Enhanced 1-Ch Full-Speed USB HID to UART Bridge


The XR21B1421 is an enhanced Universal Asynchronous Receiver and Transmitter (UART) bridge to HID class USB interface. The USB interface is fully compliant to the USB 2.0 (Full-Speed) specification with 12 Mbps USB data transfer rate. The USB interface also supports USB suspend, resume and remote wakeup operations. The USB Vendor ID, Product ID, power mode, remote wakeup support, maximum power, and numerous other settings may be programmed in the no-chip OTP memory via the USB interface using either HID reports or Exar-supplied OTP programming tools.

The XR21B1421 includes an internal oscillator and does not require an external crystal/oscillator. Any UART baud rate up to 12 Mbps may be generated with this internal clock and the fractional baud rate generator. The UART pins for each port may also be configured as GPIO; direction, state, output driver type and input pull-up or pull-down resistors are programmed either through on chip OTP, or on the fly via memory mapped registers.

Large 512-byte TX and RX FIFOs prevent buffer overflow errors and optimize data throughput. Automatic half-duplex direction control and optional multi drop (9-bit) mode simplify both hardware and software in half-duplex RS-485 applications.

The XR21B1421 uses the native OS HID driver using HID reports. The HID reports perform a variety of functions, including device configuration, transmitting and receiving UART data, and programming of the on-chip OTP. Exar provides a .dll as an easy to use programming interface to the HID reports.

The XR21B1421 operates from a single 5V or 3.3V power supply and is available in a 24 or 28 pin QFN package. When powered with 5V input, the XR21B1421 supplies a regulated 3.3V output. The 24 pin packagehas a separate VIO supply voltage input for the Modem / GPIO pins. The 28 pin package has VIO internally tied to 3.3V.


For UART technical support or to obtain an IBIS model for this product, please email Exar's UART Technical Support group.

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  • ±15kV ESD on USBD+/USBD-
  • USB 2.0 Compliant, Full-Speed (12Mbps)
  • Unique pre-programmed USB serial number
  • Internally generated 48MHz core clock
  • Enhanced UART features
    • Baud rates up to 12 Mbps
    • Fractional Baud Rate Generator
    • 512-byte TX and 512-byte RX FIFOs
    • Auto Hardware / Software Flow Control
    • Multidrop and Half-Duplex Modes
    • Auto RS-485 Half-Duplex Control
    • Selectable GPIO or Modem I/O
  • Up to 10 GPIOs per channel
  • 5V tolerant GPIO inputs
  • Suspend state GPIO configuration
  • Configurable clock output
  • 24-pin or 28-pin QFN package
  • Industrial -40°C to +85°C Temperature Range


  • Portable Medical Diagnostics
  • Blood Glucose Meters
  • Health and Fitness Wellness Monitors
  • Uninterruptible Power Supplies
  • Energy Management Consoles

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Parts & Purchasing

Part Number Pkg Code RoHS Min Temp Max Temp Status Buy Now Order Samples
XR21B1421IL24-F QFN24 -40 85 Active
XR21B1421IL24TR-F QFN24 -40 85 Active
XR21B1421IL28-F QFN28 -40 85 Active
XR21B1421IL28TR-F QFN28 -40 85 Active
XR21B1421IL24-0A-EVB Board Active
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Part Status Legend
Active - the part is released for sale, standard product.
EOL (End of Life) - the part is no longer being manufactured, there may or may not be inventory still in stock.
CF (Contact Factory) - the part is still active but customers should check with the factory for availability. Longer lead-times may apply.
PRE (Pre-introduction) - the part has not been introduced or the part number is an early version available for sample only.
OBS (Obsolete) - the part is no longer being manufactured and may not be ordered.
NRND (Not Recommended for New Designs) - the part is not recommended for new designs.
Distribution Date Description File
01/31/2017 Addition of qualified 12 inch wafer processing line in Global Foundries, in addition to the currently qualified 8 inch wafer processing. Note: Reliability report will be available February 17, 2017. PCN 16-0731-01-01 12 inch Global Foundries-1033.pdf
01/27/2017 Qualification of alternate assembly subcon, ANST, China. PCN 16-1145-04-01 ANST-1033.pdf