Philips Semiconductors
SC16C754B
5 V, 3.3 V and 2.5 V quad UART, 5 Mbit/s (max.) with 64-byte FIFOs
6.3.2 TX
Xoff1/Xoff2 character is transmitted when the RX FIFO has passed the HALT trigger level
programmed in TCR[3:0].
Xon1/Xon2 character is transmitted when the RX FIFO reaches the RESUME trigger level
programmed in TCR[7:4].
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 signi铿乧ant bits of Xoff1/Xoff2 and Xon1/Xon2 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.)
It is assumed that software 铿俹w control and hardware 铿俹w control will never be enabled
simultaneously.
Figure 8
shows an example of software 铿俹w control.
6.3.3 Software 铿俹w control example
UART1
UART2
TRANSMIT FIFO
RECEIVE FIFO
PARALLEL-TO-SERIAL
data
SERIAL-TO-PARALLEL
Xoff鈥揦on鈥揦off
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-2 WORD
compare
programmed
Xon-Xoff
characters
Xoff-2 WORD
002aaa229
Fig 8. Software 铿俹w control example
6.3.3.1
Assumptions
UART1 is transmitting a large text 铿乴e to UART2. Both UARTs are using software 铿俹w
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.
9397 750 14668
漏 Koninklijke Philips Electronics N.V. 2005. All rights reserved.
Product data sheet
Rev. 02 鈥?13 June 2005
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