UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
162 of 362
NXP Semiconductors
UM10208
Chapter 14: LPC2800 UART
If Auto RTS mode is disabled, the RTS bit (MCR1) controls the RTS output. If Auto RTS
mode is enabled, the Rx FIFO controls the RTS output, and software can read the state of
RTS in the RTS bit (MCR1). As long as Auto RTS is enabled, the RTS bit is read-only for
software.
Example: Suppose the UART is in FIFO mode, auto RTS is enabled, and the trigger level
in the FCR is 10. In this case, RTS is negated when the receive FIFO contains 24 bytes
(see
). RTS is reasserted when the receive FIFO hits the previous trigger
level: 16 bytes.
3.11 Auto CTS
The Auto CTS function is enabled by setting the autoCTS bit (MCR7). If Auto CTS is
enabled, the transmitter checks the CTS input before sending each character. While CTS
is active (low), the transmitter sends characters. To stop the transmitter from sending,
CTS must go high before the middle of the transmitted stop bit. In Auto CTS mode, a
change of CTS does not trigger a modem status interrupt unless the CTS Interrupt Enable
bit is set. However, the Delta CTS bit in the MSR will be set.
lists the
conditions for generating a Modem Status interrupt.
The auto CTS function reduces interrupts on the LPC288x. When flow control is enabled,
a CTS state change does not trigger an interrupt because the UART automatically
controls its own transmitter. Without Auto CTS, the transmitter sends any data present in
the transmit FIFO and a receiver overrun error can result in the remote device.
illustrates the Auto CTS functional timing.
Fig 21. Auto RTS functional timing
start
byte N
stop start
bits0..7
stop
start
bits0..7
stop
N-1
N
N-1
N-1
N-2
N-2
M+2
M+1
M
M-1
UART Rx
RTS pin
UART Rx
FIFO level
UART Rx
FIFO read
~ ~
~ ~
~ ~
~ ~
~ ~
Table 177. Modem status interrupt generation
Enable Modem
Status Interrupt
(IER3)
autoCTS
(MCR7)
CTS Interrupt
Enable (IER7)
Delta CTS
(MSR0)
Modem Status
Interrupt
0
x
x
x
No
1
0
x
0
No
1
0
x
1
Yes
1
1
0
x
No
1
1
1
0
No
1
1
1
1
Yes