TL16CP754C, TL16CM754C, TL16C754C

SLLS644E – DECEMBER 2007 – REVISED MAY 2010

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Table 1. Software Flow Control Options EFR[3:0]

BIT 3

BIT 2

BIT 1

BIT 0

Tx, Rx SOFTWARE FLOW CONTROLS

0

0

X

X

No transmit flow control

1

0

X

X

Transmit Xon1, Xoff1

0

1

X

X

Transmit Xon2, Xoff2

1

1

X

X

Transmit Xon1, Xon2: Xoff1, Xoff2

X

X

0

0

No receive flow control

X

X

1

0

Receiver compares Xon1, Xoff1 X X 0 1

X

X

0

1

Receiver compares Xon2, Xoff2

Transmit Xon1, Xoff1

1

0

1

1

Receiver compares Xon1 or Xon2, Xoff1 or Xoff2

Transmit Xon2, Xoff2

0

1

1

1

Receiver compares Xon1 or Xon2, Xoff1 or Xoff2

Transmit Xon1, Xon2: Xoff1, Xoff2

1

1

1

1

Receiver compares Xon1 and Xon2: Xoff1 and Xoff2

No transmit flow control

0

0

1

1

Receiver compares Xon1 and Xon2: Xoff1 and Xoff2

When software flow control operation is enabled, the '754C compares incoming data with Xoff1/2 programmed

received, transmission is halted after completing transmission of the current character. Xoff character detection

also sets IIR[4] and causes INT to go high (if enabled via IER[5]).

To resume transmission an Xon1/2 character must be received (in certain cases Xon1 and Xon2 must be

received sequentially). When the correct Xon characters are received IIR[4] is cleared and the Xoff interrupt

disappears.

NOTE

If a parity, framing or break error occurs while receiving a software flow control character,

this character is treated as normal data and is written to the RCV FIFO.

Xoff1/2 characters are transmitted when the RX FIFO has passed the programmed trigger level TCR[3:0].

Xon1/2 characters are transmitted when the RX FIFO reaches the trigger level programmed via TCR[7:4].

NOTE

If, after an Xoff character has been sent, software flow control is disabled, the UART

transmits Xon characters automatically to enable normal transmission to proceed. A

feature of the '754C UART design is that if the software flow combination (EFR[3:0])

changes after an Xoff has been sent, the originally programmed Xon is automatically sent.

If the RX FIFO is still above the trigger level the newly programmed Xoff1/2 is 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 are 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 are never enabled simultaneously. Figure 4

shows a software flow control example.

(1)

When pairs of Xon/Xoff characters are programmed to occur sequentially, received Xon1/Xoff1 characters will be written to the Rx FIFO

if the subsequent character is not Xon2/Xoff2.

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