XR21B1420 Datasheet, PDF (15/60 Pages) Exar Corporation – Enhanced 1-Ch Full-Speed USB UART

XR21B1420
IR Mode
The XR21B1420 supports IR mode at a maximum baud rate of 2.5 Mbaud with transmit pulses of 3/16th or 4/16th of a bit
period and centered in the bit period. Receive data may be inverted to conform to some manufacturer’s non-standard
devices. IR mode is disabled by default but may be enabled by the IR_MODE register.
USB_STAT Pins
The XR21B1420 has two USB_STAT output pins that may be used to indicate 3 different statuses in either positive or nega-
tive polarity. The SUSPEND status indicates that the XR21B1420 device has been placed into a suspended state by the
USB host. This output can then be used by external circuitry, for example, to power down devices in order to meet USB
requirements for suspend mode. The LOW_POWER status is similar to the SUSPEND status, but LOW_POWER is also
asserted for high power devices (any device that consumes more than 100 mA of VBUS power from the USB host), before
the device is configured during enumeration by the USB host. For low power devices (devices that consume 100 mA or less
of VBUS power), SUSPEND and LOW_POWER status outputs are functionally the same. Lastly, the BUS_RESET output
status is asserted any time the XR21B1420 device is being reset by the USB host. This status output could be used, for
example, by an FPGA or other logic device to synchronize this external logic with the XR21B1420 device.
Suspend Mode Settings
The USE_SUSPEND bit controls the GPIO pins when the XR21B1420 device is suspended by the USB host. If USE_SUS-
PEND is cleared to ’0,’ the GPIO pins retain their output states when the device is suspended. When USE_SUSPEND is
set to ’1’, the GPIO pin’s behavior is defined by the SUSPEND_STATE and SUSPEND_MODE registers, with the following
exceptions: GPIO0/CLK when configured as an output clock will always be driven low, i.e the clock output will stop, and
GPIO1/RTS#/RS485 or GPIO3/RS485 when configured as auto. RS-485 half-duplex enable will always be de-asserted.
Note that USE_SUSPEND does not affect the UART RX and TX pins. During suspend state, RX and TX will always idle to a
logic ’1’ state.
The SUSPEND_STATE field will set or clear the GPIO pins and the SUSPEND_MODE field will configure GPIO outputs as
either open drain or push-pull outputs. SUSPEND_STATE and SUSPEND_MODE may be configured through registers or
OTP. As opposed to OTP configuration, register configurations are not retained if the power is lost or the bus is reset.
TXT and RXT Pins
The Transmit toggle and Receive toggle pins "toggle" at a rate of approximately 10 Hz whenever the UART transmit and
receive pins (respectively) are active.
OTP
The OTP is an on-chip non-volatile memory, that is incrementally one-time programmable via the USB interface. Some bits
are pre-programmed at the factory and caution must be taken not to program any locations except those user defined
addresses given in this data sheet. Once a specific portion of the OTP is programmed, the PROG bit for that section of the
OTP must be set and further changes to that section will not be allowed.
© 2014 Exar Corporation
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exar.com/XR21B1420
Rev 1A