XR21B1420 Datasheet, PDF (8/60 Pages) Exar Corporation – Enhanced 1-Ch Full-Speed USB UART

XR21B1420
Table 1: USB String Descriptor Defaults
Descriptor
Value
Exar USB Vendor ID 0x04E2
Exar USB Product ID 0x1420
Manufacturer String Exar Corp.
Product String
Exar USB
UART
Device Driver
The XR21B1420 device may be used with either a standard CDC-ACM driver or an Exar supplied custom driver. The CDC-
ACM driver is native to the Operating System. In Linux, the CDC-ACM driver will automatically load for the XR21B1420, but
in the Windows OS, an extra INF file is required to install the CDC-ACM driver. The custom drivers must also be installed,
although for Windows 7 OS and newer with Internet access and Windows updates set to automatic, the latest Windows-
Certified (WHQL/HCK) driver will be downloaded and installed automatically.
CDC-ACM Driver
Because the CDC-ACM driver has no ability to access the XR21B1420 internal device registers, the device is initialized to
certain hardware defaults. By default the XR21B1420 enables hardware RTS/CTS flow control, GPIO7 is set as active high
auto RS-485 half-duplex control, and RI, CD and DSR pins are enabled to be interrupt sensitive. These settings are listed in
Table 2. Additionally, the low latency threshold in CDC mode is automatically set to 40,960 bps. Refer to “RX FIFO Low
Latency” on page 12. This threshold may be modified in the OTP CDC_ACM_BAUD_THRESH locations.
Table 2: XR21B1420 Register Defaults with CDC-ACM Driver
Register
Value
FLOW_CONTROL
GPIO_MODE
0x0001
0x0339
GPIO_DIRECTION 0x0028
GPIO_INT_MASK 0x03F0
Notes
Hardware flow control
RTS / CTS flow control, GPIO7 is used as RS-485 half-duplex enable
(RS485) with active high polarity. GPIO6 is a GPIO input, RXT and TXT
remain enabled.
DTR / RTS are configured as outputs (TXT, RXT, CLK and RS485 are
also special function outputs). All other GPIOs are configured as
inputs.
RI, CD and DSR are interrupt sensitive, i.e. can cause a USB interrupt
to be generated.
Custom Exar Driver
Custom Windows and Linux drivers are available from Exar. The custom driver allows software applications to make full use
of the XR21B1420 register set and features.
Note that a custom driver must always immediately set CUSTOM_DRIVER bit-0 = 1. Once CUSTOM_DRIVER bit-0 is set,
the custom driver can use standard CDC-ACM commands without the XR21B1420 automatically changing to the settings in
the Table 2.
© 2014 Exar Corporation
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exar.com/XR21B1420
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