XR21B1424_17 Datasheet, PDF (11/64 Pages) Exar Corporation – Enhanced 4-Ch Full-Speed USB UART

XR21B1424
Table 1: USB String Descriptor Defaults
Descriptor
Exar USB Vendor ID
Exar USB Product ID
Manufacturer String
Product String
Value
0x04E2
0x1424
Exar Corp.
Exar USB UART
Device Driver
The XR21B1424 device may be used with either a standard CDC-ACM driver or a MaxLinear supplied custom driver. The
CDC-ACM driver is native to the Operating System. In Linux, the CDC-ACM driver will automatically load for the
XR21B1424, 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 XR21B1424 internal device registers, the device is initialized to
certain hardware defaults. By default the XR21B1424 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 16. This threshold may be modified in the OTP CDC_ACM_BAUD_THRESH locations.
Table 2: XR21B1424 Register Defaults with CDC-ACM Driver
Register
FLOW_CONTROL
GPIO_MODE
GPIO_DIRECTION
GPIO_INT_MASK
Value
0x0001
0x0339
0x0028
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 spe-
cial 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 MaxLinear Driver
Custom drivers for all major Operating Systems are available from MaxLinear. The custom driver allows software applica-
tions to make full use of the XR21B1424 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 XR21B1424 automatically changing to the settings in
the Table 2.
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exar.com/XR21B1424
Rev 1C