TL16C752B-EP

3.3-V DUAL UART WITH 64-BYTE FIFO

SGLS153 – FEBRUARY 2003

8

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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 transmits Xon characters automatically to enable normal transmission to proceed. A feature of the

TL16C752B-EP 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 is 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.)

It is assumed that software flow control and hardware flow control are never enabled simultaneously. Figure 4

shows an example of software flow control.

UART 1

Parallel to Serial

Serial to Parallel

Xon-1 Word

Xon-2 Word

Xoff-1 Word

Xoff-1 Word

Transmit

FIFO

Serial to Parallel

Parallel to Serial

Xon-1 Word

Xoff-1 Word

Xoff-2 Word

Receive

FIFO

Data

Xoff – Xon – Xoff

Compare

Programmed

Xon–Xoff

Characters

UART 2

Xon-2 Word

Figure 4. Software Flow Control Example