UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
163 of 362
NXP Semiconductors
UM10208
Chapter 14: LPC2800 UART
Data is sent as long it’s available and CTS is low. Transmission stalls when CTS goes
high and the current Tx character is complete. The UART keeps TXD high as long as CTS
is high. When CTS goes low, transmission resumes and a start bit is sent followed by the
data bits of the next character.
3.12 Line Status Register (LSR - 0x8010 1014, Read Only)
The LSR is a read-only register that provides status information on the TX and RX blocks.
Fig 22. Auto CTS functional timing
start
bits0..7
start
bits0..7
stop
start
bits0..7
stop
UART TX
CTS pin
~ ~
~ ~
~ ~
~ ~
stop
Table 178. Line Status Register (LSR - 0x8010 1014, read only)
Bit
Name
Description
Reset
Value
0
Receiver Data
Ready (RDR)
This bit is 1 if the RBR holds an unread character, 0 if the Rx FIFO
is empty.
0
1
Overrun Error
(OE)
This bit is set when the receive shift register has a new character
assembled and the Rx FIFO is full. In this case, the Rx FIFO is not
overwritten and the new character is lost. This bit is set as soon the
overrun condition occurs. Reading the LSR clears this bit.
0
2
Parity Error
(PE)
This bit is 1 if LCR3 is 1, and the parity bit of the character at the
top of the Rx FIFO does not match the checking criterion in
LCR5:4. Reading the LSR clears this bit. This bit is significant only
when RDR (LSR0) is 1.
0
3
Framing Error
(FE)
This bit is 1 if the UART sampled the RXD signal low at the center
of the stop bit of the character at the top of the Rx FIFO. Reading
this register clears this bit. This bit is significant only when RDR
(LSR0) is 1 and BI (LSR4) is 0.
Upon detecting a framing error, the receiver attempts to
re-synchronize to the data by assuming that the bad stop bit is
actually an early start bit. However, the next received byte may not
be correct, even if it has no Framing Error. To minimize Framing
errors, send more than one stop bit.
0
4
Break Indicator
(BI)
This bit is 1 if the character at the top of the Rx FIFO has all zero
data bits, and the receiver also sampled the Stop bit low (and the
parity bit low if LCR3 is 1). Once a break condition has been
detected, the receiver goes idle until RXD goes high. Reading this
register clears this bit. This bit is significant only when RDR (LSR0)
is 1.
0
5
Transmit
Holding
Register Empty
(THRE)
In ’450 mode this bit is 1 if the UART is ready to accept a character
for transmission. In the FIFO mode, this bit is 1 if the Tx FIFO is
empty.
1