Role classification and definition of USB Type-C

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Role classification and definition of USB Type-C

The USB cable is a bus with a master-slave relationship. In a USB cable, there can only be one USB host, and the other devices are slaves.

Before connecting two devices in a USB cable, their master-slave relationship is generally determined (except for OTG). For example, the DownStreaming Face Port (DFP) of USB is generally the host, while the UpStreaming Face Port (UFP) of USB is the slave device. In the USB2.0 and USB3.0 eras, most devices still used interfaces such as TYPE-A and TYPE-B, which are easy to distinguish their roles by type. Generally, TYPE-A is the host, TYPE-B, MICRO B, and MINI are the slave devices.

Starting from USB3.1, both ends of the USB cable may be TYPE-C interfaces. The more advanced USB4 can only use TYPE-C interfaces. This type of interface is the same on the device side and the host side. This is to a certain extent that it is impossible to tell which one is the host and which one is the slave device.

In the previous USB protocol, if the interface of a device is DFP, it must be a host or have the function of a host. If the interface of a device is UFP, it must be a slave device or have the function of a slave device. However, because TYPE-C introduces the PD charging protocol, it realizes switching between master and slave devices, and expands and redefines the original two roles. A DFP interface is not just a USB host, and a UFP interface is also not just a UFP.


TYPE-C defines more role types compared to the previous USB specification, which are:

• Pure power supply party: It can only be used as a power supply party (Source) and cannot be switched to a power consumer (Sink) through the power supply agreement.
• Default power supply party: It is the power supply party by default and can be switched to the power consumer through the power supply protocol.
• Pure power consumer: It can only be used as a power consumer and cannot be switched to a power provider through the power supply agreement.
• Default power consumer: It is the power consumer by default and can be switched to the power provider through the power supply protocol.
• Switchable dual role: Defaults to dual role (DRP), roles can be switched via power supply protocol.
• Dual-role power-supply device: It works as dual-role by default and can switch the power-supply role through the power supply protocol. It has DFP and UFP functions but cannot be used as a USB host, such as the upstream Face port (UFP) of a HUB.
• Dual-role power-consuming host: It works as a dual-role by default, and can switch the power supply role through the power supply protocol. It has DFP and UFP functions but cannot be used as a USB device (detico), such as the Hub's downstream Face Port (DFP).

 

Roles	Initial Connection Communication Function Role	illustrate
power supply party （Source）	DownStreaming Face Port (DFP)	Interface output power
power consumption party （Sink）	UpStreaming Face Port (UFP)	Interface input power
Dual-role devices （Dual Role Port,DRB）	Dual-role devices (DFP or UFP)	1. The interface can be either a power supply party or a power consumption party. Dual role switching (DFP or UFP) occurs during the connection. Both parties identify and determine the roles when the two devices are connected.  2. When another connected device is the power supply, this interface is the power consumer;  3. When another connected device is a power consumer, this interface is the power supply;  4. When the connected device also has dual roles, this interface may be a power supply or a power consumer.

 

• From the perspective of power supply, the Type-C interface can be a power supply (Source) or a power consumer (Sink) or a dual role part (DRP).
• From the perspective of communication functions, the Type-C interface can be a downstream Face port (DFP) or an upstream Face port (UFP) or a dual role part.
• For their functions and roles, it is determined based on the power supply role when connecting. When the power supply role is the power supply party, the functional role defaults to DFP; when the power supply role is the power consumer, the functional role defaults to UFP.

The role of Type-C devices is generally determined before manufacturing, and whether two Type-C devices can recognize and work with each other when connected.
 

• "Not working" means that after the devices are connected, neither party can recognize the other, and there is no power supply and signal transmission, but it will not cause damage to the devices.
• "Working" means that after the devices are interconnected, both parties can identify each other and perform power supply and communication.

As can be seen from the above table, the power supply party cannot work in connection with the power supply party, and the power consumption party cannot work in connection with the power consumption party. Dual-role devices can connect to any device, if a dual-role device is connected to a power-supplying device, the device operates as a power consumer; if connected to a power-consuming device, the device operates as a power provider. If connected to another dual-role device, the device will randomly work as a power supply or power consumer.
 

Roles	Power Supply Party	Power Consumption Party	Dual-role Devices
power supply party	Not working	Working	Working
power consumption party	Working	Not working	Working
Dual-role devices	Working	Working	Working

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