XR21B1422IL40-0A Datasheet, PDF (9/60 Pages) Exar Corporation – Enhanced 2-Ch Full-Speed USB UART

XR21B1422
Functional Description
USB Interface
The USB interface of the XR21B1422 is compliant with the USB 2.0 Full-Speed Specifications.
The XR21B1422 uses the following set of parameters:
• 1 Control Endpoint
• Endpoint 0 as outlined in the USB specifications
• 1 Configuration is supported
• 1 Interface for each UART channel
• Bulk-in and bulk-out endpoints
• Interrupt-in endpoint for notifications
USB Vendor and Product IDs
Exar’s USB Vendor ID is 0x04E2. This is the default Vendor ID that is used for the XR21B1422. Customers may obtain their
own Vendor ID from USB.org. The default USB Product ID for the XR21B1422 is 0x1422. Upon request, Exar will provide up
to 8 PID values for use with Exar’s VID. The VID and PID can be changed using the VID and PID fields. Refer to Table 1.
USB Suspend
All USB peripheral devices must support the USB suspend mode. Per USB standard, the XR21B1422 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 peripheral 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 XR21B1422) that
draws power from the host VBUS.
Remote Wakeup
If the XR21B1422 device has been placed into the suspend state by the USB host, a high to low transition on the RI#/RWK#
pins can be used to request that the host exit the suspended state. By default the XR21B1422 device reports in its USB
device attributes that it supports remote wakeup. The RI#/RWK# pins of each UART channel are enabled for remote
wakeup signaling if their default configuration as an input pin has not been changed. The RI#/RWK# pins from each UART
channel are logically ANDed, such that a logic ’0’ on any of the two pins will prevent the remote wakeup signaling. Addition-
ally, the RX pins of each UART channel may also be enabled via OTP to support remote wakeup. Again all RX pins that are
enabled to support remote wakeup signaling are also logically ANDed. Note that the CDC driver does not support remote
wakeup.
USB Strings
USB specifies three character string descriptors that are provided to the USB host during enumeration in string descriptors:
the manufacturer, product and serial strings. The default manufacturer and product strings for the XR21B1422 device are
"Exar Corp." and, "Exar USB UART", respectively. The serial number string is a unique alpha-numeric string programmed
into the device at the factory. All character strings use Unicode UTF-16LE format by default, but the Unicode language ID
may be changed for the manufacturer and product strings. The default character string language ID is US English. If the lan-
guage ID is modified via OTP, the serial number string should also be modified accordingly. To ensure unique serial number
strings, it is recommended that the factory pre-programmed serial number string be used.
© 2014 Exar Corporation
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