UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 252 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
12.5.8.1.2 Auto-CTS
The Auto-CTS function is enabled by setting the CTSen bit. If Auto-CTS is enabled, the
transmitter circuitry checks the CTS
input before sending the next data byte. When CTS is
active (low), the transmitter sends the next byte. To stop the transmitter from sending the
following byte, CTS
must be released before the middle of the last stop bit that is currently
being sent. In Auto-CTS mode, a change of the CTS
signal does not trigger a modem
status interrupt unless the CTS Interrupt Enable bit is set, but the Delta CTS bit in the
MSR will be set. Table 240
lists the conditions for generating a Modem Status interrupt.
The auto-CTS function typically eliminates the need for CTS interrupts. When flow control
is enabled, a CTS
state change does not trigger host interrupts because the device
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. Figure 26
illustrates the Auto-CTS functional timing.
Fig 25. Auto-RTS Functional Timing
start byte N stop start bits0..7 stop start bits0..7 stop
N-1 N N-1 N-1N-2 N-2 M+2 M+1 M M-1
UART1 Rx
RTS1 pin
UART1 Rx
FIFO level
UART1 Rx
FIFO read
~
~
~
~
~
~
~
~
~
~
Table 240. Modem status interrupt generation
Enable
modem
status
interrupt
(IER[3])
CTSen
(MCR[7])
CTS
interrupt
enable
(IER[7])
Delta CTS
(MSR[0])
Delta DCD or trailing edge
RI or
Delta DSR (MSR[3:1])
Modem
status
interrupt
0x x x x No
10 x 0 0 No
10 x 1 x Yes
10 x x 1 Yes
11 0 x 0 No
11 0 x 1 Yes
11 1 0 0 No
11 1 1 x Yes
11 1 x 1 Yes
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