
Advanced Notebook I/O for ISA or LPC Designs
Datasheet
Rev. 0.5 (03-24-05)
Page 98
SMSC SIO10N268
DATASHEET
These bits are permanently set to logic zero.
8.8.1.8
Line Status Register (LSR)
Address Offset = 5H, DLAB = X, READ/WRITE
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 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
overrunn 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 overwritten but not transferred to the FIFO. The OE indicator
is set to a logic "1" immediately upon detection of an overrun condition, and reset whenever the Line Status
Register is read.
Bit 2
Parity Error (PE). Bit 2 indicates that the received data character does not have the correct even or odd
parity, as selected by the even parity select bit. The PE is set to a logic "1" upon detection of a parity error
and is reset to a logic "0" whenever the Line Status Register is read. In the FIFO mode this error is associated
with the particular character in the FIFO it applies to. This error is indicated when the associated character is
at the top of the FIFO.
Bit 3
Framing Error (FE). Bit 3 indicates that the received character did not have a valid stop bit. Bit 3 is set to a
logic "1" whenever the stop bit following the last data bit or parity bit is detected as a zero bit (Spacing level).
The FE is reset to a logic "0" whenever the Line Status Register is read. In the FIFO mode this error is
associated with the particular character in the FIFO it applies to. This error is indicated when the associated
character is at the top of the FIFO. The Serial Port will try to resynchronize after a framing error. To do this, it
assumes that the framing error was due to the next start bit, so it samples this 'start' bit twice and then takes in
the 'data'.
Bit 4
Break Interrupt (BI). Bit 4 is set to a logic "1" whenever the received data input is held in the Spacing state
(logic "0") for longer than a full word transmission time (that is, the total time of the start bit + data bits + parity
bits + stop bits). The BI is reset after the CPU reads the contents of the Line Status Register. In the FIFO
mode this error is associated with the particular character in the FIFO it applies to. This error is indicated when
the associated character is at the top of the FIFO. When break occurs only one zero character is loaded into
the FIFO. Restarting after a break is received, requires the serial data (RXD) to be logic "1" for at least 1/2 bit
time.
NOTE:
Bits 1 through 4 are the error conditions that produce a 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