XR21B1424_17 Datasheet, PDF (17/64 Pages) Exar Corporation – Enhanced 4-Ch Full-Speed USB UART

XR21B1424
Local UART
UARTA
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Transmitter
Auto CTS
Monitor
RXA
RTSA#
TXA
CTSA#
RTSA#
1
ON
6
OFF
TXB
CTSB#
RXB
RTSB#
9
Remote UART
UARTB
Transmitter
Auto CTS
Monitor
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
ON
CTSB#
2
ON
7
OFF
10
ON
3
TXB
4
RXA
8
11
5
1) COM port opened, RX FIFO empty, RTSA# output is asserted
2) Signal propagated to CTSB# input
3) Data bytes enter TX FIFO, begin transmitting on TXB
4) Data propagates to Receiving device RXA
5) RX FIFO reaches threshold
6) RTSA# de-asserts
7) Signal propagates to CTSB# input
8) Transmission stops on TXB
9) USB Bulk-In empties RX FIFO below threshold, RTSA# is asserted
10) Signal propagated to CTSB# input
11) Data bytes resume transmitting on TXB
Figure 6: Auto RTS and CTS Flow Control Operation
Automatic DTR/DSR Hardware Flow Control
Auto DTR/DSR hardware flow control behaves the same as the Auto RTS/CTS hardware flow control described above
except that it uses the DTR# and DSR# signals. GPIO2 and GPIO3 become DSR# and DTR#, respectively, when the GPI-
O_MODE register is configured for DTR/DSR hardware flow control.
Automatic XON/XOFF Software Flow Control
When software flow control is enabled, the XR21B1424 compares the receive data characters with the programmed XON or
XOFF characters. If the received character matches the programmed XOFF character, the XR21B1424 will halt transmis-
sion as soon as the current character has completed transmission. Data transmission is resumed when a received charac-
ter matches the XON character.
In the receive data direction, the XOFF character will be sent when there are 450 bytes in the receive FIFO. When there are
again less than 450 bytes in the RX FIFO, the XON character will be sent. This threshold may be changed using the RX_-
THRESHOLD register.
Software flow control is enabled / disabled by the FLOW_CONTROL register. Additionally, the XON_CHAR and
XOFF_CHAR registers may be used to configure the start (XON) and stop (XOFF) characters.
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exar.com/XR21B1424
Rev 1C