UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 328 of 841
NXP Semiconductors
UM10360
Chapter 15: LPC176x/5x UART1
15.4.9 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 U1TSR hardware will
only start transmitting if the CTS1 input signal is asserted.
15.4.9.1
Auto-RTS
The auto-RTS function is enabled by setting the RTSen bit. Auto-RTS data flow control
originates in the U1RBR 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.
Table 299: UART1 Modem Control Register (U1MCR - address 0x4001 0010) bit description
Bit Symbol Value Description Reset
value
0 DTR Control Source for modem output pin, DTR. This bit reads as 0 when modem loopback mode
is active.
0
1 RTS Control Source for modem output pin RTS. This bit reads as 0 when modem loopback mode is
active.
0
3-2 - NA Reserved, user software should not write ones to reserved bits. The value read from a
reserved bit is not defined.
0
4 Loopback
Mode Select
The modem loopback mode provides a mechanism to perform diagnostic loopback
testing. Serial data from the transmitter is connected internally to serial input of the
receiver. Input pin, RXD1, has no effect on loopback and output pin, TXD1 is held in
marking state. The 4 modem inputs (CTS, DSR, RI and DCD) are disconnected
externally. Externally, the modem outputs (RTS, DTR) are set inactive. Internally, the 4
modem outputs are connected to the 4 modem inputs. As a result of these
connections, the upper 4 bits of the U1MSR will be driven by the lower 4 bits of the
U1MCR rather than the 4 modem inputs in normal mode. This permits modem status
interrupts to be generated in loopback mode by writing the lower 4 bits of U1MCR.
0
0 Disable modem loopback mode.
1 Enable modem loopback mode.
5 - NA Reserved, user software should not write ones to reserved bits. The value read from a
reserved bit is not defined.
0
6 RTSen 0 Disable auto-rts flow control. 0
1 Enable auto-rts flow control.
7 CTSen 0 Disable auto-cts flow control. 0
1 Enable auto-cts flow control.
31:8 - Reserved, user software should not write ones to reserved bits. The value read from a
reserved bit is not defined.
NA
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