XR22804 Datasheet, PDF (15/47 Pages) Exar Corporation – Hi-Speed USB to 10/100 Ethernet Bridge with 4 UARTs

XR22804
the RX FIFO, the XR22804 may NAK the bulk-in request indicating that data is not ready to transfer at that time. However, if
there is less than 512 bytes in the RX FIFO and no data has been received for more than 3 character times, the XR22804
will acknowledge the bulk-in request and transfer any data in the RX FIFO to the USB host.
In some cases, especially when the baud rate is low, this increases latency unacceptably. The XR22804 has a low latency
register bit that will cause the XR22804 to immediately transfer any received data in the RX FIFO to the USB host, i.e. it will
not wait for 3 character times. The custom driver may automatically set the RX_CONTROL register to force the XR22804 to
be in the low latency mode, or the user may manually set this bit. With the CDC-ACM driver, the low latency mode is auto-
matically set whenever the baud rate is set to a value of less than 46921 bps using the CDC_ACM_IF_SET_LINE_COD-
ING command.
GPIO
There can be up to 8 GPIO pins in the XR22804 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 17.
Automatic RTS / CTS hardware flow control
E[n]/RTS#/RS485/G[n] and E[n]/CTS#/G[n] of each 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
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