NXP Semiconductors UM10237 - page 429

UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 429 o f 792
NXP Semiconductors UM10237
Chapter 16: LPC24XX Universal Asynchronous Receiver/Transmitter
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 16–4.8 “UARTn Line Status Register (U0LSR - 0xE000C014, U2LSR -
0xE007 8014, U3LSR - 0xE007C014, Read Only)”
[3] For details see Section 16–16.4.1 “UARTn Receiver Buffer Register (U0RBR - 0xE000C000, U2RBR -
0xE007 8000, U3RBR - 0xE007C000 when DLAB = 0, Read Only)”
[4] For details see Section 16–4.5 “UARTn Interrupt Identification Register (U0IIR - 0xE000 C008, U2IIR -
0xE007 8008, U3IIR - 0x7008C008, Read Only)” and Section 16–4.2 “UARTn Transmit Holding Register
(U0THR - 0xE000 C000, U2THR - 0xE0078000, U3THR - 0xE007C000 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 384: UARTn Interrupt Handling
U0IIR[3:0]
value[1] Priority Interrupt Type Interrupt Source Interrupt Reset
0001 - None None -
0110 Highest RX Line Status
/ Error OE[2] or PE[2] or FE[2] or BI[2] UnLSR Read[2]
0100 Second RX Data
Available Rx data available or trigger level reached in FIFO (UnFCR0=1) UnRBR Read[3] 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[3]
0010 Third THRE THRE[2] UnIIR Read (if
source of
interrupt) or
THR write[4]