XR17D154
xr
UNIVERSAL (3.3V AND 5V) PCI BUS QUAD UART
REV. 1.2.0
EFR[6]: Auto RTS or DTR Flow Control Enable
RTS#/DTR# output may be used for hardware flow control by setting EFR bit-6 to logic 1. When Auto RTS/
DTR is selected, an interrupt will be generated when the receive FIFO is filled to the programmed trigger level
and RTS#/DTR# will de-assert HIGH at the next upper trigger or selected hysteresis level. RTS#/DTR# will
return LOW when FIFO data falls below the next lower trigger or selected hysteresis level (see FCTR bits 4-7).
The RTS# or DTR# output must be asserted (LOW) before the auto RTS/DTR can take effect. The selection
for RTS# or DTR# is through MCR bit-2. RTS/DTR# pin will function as a general purpose output when
hardware flow control is disabled.
• Logic 0 = Automatic RTS/DTR flow control is disabled (default).
• Logic 1 = Enable Automatic RTS/DTR flow control.
EFR[7]: Auto CTS Flow Control Enable
Automatic CTS or DSR Flow Control.
• Logic 0 = Automatic CTS/DSR flow control is disabled (default).
• Logic 1 = Enable Automatic CTS/DSR flow control. Transmission stops when CTS#/DSR# pin de-asserts
HIGH. Transmission resumes when CTS/DSR# pin returns LOW. The selection for CTS# or DSR# is through
MCR bit-2.
5.8.15 TXCNT[7:0]: Transmit FIFO Level Counter - Read-Only
Transmit FIFO level byte count from 0x00 (zero) to 0x40 (64). This 8-bit register gives an indication of the
number of characters in the transmit FIFO. The FIFO level Byte count register is read only. The user can take
advantage of the FIFO level byte counter for faster data loading to the transmit FIFO, which reduces CPU
bandwidth requirements. Please see the Application Note DAN119 on Exar’s website for a detailed discussion
of FIFO level counters. Due to the dynamic nature of the FIFO counters, this register should be read until the
same value is returned twice.
5.8.16 TXTRG [7:0]: Transmit FIFO Trigger Level - Write-Only
An 8-bit value written to this register sets the TX FIFO trigger level from 0x00 (zero) to 0x40 (64). The TX FIFO
trigger level generates an interrupt whenever the data level in the transmit FIFO falls below this preset trigger
level.
5.8.17 RXCNT[7:0]: Receive FIFO Level Counter - Read-Only
Receive FIFO level byte count from 0x00 (zero) to 0x40 (64). It gives an indication of the number of characters
in the receive FIFO. The FIFO level byte count register is read only. The user can take advantage of the FIFO
level byte counter for faster data unloading from the receiver FIFO, which reduces CPU bandwidth
requirements. Please see the Application Note DAN119 on Exar’s website for a detailed discussion of FIFO
level counters. Due to the dynamic nature of the FIFO counters, this register should be read until the same
value is returned twice.
5.8.18 RXTRG[7:0]: Receive FIFO Trigger Level - Write-Only
An 8-bit value written to this register, sets the RX FIFO trigger level from 0x00 (zero) to 0x40 (64). The RX
FIFO trigger level generates an interrupt whenever the receive FIFO level rises to this preset trigger level.
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EXAR [ EXAR CORPORATION ]
