How does USB Type-C distinguish between connected devices? Connection detection is determined by the CC terminal and Rp/Rd

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Design Interface Parade Technologies

With USB Type-C, simply plugging in the connector allows the connection to be recognized, and power supply and communication to begin. But how does USB Type-C distinguish between connected devices? That's where the CC (Configuration Channel) terminal comes in. USB Type-C uses Rp and Rd connected to the CC terminal to determine the presence and role of the connected device, before proceeding to 5V power supply or USB PD negotiation.

This article explains the mechanism of connection detection using CC terminals and Rp/Rd.

How do negotiations for USB PD even begin?

USB Power Delivery (USB PD) is a system where the Source (power supply side) and Sink (power receiver side) communicate to determine the optimal voltage and current. However, this negotiation doesn't begin the moment the cable is connected.

For example, when connecting a USB PD-compatible AC adapter to a laptop, the device doesn't immediately start operating at 9V or 20V. Instead, it first checks the connection status. After that, power is supplied at 5V, and only then does it proceed to USB PD negotiation.

The operation of USB Type-C can be roughly described as follows:

A general overview of how USB Type-C works

While USB PD is often described as a "power negotiation standard," in reality, there is a preliminary process of confirming the existence and role of the connected device. In other words, to understand USB PD negotiation, it is important to first understand "how the connection is detected."

USB Type-C detects the connected device using the CC terminal.

With USB Type-C, the presence of the connected device must be confirmed before negotiation via USB PD begins. This role is handled by the CC (Configuration Channel) terminal. In conventional USB Type-A, the roles of the power supply and power receiver were predetermined, so a dedicated signal to determine the connected device was not necessary. However, with USB Type-C, various devices are connected using the same connector shape, so a mechanism is required to confirm the presence and role of the connected device immediately after connection.

That's where the CC terminal comes in. The CC terminal is a dedicated terminal located on the USB Type-C connector and primarily serves the following roles:
・Detection of the connection partner
- Determining the power supply side (Source) and the power receiving side (Sink)
USB PD communication
- Determining the insertion direction of the connector
Of these, the first thing USB Type-C does is detect the connected device. In other words, before USB PD communication takes place, it first uses the CC terminal to check "whether the other device exists" and "which device is the power source," and only then does it proceed to 5V power supply and USB PD negotiation.

The CC terminal immediately after a USB Type-C cable is connected is not just a communication line. It first functions as a detection line to confirm the connection status. So how does USB Type-C determine the presence and role of the connected device? The Rp and Rd terminals are used for this determination.

The connection status is determined by Rp and Rd.

So, how does USB Type-C determine the presence and role of the connected device?

The Rp and Rd resistors connected to the CC terminal are used for this determination. In USB Type-C, the power supply side (Source) connects Rp (pull-up resistor) to the CC terminal, and the power receiving side (Sink) connects Rd (pull-down resistor). The Source monitors the voltage of the CC terminal and recognizes the connection of the other party from the voltage change caused by the connection of Rd. For example, when nothing is connected, from the Source's perspective, only Rp is connected to the CC terminal. However, when the Sink is connected, Rd is added, and the voltage of the CC terminal changes.

Image of connection detection using Rp and Rd

The Source detects this voltage change to determine that "the connected device exists" and "the connection has been successfully established." It's important to note that USB Type-C doesn't initially confirm the connection through communication. Immediately after connection, the connected device's circuitry and software may not yet be operational.

Therefore, the connection status is first checked using a simple analog circuit utilizing Rp and Rd, and only then does it proceed to 5V power supply and USB PD communication. In other words, what USB Type-C does first is not complex communication, but connection detection using the voltage change of the CC terminal.

USB PD will not start if the CC terminal is not functioning correctly.

As we have seen, USB Type-C uses the CC terminal to determine the presence and role of the connected device. In other words, if connection detection by the CC terminal is not performed correctly, the subsequent USB PD negotiation cannot proceed. When thinking about USB PD, we tend to focus on the communication and negotiation that takes place on the CC terminal. However, in reality, connection detection must be successful as a prerequisite. Therefore, if the connection is not recognized by the CC terminal, USB PD communication itself will not start.

for example,
- The device is USB PD compatible but does not switch to high voltage.
- USB PD negotiation does not start.
- It will always remain at 5V.
When phenomena like these occur, simply checking USB PD communication may not lead to the cause. Of course, the CC terminal is not the only reason why the voltage remains at 5V. However, since USB PD relies on connection detection via the CC terminal, it is important to first check whether the connected device is being recognized correctly.

When troubleshooting USB Type-C or USB PD issues, it's helpful to consider not only whether "USB PD is working correctly," but also whether "the CC terminal is correctly detecting the connection." This perspective will make it easier to pinpoint the problem.

Summary

With USB Type-C, connection detection is performed using the CC terminal before negotiation via USB PD begins. The presence and role of the connected device are determined by the voltage change on the CC terminal caused by Rp on the Source side and Rd on the Sink side. USB Type-C proceeds to 5V power supply and USB PD communication only after this connection detection is successfully completed.

USB PD is often described as a "power negotiation mechanism," but its foundation lies in connection detection via the CC terminal. To understand how USB Type-C and USB PD work, it's crucial to first understand how the CC terminal recognizes the connected device.