XR16L784_08 Datasheet, PDF (15/51 Pages) Exar Corporation – HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART

XR16L784
REV. 1.2.3
HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART
2.13 Auto RS485 Half-duplex Control
The auto RS485 half-duplex direction control changes the behavior of the transmitter when enabled by FCTR
bit-5. It asserts RTS# or DTR# (LOW) after a specified delay indicated in MSR[7:4] following the last stop bit of
the last character that has been transmitted. This helps in turning around the transceiver to receive the remote
station’s response. The delay optimizes the time needed for the last transmission to reach the farthest station
on a long cable network before switching off the line driver. This delay prevents undesirable line signal
disturbance that causes signal degradation. When the host is ready to transmit next polling data packet again,
it only has to load data bytes to the transmit FIFO. The transmitter automatically de-asserts RTS# or DTR#
output (HIGH) prior to sending the data. The auto RS485 half-duplex direction control also changes the
transmitter empty interrupt to TSR empty instead of THR empty.
2.14 Infrared Mode
Each UART in the 784 includes the infrared encoder and decoder compatible to the IrDA (Infrared Data
Association) version 1.0. The input pin ENIR conveniently activates all 8 UART channels to start up in the
infrared mode. Note that the ENIR pin is sampled when the RST# input is de-asserted. This global control pin
enables the MCR bit-6 function in every UART channel register. After power up or a reset, the software can
overwrite MCR bit-6 if so desired. ENIR and MCR bit-6 also disable the receiver while the transmitter is
sending data. This prevents echoed data from reaching the receiver. The global activation ENIR pin prevents
the infrared emitter from turning on and drawing large amount of current while the system is starting up. When
the infrared feature is enabled, the transmit data outputs, TX[3:0], would idle at logic zero level. Likewise, the
RX [3:0] inputs assume an idle level of logic zero.
The infrared encoder sends out a 3/16 of a bit wide HIGH-pulse for each “0” bit in the transmit data stream.
This signal encoding reduces the on-time of the infrared LED, hence reduces the power consumption. See
Figure 11.
The infrared decoder receives the input pulse from the infrared sensing diode on RX pin. Each time it senses a
light pulse, it returns a logic zero to the data bit stream. The decoder also accepts (when FCTR bit-4 = 1) an
inverted IR-encoded input signal. This option supports active low instead of normal active high pulse from
some infrared modules on the market.
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