
SMSC DS – FDC37N958FR
Page 88
Rev. 09/01/99
Receiver Line Status Interrupt whenever any of
the corresponding conditions are detected and the
interrupt is enabled.
BIT 5
Transmitter Holding Register Empty (THRE). Bit 5
indicates that the Serial Port is ready to accept a
new character for transmission. In addition, this
bit causes the Serial Port to issue an interrupt
when the Transmitter Holding Register interrupt
enable is set high. The THRE bit is set to a logic
"1" when a character is transferred from the
Transmitter Holding Register into the Transmitter
Shift Register. The bit is reset to logic "0"
whenever the CPU loads the Transmitter Holding
Register. In the FIFO mode this bit is set when
the XMIT FIFO is empty, it is cleared when at least
1 byte is written to the XMIT FIFO. Bit 5 is a read
only bit.
BIT 6
Transmitter Empty (TEMT). Bit 6 is set to a logic
"1" whenever the Transmitter Holding Register
(THR) and Transmitter Shift Register (TSR) are
both empty. It is reset to logic "0" whenever either
the THR or TSR contains a data character. Bit 6
is a read only bit. In the FIFO mode this bit is set
whenever the THR and TSR are both empty,
BIT 7
This bit is permanently set to logic "0" in the 450
mode. In the FIFO mode, this bit is set to a logic
"1" when there is at least one parity error, framing
error or break indication in the FIFO. This bit is
cleared when the LSR is read if there are no
subsequent errors in the FIFO.
MODEM STATUS REGISTER (MSR)
Address Offset = 6H, DLAB = X, READ/WRITE
This 8 bit register provides the current state of the
control lines from the MODEM (or peripheral
device). In addition to this current state
information, four bits of the MODEM Status
Register (MSR) provide change information.
These bits are set to logic "1" whenever a
control input from the MODEM changes state.
They are reset to logic "0" whenever the MODEM
Status Register is read.
BIT 0
Delta Clear To Send (DCTS). Bit 0 indicates that
the nCTS input to the chip has changed state
since the last time the MSR was read.
BIT 1
Delta Data Set Ready (DDSR). Bit 1 indicates
that the nDSR input has changed state since the
last time the MSR was read.
BIT 2
Trailing Edge of Ring Indicator (TERI). Bit 2
indicates that the nRI input has changed from
logic "0" to logic "1".
BIT 3
Delta Data Carrier Detect (DDCD). Bit 3 indicates
that the nDCD input to the chip has changed
state.
NOTE: Whenever bit 0, 1, 2, or 3 is set to a logic
"1", a MODEM Status Interrupt is generated.
BIT 4
This bit is the complement of the Clear To Send
(nCTS) input. If bit 4 of the MCR is set to logic
"1", this bit is equivalent to nRTS in the MCR.
BIT 5
This bit is the complement of the Data Set Ready
(nDSR) input. If bit 4 of the MCR is set to logic
"1", this bit is equivalent to DTR in the MCR.
BIT 6
This bit is the complement of the Ring Indicator
(nRI) input. If bit 4 of the MCR is set to logic "1",
this bit is equivalent to OUT1 in the MCR.