
For details see
Section 14–4.5 “UARTn Interrupt Identification Register (U0IIR - 0x4000 C008, U2IIR -
0x4009 8008, U3IIR - 0x4009 C008, Read Only)”
Section 14–4.2 “UARTn Transmit Holding Register
(U0THR - 0x4000 C000, U2THR - 0x4009 8000, U3THR - 0x4009 C000 when DLAB = 0, Write Only)”
The UARTn THRE interrupt (UnIIR[3:1] = 001) is a third level interrupt and is activated
when the UARTn THR FIFO is empty provided certain initialization conditions have been
met. These initialization conditions are intended to give the UARTn 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
Table 255: UARTn Interrupt Handling
U0IIR[3:0]
value
Priority Interrupt Type
Interrupt Source
Interrupt Reset
0001
-
None
None
-
0110
Highest RX Line Status
/ Error
or BI
UnLSR Read
0100
Second RX Data
Available
Rx data available or trigger level reached
in FIFO (UnFCR0=1)
UnRBR Read
or UARTn FIFO
drops below
trigger level
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
UnRBR Read
0010
Third
THRE
THRE
UnIIR Read (if
source of
interrupt) or
THR write