XR21B1411IL-0A-EB Datasheet, PDF (9/30 Pages) Exar Corporation – ENHANCED 1-CH FULL-SPEED USB UART

XR21B1411
REV. 1.2.0
ENHANCED 1-CH FULL-SPEED USB UART
multidrop applications. See ”Section 3.1.1.7, FLOW_CONTROL Register Description (Read / Write)” on
page 15. If enabled, the transmitter automatically asserts the GPIO5/RTS#/RS485 output prior to sending the
data. By default, it de-asserts GPIO5/RTS#/RS485 following the last stop bit of the last character that has
been transmitted, but the RS485_DELAY register may be used to delay the deassertion. The polarity of the
GPIO5/RTS#/RS485 signal may also be modified using the GPIO_MODE register bit 3.
1.3.7 Multidrop mode with address matching
The B1411 device has two address matching modes which are also set by the flow mode control register using
modes 3 and 4. These modes are intended for a multi-drop network application. In these modes, the
XON_CHAR register holds a unicast address and the XOFF_CHAR holds a multicast address. A unicast
address is used by a transmitting master to broadcast an address to all attached slave devices that is intended
for only one slave device. A multicast address is used to broadcast an address intended for more than one
recipient device. Each attached slave device should have a unique unicast address value stored in the
XON_CHAR register, while multiple slaves may have the same multicast adderss stored in the XOFF_CHAR
register. An address match occurs when an address byte (9th bit or parity bit is ’1’) is received that matches
the value stored in either the XON_CHAR or XOFF_CHAR register.
1.3.7.1
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.
1.3.7.2
Transmitter
In flow control mode 3, the UART transmitter will transmit irrespective of the Rx address match. In flow control
mode 4, the UART will only transmit following an Rx address match.
1.3.8 Programmable Turn-Around Delay
By default, the GPIO5/RTS#/RS485 pin will be de-asserted immediately after the stop bit of the last byte has
been shifted when auto RS-485 half-duplex control is enabled by the GPIO_MODE register. However, this
may not be ideal for systems where the signal needs to propagate over long cables. Therefore, the de-
assertion of GPIO5/RTS#/RS485 pin can be delayed from 1 to 15 bit times via the RS485_DELAY register to
allow for the data to reach distant UARTs.
1.3.9 Half-Duplex Mode
Half-duplex mode is enabled when FLOW_CONTROL[3] = 1. In half duplex mode, the UART will ignore any
data on the RX input when the UART is transmitting data.
1.3.10 USB Suspend
All USB peripheral devices must support the USB suspend mode. Per USB standard, the B1411 device will
begin to enter the Suspend state if it does not detect any activity (including Start of Frame or SOF packets) on
its USB data lines for 3 ms. The device must then reduce power consumption from VBUS power within the
next 7 ms to the allowed limit of 2.5 mA for the suspended state. Note that in this context, the "device" is all
circuitry (including the B1411) that draws power from the host VBUS.
1.3.11 Remote Wakeup
When the B1411 is suspended, the GPIO0/RI#/RWK# pin 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 B1411 device must have the remote wakeup feature support
indicated in the USB attributes - See ”Section 3.2.1.11, USB_ATTRIBUTES (Read / Write OTP)” on
page 25. Lastly, the host must detect the B1411 support for remote wake up and enable this feature. Note
that per USB standard, any remote wakeup signaling to the host will be suppressed for the first 5 ms after the
device enters the suspend state.
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