USB serial driver (Usbser.sys)

Use the Microsoft-provided USB driver (Usbser.sys) for your communications and communications device class (CDC) control devices. Use the drivers included with Windows whenever possible.

Usbser.sys is implemented using the Kernel-Mode Driver Framework. The driver supports Plug and Play, and power management features like USB Selective Suspend.

Universal Windows Platform (UWP) applications can use the APIs provided by the Windows.Devices.SerialCommunication namespace, allowing apps to talk to CDC devices.

Usbser.sys installation

Load the Microsoft-provided in-box driver (Usbser.sys) for your communications and CDC control device.

Usbser.inf is located in the %Systemroot%\INF directory. This setup information (INF) file loads Usbser.sys as the functional device object (FDO) in the device stack. If your device belongs to the communications and CDC control device class, Usbser.sys loads automatically. You don't need to write your own INF file to reference the driver. Windows loads the driver based on a compatible ID match, similar to other USB device class drivers included in Windows.

USB\Class_02

USB\Class_02&SubClass_02

To load Usbser.sys automatically, set the class code to 02 and subclass code to 02 in the Device Descriptor. With this approach, you don't need to distribute INF files for your device because the system uses Usbser.inf. For more information, see Class definitions for Communication Devices 1.2.

If your device specifies class code 02 but a subclass code other than 02, Usbser.sys doesn't load automatically. The Plug and Play manager tries to find a driver. If Windows doesn't find a suitable driver, the device might not have a driver loaded. You might need to load your own driver or write an INF file that references another in-box driver.

If your device specifies class and subclass codes of 02, and you want to load another driver instead of Usbser.sys, write an INF file. In the INF file, specify the hardware ID of the device and the driver to install.

Configure selective suspend for Usbser.sys

Usbser.sys supports USB Selective Suspend. This driver lets the attached USB-to-serial device enter a low power state when not in use, while the system stays in the S0 state. When communication with the device resumes, the device leaves the suspend state and resumes the working state. The feature is disabled by default, but can be enabled and configured by setting the IdleUsbSelectiveSuspendPolicy entry under this registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB\<hardware id>\<instance id>\Device Parameters

To configure power management features of Usbser.sys, set the IdleUsbSelectiveSuspendPolicy parameter to:

Add that entry in one of two ways:

1. Write an INF that references the install INF and add the registry entry in the HW.AddReg section.

2. Describe the registry entry in an extended properties OS feature descriptor. Add a custom property section that sets the bPropertyName field to a Unicode string: IdleUsbSelectiveSuspendPolicy. Set the wPropertyNameLength to 62 bytes. Set the bPropertyData field to 0x00000001 or 0x00000000. The property values store as little-endian 32-bit integers.

   For more information, see Microsoft OS Descriptors.

Develop Windows applications for a USB CDC device

A Windows app sends requests to Usbser.sys by using the Windows.Devices.SerialCommunication namespace. The namespace defines Windows Runtime classes that communicate with a USB CDC device through a serial port or an abstraction of a serial port. The classes let you discover serial devices, read and write data, and control serial-specific properties for flow control, such as setting the baud rate and signal states.

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