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10-18 Intel® PXA255 Processor Developer’s Manual
UARTs
10.4.2.9 Modem Control Register (MCR)
The MCR, shown in Table 10-14, uses the modem control pins nRTS and nDTR to control the
interface with a modem or data set. The MCR also controls the Loopback mode. Loopback mode
must be enabled before the UART is enabled. The differences between UARTs specific to this
register are described in Section 10.5.1.
This is a read/write register. Ignore reads from reserved bits. Write zeros to reserved bits.
Table 10-14. MCR Bit Definitions (Sheet 1 of 2)
Base+0x10 Modem Control Register UART
Bit
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
reserved
LOOP
OUT2
OUT1
RTS
DTR
Reset 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Read/Write
Bits Name Description
31:5 — reserved
4 LOOP
Loopback Mode: This bit provides a local loopback feature for diagnostic testing of the
UART. When LOOP is set to a logic 1, the following will occur: The transmitter serial output
is set to a logic 1 state. The receiver serial input is disconnected from the pin. The output of
the Transmitter Shift register is “looped back” into the receiver shift register input. The four
modem control inputs (nCTS, nDSR, nDCD, and nRI) are disconnected from the pins and
the modem control output pins (nRTS and nDTR) are forced to their inactive state.
Coming out of the loopback mode may result in unpredictable activation of the delta bits
(bits 3:0) in the Modem Status Register. It is recommended that MSR is read once to clear
the delta bits in the MSR.
Loopback mode must be configured before the UART is enabled.
The lower four bits of the MCR are connected to the upper four Modem Status Register
bits:
• DTR = 1 forces DSR to a 1
• RTS = 1 forces CTS to a 1
• OUT1 = 1 forces RI to a 1
• OUT2= 1 forces DCD to a 1
In loopback mode, data that is transmitted is immediately received. This feature allows the
processor to verify the transmit and receive data paths of the UART. The transmit, receive
and modem control interrupts are operational, except the modem control interrupts are
activated by MCR bits, not the modem control pins. A break signal can also be transferred
from the transmitter section to the receiver section in loopback mode.
0 – normal UART operation
1 – loopback mode UART operation
3OUT2
OUT2 signal control: OUT2 connects the UART’s interrupt output to the Interrupt Controller
unit. When LOOP=0:
0 – UART interrupt is disabled.
1 – UART interrupt is enabled.
When LOOP=1, interrupts always go to the processor:
0 – MSR[DCD] forced to a 0
1 – MSR[DCD] forced to a 1