Datasheet

WR THR
TX
DTRx
1 Baud Time Controlled by DLY[3:0]
TL16CP754C, TL16C754C
www.ti.com SLLS644G –DECEMBER 2007– REVISED MAY 2011
Table 24. LOOP and RCVEN Functionality
LOOP MODE RCVEN AFR MODE DESCRIPTION
Receive threshold, timeout, and error detection interrupts available.
AFR = 10 RS-232
Data stored in receive FIFO.
Receive threshold, timeout, and error detection interrupts available.
RCVEN = 1 AFR = 14 RS-485
Data stored in receive FIFO.
LOOP mode off,
Receive threshold, timeout, and error detection interrupts available.
AFR = 12 IrDA
MCR4 = 0,
Data stored in receive FIFO.
RX, TX active
Receive threshold and error detection interrupts available.
AFR = 00 RS-232
Data stored in receive FIFO.
RCVEN = 0
AFR = 04 RS-485 No data stored in receive FIFO, hence no interrupts available.
AFR = 02 IrDA No data stored in receive FIFO, hence no interrupts available.
Receive threshold, timeout, and error detection interrupts available.
AFR = 10 RS-232
Data stored in receive FIFO.
Receive threshold, timeout, and error detection interrupts available.
RCVEN = 1 AFR = 14 RS-485
Data stored in receive FIFO.
Receive threshold, timeout, and error detection interrupts available.
AFR = 12 IrDA
LOOP mode on,
Data stored in receive FIFO.
MCR4 = 1,
Receive threshold and error detection interrupts available.
RX, TX inactive
AFR = 00 RS-232
Data stored in receive FIFO.
Receive threshold and error detection interrupts available.
RCVEN = 0 AFR = 04 RS-485
Data stored in receive FIFO.
Receive threshold and error detection interrupts available.
AFR = 02 IrDA
Data stored in receive FIFO.
RS-485 Mode
The RS-485 mode is intended to simplify the interface between the UART channel and an RS-485 driver or
transceiver. When enabled by setting 485EN, the DTRx output goes high one bit time before the first start bit of
the first data byte being sent, and remains high as long as there is pending data in the transmitter shift register
(TSR) or transmitter holding register (THR, xmt fifo). Once both are empty (after the last stop bit of the last data
byte), the DTRx output stays high for a programmable delay of 0 to 15 bit times, as set by DLY[3:0]. This helps
preserve data integrity over long signal lines. This is illustrated in the following.
Often RS-485 packets are relatively short and the entire packet can fit within the 64 byte xmt fifo. In this case, it
goes empty when the TSR goes empty. But in cases where a larger block needs to be sent, it is advantageous to
reload the xmt fifo as soon as it is depleted. Otherwise, the transmission stalls while waiting for the xmt fifo to be
reloaded, which varies with processor load. In this case, it is best to also set 485LG (large block) which causes
the transmit interrupt to occur wither when the THR becomes empty (if the xmt fifo level was not above the
threshold), or when the xmt fifo threshold is crossed. The reloading of the xmt fifo occurs while some data is
being shifted out, eliminating fifo underrun. If desired, when the last bytes of a current transmission are being
loaded in the xmt fifo, 485LG can be cleared before the load and the transmit interrupt occurs on the TSR going
empty.
A. Waveforms are not shown to scale, as the WR THR pulses typically are less than 100 ns, where the TX waveform
varies with baud rate but is typically in the microsecond range.
Figure 22. DTRx and Transmit Data Relationship
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