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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1170 of 1441
NXP Semiconductors
UM10503
Chapter 41: LPC43xx/LPC43Sxx UART1
41.7 Functional description
41.7.1 Auto-flow control
If auto-RTS mode is enabled the UART1‘s receiver FIFO hardware controls the RTS1
output of the UART1. If the auto-CTS mode is enabled the UART1‘s TSR hardware will
only start transmitting if the CTS1 input signal is asserted.
41.7.1.1
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, RTS1 is de-asserted
(to a high value). It is possible that the sending UART sends an additional byte after the
trigger level is reached (assuming the sending UART has another byte to send) because it
might not recognize the de-assertion of RTS1 until after it has begun sending the
additional byte. RTS1 is automatically reasserted (to a low value) once the receiver FIFO
has reached the previous trigger level. The re-assertion of RTS1 signals to the sending
UART to continue transmitting data.
If Auto-RTS mode is disabled, the RTSen bit controls the RTS1 output of the UART1. If
Auto-RTS mode is enabled, hardware controls the RTS1 output, and the actual value of
RTS1 will be copied in the RTS Control bit of the UART1. As long as Auto-RTS is enabled,
the value of the RTS Control bit is read-only for software.
Example:
Suppose the UART1 operating in ‘550 mode has trigger level in FCR set to 0x2
then if Auto-RTS is enabled the UART1 will de-assert the RTS1 output as soon as the
receive FIFO contains 8 bytes (
). The RTS1 output will be
reasserted as soon as the receive FIFO hits the previous trigger level: 4 bytes.
Table 970: UART1 Transmit Enable Register (TER - address 0x4008 205C) bit description
Bit
Symbol
Description
Reset
value
0
TXEN
Transmit enable.
After reset transmission is enabled. When the TXEN bit is de-asserted,
no data will be transmitted although data may be pending in the TSR
or THR.
1
31:1
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-