XR21B1421 Datasheet, PDF (10/50 Pages) Exar Corporation – Enhanced 1-Ch Full-Speed

XR21B1421
Functional Description
USB Interface
The USB interface of the XR21B1421 is compliant with the USB 2.0 Full-Speed Specifications.
The XR21B1421 uses the following set of parameters:
• 1 Control Endpoint
• Endpoint 0 as outlined in the USB specifications
• 1 Configuration is supported
• 1 HID Interface for the UART channel
• Interrupt-in endpoint for UART receive data
Interrupt-out endpoint for UART transmit data
USB Vendor and Product IDs
Exar’s USB Vendor ID is 0x04E2. This is the default Vendor ID that is used for the XR21B1421. Customers may obtain their
own Vendor ID from USB.org. The default USB Product ID for the XR21B1421 is 0x1421. Upon request, Exar will provide up
to 8 PID values for use with Exar’s VID. The VID and PID may be modified by the SET_USB_CONFIG HID report. However,
the Exar VID and PID values will always be returned in the GET_CHIP_ID report (0x4F) irrespective of any changes to the
VID and PID using the SET_USB_CONFIG report. Refer to Table 1.
USB Suspend
All USB peripheral devices must support the USB suspend mode. Per USB standard, the XR21B1421 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 XR21B1421) that
draws power from the host VBUS.
Remote Wakeup
If the XR21B1421 device has been placed into suspend state by the USB host, the RI#/RWK# and / or RX pins may be
used to request that the host exit the suspended state. By default the XR21B1421 device reports in its device attributes that
it does not support remote wakeup. If remote wakeup is enabled via the OTP, the device will respond to a high to low transi-
tion on the RI#/RWK# pin if it is configured as an input, and remotely wake up the USB host. Note that Windows OS does
not support remote wakeup for HID class devices that are not identified as either keyboard or mouse. RX pin remote
wakeup is disabled by default and must also be separately enabled.
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 defaults manufacturer, and product strings for the XR21B1421 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. However, note that USB.org
requires any serial number string for a specific VID/PID combination to be unique for each device.
© 2014 Exar Corporation
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