TL16C754

QUAD UART WITH 64-BYTE FIFO

SLLS279A – OCTOBER 1998 – REVISED OCTOBER 1999

8

POST OFFICE BOX 655303

functional description (continued)

NOTE:

It is possible that an Xon1 character is recognized as an Xon Any character, which could

cause an Xon2 character to be written to the RX FIFO.

–

Special Character [EFR(5)]: Incoming data is compared to Xoff2. Detection of the special character

sets the Xoff interrupt {IIR(4)] but does not halt transmission. The Xoff interrupt is cleared by a read of the

IIR. The special character is transferred to the RX FIFO.

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

Receiver compares Xon2, Xoff2

1

0

1

1

Transmit Xon1, Xoff1

Receiver compares Xon1 or Xon2, Xoff1 or Xoff2

0

1

1

1

Transmit Xon2, Xoff2

Receiver compares Xon1 or Xon2, Xoff1 or Xoff2

1

1

1

1

Transmit Xon1, Xon2: Xoff1, Xoff2

Receiver compares Xon1 and Xon2: Xoff1 and Xoff2

0

0

1

1

No transmit flow control

Receiver compares Xon1 and Xon2: Xoff1 and Xoff2

When software flow control operation is enabled, the TL16C754 will compare incoming data with Xoff1/2

character is 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 will be treated as normal data and will be 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].

An important note here is that if, after an Xoff character has been sent, software flow control is disabled, the

UART will transmit Xon characters automatically to enable normal transmission to proceed. A feature of the

TL16C754 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 will be transmitted.

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.