UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
161 of 362
NXP Semiconductors
UM10208
Chapter 14: LPC2800 UART
3.9 Modem Control Register (MCR - 0x8010 1010)
The MCR enables the modem loopback mode and controls the RTS output signal.
3.10 Auto-Flow Control
If auto RTS mode is enabled, the UART’s receiver FIFO hardware controls the RTS
output. If auto CTS mode is enabled, the UART’s transmitter will only send characters
when the CTS input is active (low).
3.10.1 Auto RTS
The Auto RTS function is enabled by setting the autoRTS bit (MCR6). Auto RTS data flow
control is linked to the programmed Rx FIFO trigger level. If auto RTS is enabled, and if
the Rx FIFO level reaches the programmed trigger level, RTS is negated (made high).
The sending UART may send an additional byte after the trigger level is reached
(assuming the sending UART has another byte to send) because it may not recognize the
negation of RTS until after it has begun sending the additional byte. RTS is automatically
reasserted (made low) once the Rx FIFO has reached the previous trigger level. The
assertion of RTS signals the sending UART to continue transmitting data.
Table 176. Modem Control Register (MCR - address 0x8010 1010)
Bit
Name
Description
Reset
value
0
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-
1
RTS
If the autoRTS bit (MCR6) is 1, this bit is read-only and reflects the
current state of the RTS pin. If autoRTS is 0, this bit controls the RTS
pin. In either case, a 1 in this bit is equivalent to RTS low, a 0 to RTS
high. This bit reads as 0 when modem loopback mode is active.
0
3:2
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-
4
Loopback
Mode
Select
A 1 in this bit enables a mechanism for diagnostic loopback testing. In
this mode:
Serial data from the transmitter is connected internally to serial input of
the receiver
Input pin RXD has no effect on loopback and output pin,
TXD is held in marking state,
The four modem inputs (CTS, DSR, RI and DCD) are disconnected
externally,
Externally, the modem outputs (RTS, DTR) are set inactive,
Internally, the four modem outputs are connected to the four modem
inputs, and
The upper four bits of the MSR are driven by the lower four bits of the
MCR, rather than the four modem inputs as in normal mode.
This permits modem status interrupts to be generated in loopback mode
by writing the lower four bits of the MCR.
0
6
autoRTS
A 1 in this bit enables automatic RTS flow control.
0
7
autoCTS
A 1 in this bit enables automatic CTS flow control.
0
31:8 -
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-