TL16C754 Datasheet, PDF (9/39 Pages) Texas Instruments – QUAD UART WITH 64-BYTE FIFO

TL16C754
QUAD UART WITH 64-BYTE FIFO
SLLS279A – OCTOBER 1998 – REVISED OCTOBER 1999
functional description (continued)
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. 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.
It is assumed that software flow control and hardware flow control will never be enabled simultaneously. Figure 4
shows a software flow control example.
UART 1
UART 2
Transmit
FIFO
Receive
FIFO
Parallel to Serial
Serial to Parallel
Data
Xoff – Xon – Xoff
Serial to Parallel
Parallel to Serial
Xon-1 Word
Xon-1 Word
Xon-2 Word
Xon-2 Word
Xoff-1 Word
Xoff-1 Word
Xoff-1 Word
Compare
Programmed
Xon–Xoff
Characters
Xoff-2 Word
Figure 4. Software Flow Control Example
software flow control example
Assumptions: UART1 is transmitting a large text file to UART2. Both UARTs are using software flow control with
single character Xoff (0F) and Xon (0D) tokens. Both have Xoff threshold (TCR [3:0]=F) set to 60 and Xon
threshold (TCR[7:4]=8) set to 32. Both have the interrupt receive threshold (TLR[7:4]=D) set to 52.
UART1 begins transmission and sends 52 characters, at which point UART2 will generate an interrupt to its
processor to service the RCV FIFO, but assume the interrupt latency is fairly long. UART1 will continue sending
characters until a total of 60 characters have been sent. At this time UART2 will transmit a 0F to UART1,
informing UART1 to halt transmission. UART1 will likely send the 61^st character while UART2 is sending the
Xoff character. Now UART2 is serviced and the processor reads enough data out of the RCV FIFO that the level
drops to 32. UART2 will now send a 0D to UART1, informing UART1 to resume transmission.
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