XR22802 Datasheet, PDF (14/46 Pages) Exar Corporation – Hi-Speed USB to 10/100 Ethernet Bridge

XR22802
GPIO
There can be up to 8 GPIO pins in the XR22802 UART including the UART RX and TX pins. These GPIO pins may be con-
figured as UART GPIO, or for other UART functions, e.g. RTS# function, or be assigned to the EDGE. Refer to Enhanced
Dedicated GPIO Entity section on page 15.
Automatic RTS / CTS hardware flow control
E[n]/RTS#/RS485/G[n] and E[n]/CTS#/G[n] of the UART channel may be enabled as the RTS# and CTS# signals for Auto
RTS/CTS flow control when GPIO_MODE[2:0] = ’001’ and FLOW_CONTROL[2:0] = ’001’. Automatic RTS flow control is
used to prevent data overrun errors in local RX FIFO by de-asserting the RTS signal to the remote UART. When there is
room in the RX FIFO, the RTS pin will be re-asserted. Automatic CTS flow control is used to prevent data overrun to the
remote RX FIFO. The CTS# input is monitored to suspend / restart the local transmitter (see Figure 3):
Local UART
UARTA
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Transm itter
Auto CTS
M onitor
RXA
RTSA#
TXA
CTSA#
RTSA#
1
ON
6
OFF
TXB
CTSB#
RXB
RTSB#
9
Rem ote UART
UARTB
Transm itter
Auto CTS
M onitor
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
ON
CTSB#
2
ON
7
OFF
10
ON
3
TXB
4
RXA
8
11
5
1) CO M port opened, RX FIFO em pty, RTSA# output is asserted
2) Signal propagated to CTSB # input
3) Data bytes enter TX FIFO , begin transm itting 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) Transm ission stops on TXB
9) USB Bulk-In em pties RX FIFO below threshold, RTSA# is asserted
10) Signal propagated to CTSB # input
11) Data bytes resum e transm itting on TXB
Figure 3: Auto RTS / CTS Hardware Flow Control
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. For Auto hardware flow control, FLOW_CONTROL[2:0] = ’001’. E[n]/DTR#/
G[n] and E[n]/DSR#/G[n] become DTR# and DSR#, respectively, when GPIO_MODE[2:0] = ’010.’
Automatic XON / XOFF software flow control
When software flow control is enabled, the XR22802 compares the receive data characters with the programmed Xon or
Xoff characters. If the received character matches the programmed Xoff character, the XR22802 will halt transmission as
soon as the current character has completed transmission. Data transmission is resumed when a received character
matches the Xon character. Software flow control is enabled when FLOW_CONTROL[2:0] = ’010.’
© 2015 Exar Corporation
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Rev 1B