© 2014 Exar Corporation
XR21B1421
Enhanced 1-Ch Full-Speed
USB HID to UART Bridge
exar.com/XR21B1421
Rev 1B
1 / 50
General Description
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 program-
ming 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 pro-
grammed 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 opti-
mize 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 package
has a separate V
IO
supply voltage input for the Modem / GPIO pins. The
28 pin package has V
IO
internally tied to 3.3V.
FEATURES
• ±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
• 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
APPLICATIONS
• Portable Medical Diagnostics
• Blood Glucose Meters
• Health and Fitness Wellness Monitors
• Uninterruptible Power Supplies
• Energy Management Consoles
Ordering Information – page 50
Block Diagram
USB Slave Interface
512-byte
TX FIFO
GPIOs/
Modem IO
Internal
Oscillator
(48MHz)
512-byte
RX FIFO
UART
Fractional
BRG
Internal
Status and
Control
Registers
OTP
USB
Descriptors
LDO 3V3
USB
TX
RX
