USB Type-C PD CC role definition in logic chip

USB Type-C, with its powerful functions and the strong promotion of Apple, Intel, Google and other manufacturers, will quickly lead to a revolution in USB interfaces and will have a positive impact on all aspects of our daily lives. This article discusses an important professional issue: Do USB Type-C devices need CC logic detection and control chips?

To answer this question, we have to start with the basic concepts.

DFP (Downstream Facing Port): Downstream port, which can be understood as Host, DFP provides VBUS and can also provide data. The typical DFP device is a power adapter, because it always only provides power.

UFP (Upstream Facing Port): Upstream port, which can be understood as a device, draws power from VBUS and can provide data. Typical devices are USB flash drives and mobile hard drives, because they always read data and draw power from VBUS. Of course, it is not ruled out that USB flash drives that can be used as hosts may appear in the future.

DRP (Dual Role Port): Dual role port, DRP can be used as DFP (Host) or UFP (Device), and can also switch between DFP and UFP dynamically. Typical DRP devices are computers (computers can be used as USB hosts or as devices to be charged (Apple's new MAC Book Air)), mobile phones with OTG function (mobile phones can be used as devices to be charged and read data, or as hosts to provide power to other devices or read data from USB flash drives), and mobile power supplies (discharging and charging can be done through a USB Type-C, that is, this port can discharge and charge).

CC (Configuration Channel): Configuration channel, which is a new key channel in USB Type-C. Its functions include detecting USB connection, detecting forward and reverse insertion, establishing and managing the connection between data and VBUS between USB devices, etc.

USB PD (USB Power Delivery): PD is a communication protocol. It is a new power and communication connection method. It allows USB devices to transmit up to 100W (20V/5A) of power. At the same time, it can change the properties of the port and switch the port between DFP and UFP. It can also communicate with the cable to obtain the properties of the cable.

Electronically Marked Cable: A USB Type-C active cable encapsulated with an E-Marker chip. DFP and UFP can use the PD protocol to read the properties of the cable: power transmission capability, data transmission capability, ID and other information. All full-featured Type-C cables should be encapsulated with an E-Marker, but USB2.0 Type-C cables do not need to be encapsulated with an E-Marker.

The USB Type-C device DFP-to-UFP configuration process and VBUS management have the following main processes:

Device connection and separation detection: DFP needs to detect a certain voltage on the CC pin to determine whether the UFP device has been inserted or unplugged to provide and manage VBUS. When no UFP device is inserted, VBUS must be turned off. Therefore, all DFP devices require CC logic detection and control chips.

Insertion direction detection: As shown in Figure 1, although the two rows of pins of the USB Type-C socket and plug are symmetrical, and the USB data signal has two sets of repeated channels, the main control chip usually has only one set of TX/RX and D+/- channels. Since the data rate of USB2.0 is only 480Mbps at most, the impedance continuity of the signal routing can be ignored to obtain better data transmission quality. Therefore, the D+/- signal of USB2.0 can be directly connected to the two sets of D+/- pins of the USB Type-C socket from the main control chip without being controlled by the MUX. However, the data rate of USB3.0 or USB3.1 is as high as 5Gbps or 10Gbps. If the signal line is still simply divided into two, the discontinuous signal line impedance will seriously damage the data transmission quality. Therefore, the MUX must be switched to ensure the consistency of the signal path impedance to ensure the signal transmission quality. The MUX shown on the right side of the figure below selects one from TX1/RX1 and TX2/RX2 to connect to the main control chip, and this MUX must be controlled by CC Logic.

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Therefore, in USB2.0 applications, there is no need to consider the direction detection problem, but in USB3.0 or USB3.1 applications, the direction detection problem must be considered.

Figure 1 USB Type-C data routing logic model

However, it must be noted that in the application of USB3.0/USB3.1, there is a situation where MUX is not required, that is, direction detection is not required. As shown in Figure 2, whether it is forward or reverse insertion, the left host can switch MUX according to the status of the CC pin to connect the USB3.0/USB3.1 signal. This scenario occurs when the right device is always UFP, such as a USB flash drive, mobile hard disk, etc.

Therefore, in USB3.0/USB3.1 applications, all devices except UFP devices require CC logic detection and control chips.

Figure 2 USB Type-C direct connection data routing logic model

Establishing DFP-to-UFP and VBUS management and detection

DRP switches between DFP and UFP every 50ms in standby mode. When switching to DFP, the CC pin must have a resistor Rp pulled up to VBUS or output a current source. When switching to UFP, the CC pin must have a resistor Rd pulled down to GND. This switching action must be completed by the CC Logic chip.

DFP can output VBUS only after it detects that UFP is inserted, and VBUS must be turned off when UFP is unplugged. This action must be completed by CC Logic chip.

USB Type-C VBUS current detection and use USB Type-C adds current detection and use functions, and adds three new current modes: default USB power mode (500mA/900mA), 1.5A, and 3.0A. The three current modes are transmitted and detected by the CC pin. For DFPs that need to broadcast current output capabilities, they need to be implemented through CC pull-up resistors Rp of different values; for UFPs, it is necessary to detect the voltage value on the CC pin to obtain the current output capability of the other DFP.

USB PD communication

USB PD seems to be just a protocol for power transmission and management. In fact, it can change the role of the port, communicate with active cables, allow DFP to become a powered device, and many other advanced functions. Therefore, devices that support PD must use CC Logic chips.

Discover and configure other peripherals (Audio, Debug)

USB Type-C supports voice attachments and debug mode. If the headset with USB Type-C interface is only used as UFP and does not need to detect the power supply capability of DFP because of its low power consumption, CC Logic chip is not required.

In summary, all DFPs (such as power adapters), all DRPs (such as computers, mobile phones, tablets, mobile power supplies), all UFPs that need to detect the current output capability of DFPs, and all devices that support PDs require CC logic detection and port control chips. In other words, only UFPs (U disks, headphones, mice, etc.) that do not need to detect current capabilities due to low power consumption do not need CC logic detection port control chips.