XR17V258
66MHZ PCI BUS OCTAL UART WITH POWER MANAGEMENT SUPPORT
5.0 UART CONFIGURATION REGISTERS
5.1
5.2
5.3
Receive Holding Register (RHR) - Read only
Transmit Holding Register (THR) - Write only
Baud Rate Generator Divisors (DLM, DLL and DLD)
REV. 1.0.1
The Baud Rate Generator (BRG) generates the data rate for the transmitter and receiver. The rate is
programmed through registers DLM, DLL and DLD which are only accessible when LCR bit [7] is set to logic 1.
Refer to
for more
details.
5.4
Interrupt Enable Register (IER) - Read/Write
The Interrupt Enable Register (IER) masks the interrupts from receive data ready, transmit empty, line status
and modem status registers. These interrupts are reported in the Interrupt Status Register (ISR) and also
encoded in INT (INT0-INT3) register in the Device Configuration Registers.
5.4.1
IER versus Receive FIFO Interrupt Mode Operation
When the receive FIFO (FCR bit [0] = logic 1) and receive interrupts (IER bit [0] = logic 1) are enabled, the
RHR interrupts (see ISR bits [4:3]) status will reflect the following:
A.
The receive data available interrupts are issued to the host when the FIFO has reached the programmed
trigger level. It will be cleared when the FIFO drops below the programmed trigger level.
B.
FIFO level will be reflected in the ISR register when the FIFO trigger level is reached. Both the ISR register
status bit and the interrupt will be cleared when the FIFO drops below the trigger level.
C.
The receive data ready bit (LSR bit [0]) is set as soon as a character is transferred from the shift register to
the receive FIFO. It is reset when the FIFO is empty.
5.4.2
IER versus Receive/Transmit FIFO Polled Mode Operation
When FCR bit [0] equals a logic 1 for FIFO enable; resetting IER bits [3:0] enables the XR16V258 in the FIFO
polled mode of operation. Since the receiver and transmitter have separate bits in the LSR either can be used
in the polled mode by selecting respective transmit or receive control bit(s).
A.
LSR BIT-0 indicates there is data in RHR (non-FIFO mode) or RX FIFO (FIFO mode).
B.
LSR BIT-1 indicates an overrun error has occurred and that data in the FIFO may not be valid.
C.
LSR BIT 2-4 provides the type of receive data errors encountered for the data byte in RHR, if any.
D.
LSR BIT-5 indicates THR (non-FIFO mode) or TX FIFO (FIFO mode) is empty.
E.
LSR BIT-6 indicates when both the transmit FIFO and TSR are empty.
F.
LSR BIT-7 indicates a data error in at least one character in the RX FIFO.
IER[7]: CTS# Input Interrupt Enable (requires EFR bit [4]=1)
•
Logic 0 = Disable the CTS# interrupt (default).
•
Logic 1 = Enable the CTS# interrupt. The UART issues an interrupt when CTS# pin makes a transition from
LOW to HIGH.
IER[6]: RTS# Output Interrupt Enable (requires EFR bit [4]=1)
•
Logic 0 = Disable the RTS# interrupt (default).
•
Logic 1 = Enable the RTS# interrupt. The UART issues an interrupt when RTS# pin makes a transition from
LOW to HIGH.
IER[5]: Xoff Interrupt Enable (requires EFR bit [4]=1)
•
Logic 0 = Disable the software flow control, receive Xoff interrupt (default).
•
Logic 1 = Enable the software flow control, receive Xoff interrupt. See Software Flow Control section for
details.
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EXAR [ EXAR CORPORATION ]
