TL16C752B
3.3-V DUAL UART WITH 64-BYTE FIFO
SLLS405A – DECEMBER 1999 – REVISED AUGUST 2000
functional description (continued)
TX
Xoff1/2 characters are transmitted when the RX FIFO has passed the HALT trigger level programmed in
TCR[3:0].
Xon1/2 characters are transmitted when the RX FIFO reaches the RESUME trigger level programmed in
TCR[7:4].
An important note here is that if, after an xoff character has been sent and software flow control is disabled, the
UART will transmit Xon characters automatically to enable normal transmission to proceed. A feature of the
TL16C752B UART design is that if the software flow combination (EFR[3:0]) changes after an Xoff has been
sent, theoriginallyprogrammedXonisautomaticallysent. IftheRXFIFOisstillabovethetriggerlevel, thenewly
programmed Xoff1/2 will be transmitted.
The transmission of Xoff/Xon(s) follows the exact same protocol as transmission of an ordinary byte from the
FIFO. This means that even if the word length is set to be 5, 6, or 7 characters then the 5, 6, or 7 least significant
bits of Xoff1,2/Xon1,2 will be transmitted. (Note that the transmission of 5, 6, or 7 bits of a character is seldom
done, but this functionality is included to maintain compatibility with earlier designs.)
Itisassumedthatsoftwareflowcontrolandhardwareflowcontrolwillneverbeenabledsimultaneously. Figure 4
shows an example of software flow control.
UART 1
UART 2
Transmit
FIFO
Receive
FIFO
Data
Parallel to Serial
Serial to Parallel
Xon-1 Word
Serial to Parallel
Parallel to Serial
Xon-1 Word
Xoff – Xon – Xoff
Xon-2 Word
Xon-2 Word
Xoff-1 Word
Xoff-1 Word
Compare
Programmed
Xon–Xoff
Xoff-1 Word
Xoff-2 Word
Characters
Figure 4. Software Flow Control Example
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