XR21V1412_10 Datasheet, PDF (12/30 Pages) Exar Corporation – 2-CH FULL-SPEED USB UART

XR21V1412
2-CH FULL-SPEED USB UART
REV. 1.1.0
1.5.7.1 Receiver
If an address match occurs in either flow control mode 3 or 4, the address byte will not be loaded into the RX
FIFO, but all subsequent data bytes will be loaded into the RX FIFO. The UART Receiver will automatically be
disabled when an address byte is received that does not match the values in the XON_CHAR or XOFF_CHAR
register.
1.5.7.2 Transmitter
In flow control mode 3, the UART transmitter is always enabled, irrespective of the Rx address match. In flow
control mode 4, the UART transmitter will only be enabled if there is an Rx address match.
1.5.8 Programmable Turn-Around Delay
By default, the GPIO5/RTS# pin will be de-asserted immediately after the stop bit of the last byte has been
shifted. However, this may not be ideal for systems where the signal needs to propagate over long cables.
Therefore, the de-assertion of GPIO5/RTS# pin can be delayed from 1 to 15 bit times via the
XCVR_EN_DELAY register to allow for the data to reach distant UARTs.
1.5.9 Half-Duplex Mode
Half-duplex mode is enabled when FLOW_CONTROL[3] = 1. In this mode, the UART will ignore any data on
the RX input when the UART is transmitting data.
1.5.10 RX FIFO Latency
In normal operation all bulk-in transfers will be of maxPacketSize (64) bytes to improve throughput and to
minimize USB host processing. However, in cases where the baud rate is low this may increase latency
unacceptably. To compensate, the V1412 device has a low latency mode in which received data bytes will be
immediately forwarded at the next BULK_IN packet. The Low Latency mode will be automatically set from a
CDC_ACM_IF_SET_LINE_CODING command whenever the baud rate is less than 46921 bps or alternately a
custom driver may set the RX_FIFO_LOW_LATENCY register bit to force RX data to be delivered without
delay.
1.5.11 Remote Wakeup
If the V1412 device has entered the Suspend state, the GPIO0/RI# pin of the Channel A UART can be used to
request that the host exit the Suspend state. A high to low transition on this pin will cause the device to signal a
remote wakeup request to the host via a custom driver. Note that the standard CDC-ACM driver does not
support this feature. In order for the remote wakeup to work, several things must be properly configured. First,
the GPIO0/RI# pin must be configured as an input. Additionally, the V1412 device must have the remote
wakeup feature support indicated in the USB attributes - See “Section 1.3, I2C Interface” on page 8. Lastly,
the software driver must inform the USB host that the peripheral device supports the remote wake-up feature.
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