2.2 USB CDC-ACM Class Device

The MCP2222 CDC-ACM class interface provides USB-to-UART bridge functionality. When used with Windows-based host systems, COM port assignment behavior should be considered.

When the MCP2222 is connected to a PC for the first time, Windows searches for a suitable driver and creates a registry entry for the device. This registry entry stores information about the USB-to-UART adapter, the associated driver, and the assigned COM port. On subsequent connections to the same PC, Windows uses this registry entry to load the appropriate driver and assign the previously associated COM port number.

COM port numbering is a legacy Windows feature that remains supported for compatibility. Historically, COM ports were physical ports on the computer motherboard, each assigned a unique index number. With the introduction of USB-to-UART adapters, Windows extended the COM port model to support USB serial devices.

During enumeration, the MCP2222 can provide a USB serial number. If a serial number is provided, Windows stores it in the registry and uses it to assign the same COM port number to the device, regardless of which USB port is used. If no serial number is provided, Windows may assign different COM port numbers when the device is connected to different USB ports.

Both behaviors are useful depending on the application. Using a serial number provides consistent COM port assignment across USB ports. Omitting the serial number allows Windows to treat the same device as a separate instance when connected to different USB ports.

By default, the MCP2222 includes a factory-configured unique serial number that may be used during USB enumeration. The Microchip configuration utility can be used to enable, disable, or modify the serial number used for enumeration.

All USB-related settings described in this section are part of the device configuration and are stored in NVM.