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 251 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
12.5.8.1 Auto-flow control
If auto-RTS mode is enabled, the USART‘s receiver FIFO hardware controls the RTS
output of the USART. If the auto-CTS mode is enabled, the USART‘s transmitter will only
start sending if the CTS
pin is low.
12.5.8.1.1 Auto-RTS
The auto-RTS function is enabled by setting the RTSen bit. Auto-RTS data flow control
originates in the RBR module and is linked to the programmed receiver FIFO trigger level.
If auto-RTS is enabled, the data-flow is controlled as follows:
When the receiver FIFO level reaches the programmed trigger level, RTS
is deasserted
(to a high value). It is possible that the sending USART sends an additional byte after the
trigger level is reached (assuming the sending USART has another byte to send) because
it might not recognize the deassertion of RTS
until after it has begun sending the
additional byte. RTS
is automatically reasserted (to a low value) once the receiver FIFO
has reached the previous trigger level. The reassertion of RTS
signals the sending
USART to continue transmitting data.
If Auto-RTS mode is disabled, the RTSen bit controls the RTS
output of the USART. If
Auto-RTS mode is enabled, hardware controls the RTS
output, and the actual value of
RTS
will be copied in the RTS Control bit of the USART. As long as Auto-RTS is enabled,
the value of the RTS Control bit is read-only for software.
Example: Suppose the USART operating in type ‘550 mode has the trigger level in FCR
set to 0x2, then, if Auto-RTS is enabled, the USART will deassert the RTS
output as soon
as the receive FIFO contains 8 bytes (Table 237 on page 249
). The RTS output will be
reasserted as soon as the receive FIFO hits the previous trigger level: 4 bytes.
6 RTSEN RTS enable 0
0 Disable auto-rts flow control.
1 Enable auto-rts flow control.
7 CTSEN CTS enable 0
0 Disable auto-cts flow control.
1 Enable auto-cts flow control.
31:8 - - Reserved -
Table 239. USART Modem Control Register (MCR - address 0x4000 8010) bit description
Bit Symbol Value Description Reset
value
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