The latest interface specification introduced after Type-B. Different from the traditional USB interface, Type-C adopts a symmetrical design, which does not need to distinguish the direction of the plug, avoiding the tedious operation of users plugging in the right and wrong directions. In addition, USB Type-C supports the USB PD (Power Delivery) protocol, which increases the charging power from the traditional maximum 7.5W (5V1.5A) to a maximum of 100W (20V5A). The latest USB PD3.1 specification further improves the Type-C charging power, with a maximum power of up to 240W (28V5A).
For traditional USB Type-A or Type-B devices, the power supply interface (Source) and the power receiving interface (Sink) are already standardized in the interface definition, so there is no need to worry about reverse or wrong connection. For devices with Type-C interfaces, since there are no such differences, users cannot know the type of interface, so the Type-C controller itself needs to complete it. So how do Type-C interfaces recognize each other and provide the correct power supply logic?
Pin definition of Type-C interface
Type-C interface is divided into female head (Receptacle) and male head (Plug). The complete Type-C pins are 24, and the definitions of each pin are as follows:
1. VBUS: A total of four channels, BUS voltage pins for power supply between devices, regardless of whether they are inserted forward or reverse, these four pins will provide power supply
2. GND: A total of four channels, power supply circuits between devices, regardless of whether they are inserted forward or reverse, these four pins will provide power supply circuits
3. TX+/TX- and RX+/RX-: A total of four pairs, for USB3.0 high-speed signals
4. D+/D-: A total of two pairs, for USB2.0 signals. At the female connector, these two pairs will short-circuit into one pair
5. CC/VCONN: CC pin is a configuration pin used to detect device connection and forward and reverse plugging direction, and is also the line for USB PD communication; VCONN is a pin that is obliquely symmetrical to the CC pin. When one pin is confirmed as CC, the other is defined as VCONN, which is used to power the eMark cable
6. SBU1/SBU2: Multiplexed pins, such as providing additional SBTX and SBRX for USB4
The female connector is 24 pins with oblique symmetry on the upper and lower pins to meet the user's forward and reverse plugging needs; the male connector is 22 pins. Since there is only one pair of D+/D- in the USB2.0 specification, only one pair of D+/D- pins is retained in the male connector.
Of course, in actual product design, engineers will appropriately reduce the number of pins according to the product definition to save costs. For example, for products that only provide charging, such as power adapters, such products do not require high-speed data communication of USB3.0, so only CC, VBUS, GND and D+/D- pins are retained.
In terms of power supply, Type-C devices can be divided into three categories
1. Type-C devices that can only be used as power supply (Source), such as Type-C chargers, etc.
2. Type-C devices that can only be used as power receiving (Sink), such as Type-C mobile phones, etc.
3. Type-C devices (DRP, Dual RolePort) that can be used as both power supply (Source) and power receiving (Sink), such as Type-C notebooks, two-way power banks, etc.
Obviously, when two Type-C devices are connected together through C2C cables, both parties must know what type of device the other party belongs to, otherwise it will lead to unsatisfactory charging (such as reverse charging), or no charging, and even cause safety problems.
For example, when a user uses a charger (Source) to charge a Type-C two-way power bank (DRP), ideally, the power bank should "serve" as a Sink. However, due to incorrect device type identification, the power bank may "serve" as a Source and cause "current backflow", damaging both devices.
The Type-C interface specification distinguishes between Source, Sink, and DRP through a series of "pull-up" and "pull-down" mechanisms on the CC pin. For Source devices, the CC pin is required to be configured with a pull-up resistor Rp; for Sink devices, the CC pin is required to be configured with a pull-down resistor Rd; and for DRP devices, the pull-up and pull-down are alternately switched by switching switches.
Source determines whether a device is connected by detecting the CC pin at the Rp end, and Sink determines the direction of forward and reverse insertion by detecting the CC pin at the Rd end.
Pull-down resistor Rd=5.1k, and pull-up resistor Rp is set according to its power supply capacity and pull-up voltage. The power supply capacity of USB Type-C is as follows:
1. Default USB power supply capacity (Default USB Power). USB2.0 interface is 500mA; USB3.2 interface is 900mA and 1500mA
2. BC1.2 (BatteryCharge 1.2) protocol. Supports a maximum power of 7.5W, i.e. 5V1.5A
3. USB Type-C Current 1.5A, supports a maximum power of 7.5W, i.e. 5V1.5A
4. USB Type-C Current 3A, supports a maximum power of 15W, i.e. 5V3A
5. USB PD (USB Power Delivery) protocol, supports a maximum power of 100W, i.e. 20V5A
The priorities of these five power supply capabilities increase in sequence, and the power supply power also gradually increases. The power supply capability with a high priority will override the power supply capability with a low priority. Among them, Default USB Power, USB Type-C Current 1.5A and USB Type-C Current 3A can be set by configuring the Rp value.
When the two devices are connected, the Sink obtains the power supply capability of the Source by detecting the voltage divider value vRd of Rp and Rd. The following is the corresponding relationship between the Rp value, the vRd voltage range and the Source power supply capability.
At the same time, the other CC of the device has been left floating or pulled down by Ra=1k. If Ra is pulled down, it means that the USB-C cable has a built-in eMarker chip, and the Source needs to switch the pin to VCONN to power the cable.
So far, we have explained that the devices use "pull-up" or "pull-down", or alternately switch between the two, to determine the Source, Sink and DRP, and set and determine the power supply capacity of the Source by the Rp resistance value and vRd voltage value. However, how is this process implemented? How does Type-C avoid reverse charging or incorrect charging?