XR17V254_08 Datasheet, PDF (49/70 Pages) Exar Corporation – 66MHZ PCI BUS QUAD UART WITH POWER MANAGEMENT SUPPORT

XR17V254
REV. 1.0.1
66MHZ PCI BUS QUAD UART WITH POWER MANAGEMENT SUPPORT
LSR[6]: Transmitter Empty Flag
This bit is the Transmitter Empty indicator. This bit is set to a logic 1 whenever both the transmit FIFO (or THR,
in non-FIFO mode) and the transmit shift register (TSR) are both empty. It is set to logic 0 whenever either the
TX FIFO or TSR contains a data character.
LSR[5]: Transmit FIFO Empty Flag
This bit is the Transmit FIFO Empty indicator. This bit indicates that the transmitter is ready to accept a new
character for transmission. This bit is set to a logic HIGH when the last data byte is transferred from the
transmit FIFO to the transmit shift register. The bit is reset to logic 0 as soon as a data byte is loaded into the
transmit FIFO. In the non-FIFO mode this bit is set when the transmit holding register (THR) is empty; it is
cleared when at a byte is written to the THR.
LSR[4]: Receive Break Flag
• Logic 0 = No break condition (default).
• Logic 1 = The receiver received a break signal (RX was LOW for one character frame time). In the FIFO
mode, only one break character is loaded into the FIFO. The break indication remains until the RX input
returns to the idle condition, “mark” or HIGH.
LSR[3]: Receive Data Framing Error Flag
• Logic 1 = No framing error (default).
• Logic 1 = Framing error. The receive character did not have a valid stop bit(s). This error is associated with
the character available for reading in RHR.
LSR[2]: Receive Data Parity Error Flag
• Logic 0 = No parity error (default).
• Logic 1 = Parity error. The receive character in RHR (top of the FIFO) does not have correct parity
information and is suspect. This error is associated with the character available for reading in RHR.
LSR[1]: Receiver Overrun Flag
• Logic 0 = No overrun error (default).
• Logic 1 = Overrun error. A data overrun error condition occurred in the receive shift register. This happens
when additional data arrives while the FIFO is full. In this case the previous data in the receive shift register
is overwritten. Note that under this condition the data byte in the receive shift register is not transferred into
the FIFO, therefore the data in the FIFO is not corrupted by the error.
LSR[0]: Receive Data Ready Indicator
• Logic 0 = No data in receive holding register or FIFO (default).
• Logic 1 = Data has been received and is saved in the receive holding register or FIFO.
5.10 Modem Status Register (MSR) - Read Only
This register provides the current state of the modem interface signals, or other peripheral device that the
UART is connected. Lower four bits of this register are used to indicate the changed information. These bits
are set to a logic 1 whenever a signal from the modem changes state. These bits may be used as general
purpose inputs/outputs when they are not used with modem signals.
MSR[7]: CD Input Status
Normally this bit is the complement of the CD# input. In the loopback mode this bit is equivalent to bit [3] in the
MCR register. The CD# input may be used as a general purpose input when the modem interface is not used.
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