4.
5.
All MODEM Control inputs (nCTS,
nDSR, nRI and nDCD) are disconnected.
The four MODEM Control outputs
(nDTR, nRTS, OUT1 and OUT2) are
internally connected to the four MODEM
Control inputs (nDSR, nCTS, RI, DCD).
The Modem Control output pins are
forced inactive high.
MODEM CONTROL REGISTER (MCR)
Address Offset
READ/WRITE
=
4H, DLAB
=
X,
This 8 bit register controls the interface with the
MODEM or data set (or device emulating a
MODEM). The contents of the MODEM control
register are described below.
6.
7.
Data that is transmitted is immediately
received.
BIT 0
This bit controls the Data Terminal Ready
(nDTR) output. When bit 0 is set to a logic "1",
the nDTR output is forced to a logic "0". When
bit 0 is a logic "0", the nDTR output is forced to
a logic "1".
This feature allows the processor to verify the
transmit and receive data paths of the Serial
Port. In the diagnostic mode, the receiver and
the transmitter interrupts are fully operational.
The MODEM Control Interrupts are also
operational but the interrupts' sources are now
the lower four bits of the MODEM Control
Register instead of the MODEM Control inputs.
The interrupts are still controlled by the Interrupt
Enable Register.
BIT 1
This bit controls the Request To Send (nRTS)
output. Bit 1 affects the nRTS output in a
manner identical to that described above for bit
0.
BIT 2
BIT 5-7
This bit controls the Output 1 (OUT1) bit. This
bit does not have an output pin and can only be
read or written by the CPU.
These bits are permanently set to logic zero.
LINE STATUS REGISTER (LSR)
Address Offset
READ/WRITE
=
5H, DLAB
=
X,
BIT 3
Output 2 (OUT2). This bit is used to enable an
UART interrupt. When OUT2 is a logic "0", the
serial port interrupt output is forced to a high
impedance state - disabled. When OUT2 is a
logic "1", the serial port interrupt outputs are
enabled.
BIT 0
Data Ready (DR). It is set to a logic "1"
whenever a complete incoming character has
been received and transferred into the Receiver
Buffer Register or the FIFO. Bit 0 is reset to a
logic "0" by reading all of the data in the Receive
Buffer Register or the FIFO.
BIT 4
This bit provides the loopback feature for
diagnostic testing of the Serial Port. When bit 4
is set to logic "1", the following occur:
BIT 1
Overrun Error (OE). Bit 1 indicates that data in
the Receiver Buffer Register was not read before
the next character was transferred into the
register, thereby destroying the previous
character. In FIFO mode, an overrun error will
occur only when the FIFO is full and the next
character has been completely received in the
shift register, the character in the shift register is
1.
2.
3.
The TXD is set to the Marking State
(logic "1").
The receiver Serial Input (RXD) is
disconnected.
The output of the Transmitter Shift
Register is "looped back" into
the Receiver Shift Register input.
74
SMSC [ SMSC CORPORATION ]
