MCP2221 datasheet(22/80 Pages) MICROCHIP

22 / 80 page

MCP2221

DS20005292B-page 22

 2015 Microchip Technology Inc.

2.0

USB ENUMERATION PROCESS

The MCP2221 implements the CDC class to support

the USB-to-UART protocol converter functionality.

Using USB-to-UART (CDC class) adapters with

personal computers running the Windows operating

system (OS) requires some consideration because of

the way the Windows OS responds to their connection.

When a USB-to-UART (CDC class) adapter is

connected to the USB port of the PC, Windows

searches for a driver. After a suitable driver is found,

the system creates an entry in the registry. The entry

stores relevant information about the USB-to-UART

adapter, its driver and the associated COM port.

The COM port and its number are legacy-type

adapters, which are still supported by Windows OS.

Historically, the COM ports in a computer are part of the

computer’s motherboard and are assigned a different

index number. With the advent of USB-to-UART

adapters, the Windows OS kept the COM port concept

and extended it to support the USB adapters.

Whenever a USB-to-UART adapter is first connected to

a PC, the system searches the registry for an entry that

is suitable for the connected adapter. If one is not

found, the system asks for a suitable driver. If this step

is completed, it creates a registry entry, and assigns a

COM port number as well. Then, whenever the

USB-to-UART adapter is connected to that PC, the

system checks the registry entry, loads the specified

driver and assigns the given COM port number (as

found in the registry entry).

During the enumeration process, the device can

specify a serial number. If it does, this number is stored

in the registry entry and it is used to assign the same

COM port number to the adapter in question, no matter

which USB port the adapter is connected to.

USB-to-UART adapters have the option to not present

a serial number during USB enumeration. In this case,

the operating system would not be able to differentiate

between two identical devices, if neither is providing its

serial number. Each time one of these two devices

(with no serial number provided during enumeration) is

connected to the same USB port, they will have the

same COM port number assigned.

Both functionalities (with or without serial numbers) are

very useful for different applications.

When the serial number is provided, an adapter using

the MCP2221 solution receives the same COM port

number from a Windows machine, no matter which

USB port they are connected to.

The case with no serial number is useful for

test/validation of products using the MCP2221. The

fact that all the tested boards are not supplying a serial

number will force Windows to assign them the same

COM port number (but only if connected to the same

USB port).

The MCP2221 is factory-set to not use a serial number.

Later in the process, if a customer wants the benefits

provided by using a serial number, the Configuration

Utility from Microchip can be used to enable the

MCP2221 to enumerate its serial number as well.

The MCP2221 comes with a uniquely-provided serial

number to be used during the USB enumeration

process; however, this can be changed by the user in

the Configuration Utility.

The serial number enumeration enable/disable can be

changed, as well, using the Configuration Utility.

All the USB-related settings mentioned above are part

of the Device Configuration (Chip Settings area) and

they reside only in Flash. When the Chip Settings area

(1st area) is being copied into the SRAM (at Power-Up),

the USB settings are skipped (not copied into SRAM).

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