The primary goal of USB4 development is to double the data speed compared to USB 3.2 (40 Gbps versus the previous 20 Gbps) and enable compatibility with Intel's Thunderbolt protocol. USB4 will exclusively use the USB Type-C port, the oval-shaped port with the blade in the center, famous for accepting cables even when inserted upside down. While cable insertion may be simpler, the charging technology behind USB4 ports must now include USB PD, which adds complexity. The specifications for the previous generation of USB, which used the Type-C port, had an option to enable PD; USB4 mandates it.
Power delivery is required in USB4.
The PD specification has been revised to include new messages for discovering and transitioning to USB4 mode, but the power schemes are the same. They use a single-wire 300 kHz bus on one of the Configuration Channel (CC) lines of the USB Type-C interface for the host and device to discover and negotiate the power and data to be transferred (Fig. 1). The other CC line takes on the role of "VCONN," a dedicated power supply to the electronic marker (an identification circuit within the USB cable). Power supplied between USB ports is transferred on a separate set of wires within the connector (labeled "VBUS"; see Fig. 2). When two PD devices connect, they use the CC wire to discover each other, communicate their power capabilities (what voltages and how much power at each voltage), determine which device should be the power source or receiver, negotiate how much power to deliver, and then provide that power on VBUS. This 300kHz digital signal is also used to identify that the USB connection can support a USB4 link, so there is no way to implement USB4 without this communication. USB4 ports do not need to supply or receive any power beyond a minimum of 5V/900mA; however, they must support PD communication to function as USB4.
History of Power Delivery on the Universal Serial Bus
To understand user charging experiences on USB4, it is helpful to understand the history of power delivery through the USB connector (see Fig. 2). The Universal Serial Bus was originally intended for serial data communication, with a maximum of 100 mA of current on the cable. The USB 2.0 specification was limited to 500 mA on the VBUS line; this was adequate for powering basic computer peripherals. The USB 3.0 standard increased the current limit to 900 mA, but with portable devices converging on a single connector for both data and power, this was insufficient.
The USB committees published the Battery Charging (BC) specifications, which culminated in the BC1.2 specification in 2010, allowing 1.5 A (7.5 W); at that time, many smartphone manufacturers opted out of USB compatibility.
This led to a proprietary charging protocol that used fixed voltage levels on the D+ and D- lines (USB data lines): 2V on one line and 2.7V on the other could deliver a 10W charge; 2.7V on both data lines could signal a 12W charge; 3.3V on each line caused the charger to deliver 20V (which could be harmful if applied accidentally). These methods were not interoperable, and the results were unpredictable. Furthermore, the data lines used to determine the charging levels were no longer available for data transfer. A port can transfer files, or it can charge faster, but not both. Anyone who has ever had a phone die while charging knows that port was likely a data connection supplying 500mA according to the USB 2.0 specifications.
Figure 2: USB power policies over time. Proprietary standards varied by manufacturer, while new USB specifications typically maintained backward compatibility.
This problem prompted the first PD specification (revision 1), which created a universal standard for charging at alternative voltages (above 5V) using traditional 4-pin USB cables. Maintaining backward compatibility required adding a hello signal to the VBUS line itself, which was complicated to implement, and the whole thing was abandoned without significant adoption. The USB-IF would prefer everyone to forget this specification was ever written, and practically speaking, everyone should. This approach is no longer valid or supported.
Today, PD versions 2.0 and 3.0 exist, and the programmable power supply (PPS) specifications are included. These were created alongside the USB Type-C port, with added signal connections. The differences between versions 2 and 3 lie primarily in the details of DC communication. Both are backward compatible with previous USB implementations (excluding PD revision 1), and the user experience will be the same. Devices negotiate load profiles, potentially in increments of up to 20mV (under PPS implementations). PD-capable devices can (but are not required to) support up to 100W of power transfer within the specification (5A at 20V). Proprietary schemes for advertising alternative load profiles using the data lines are explicitly prohibited, but USB Type-C also allows simplified 1.5A and 3A charging at 5V (recognized by resistors on the CC pin, rather than the digital signal). USB Type-C ports do not require Power Delivery (PD), but PD does require a Type-C port, which is where USB 4 changes things (see Fig. 3); USB 4 uses PD communication to enable USB 4 mode.
Figure 3: Relationship between USB data and USB power - USB 2.0 can exist on any USB cable, with or without power delivery on a Type-C cable. USB 3.x requires a cable with additional SuperSpeed lines, with or without power delivery on a Type-C cable. USB 4 can only exist on a Type-C connection, with Power Delivery communication. Power Delivery can only exist on a Type-C connection, but it does not require data.
Although the new devices will follow the new simplified specifications, the complexity of USB charging comes from all these legacy standards, all of which still exist in older ports, and a new USB4 port can plug into any of those legacy ports (see Table 1).
Table 1: USB Power Connections
Use cases and user experiences:
Trying to maintain five generations of backward compatibility creates a complicated landscape of potential power connections. It leaves a big question mark about what happens to all the older USB ports and cables, and users may continue to wonder, "Will it charge?"
Fortunately, ignoring the proprietary specifications, most of that story boils down to a few possible user experiences. While they will all load, they may not all load quickly.
There are essentially four use cases to consider in the context of USB4:
1. A legacy charging port connects to a USB4 Type-C device using an adapter cable.
2. A USB4 Type-C charger connects to a legacy port using an adapter cable.
3. A USB4 Type-C port connects to a non-USB4 Type-C port with a DC cable and places a resistive splitter on the DC line.
4. Two Type-C ports connect with a DC cable and communicate on the DC lines; either or both could be USB4 devices.
In terms of USB standards, the Apple 8-pin Lightning connector carries the same signals as a legacy USB 3.x cable. USB4 ports connected via USB-C to Lightning adapter cables will behave similarly to USB-C to micro-B or USB-C to USB-A cables in terms of power functionality. Below is a summary of what to expect in each charging use case:
Legacy charging port connected to a USB4 device via an adapter cable.
Legacy ports, such as Type-A and Type-B, may have been created before the Type-C specifications (see Figure 1). There was, and will be, no requirement for these ports to implement any specific faster charging scheme. A USB 2.0 port might default to charging at 500 mA, or a USB 3.x port might default to charging at 900 mA. The good news is that most newer USB ports support BC1.2 and provide 7.5 W. No matter what type of adapter cable is used, the USB4 or Type-C device connected to the legacy charging port cannot draw more than 7.5 W of power without violating the USB specifications.
USB4 Charging Port Connected to a Legacy Device via Adapter Cable:
When a USB4 charging port is connected to a legacy device, several possible outcomes can occur. The USB4 port might be advertised as delivering 1.5A using the BC1.2 standard, while the cable might carry 7.5W. If the USB4 port isn't configured to handle the additional overhead of BC1.2, it will default to 500mA for a USB 2.0 data connection, or 900mA for a USB 3.x data connection. This can be frustrating for users, as the more advanced USB port may result in slower charging when used with adapter cables.
USB4 to Non-USB4 Type-C, with Resistive Dividers:
If the power provider or consumer is using the Type-C resistive divider method to advertise power capabilities, this will dictate the power transfer. The USB4 device will not be able to communicate on the DC lines, but it will still recognize the accessory, the provider/consumer relationship, and the current limit (1.5A or 3A) using current sources or resistors connected to the DC lines. The bus voltage will remain at 5V, and the charging device will be able to draw up to 7.5W or 15W. Since the device knows it is not in PD mode, it can indicate that it is not fast charging for a clear user experience.
Type-C to Type-C, with PD communication:
This is potentially the most capable USB4 connection in terms of power transfer, but exact results will vary. The two connected devices will negotiate a power contract based on their capabilities, up to 20V and 5A. Part of the negotiation is the role of power provider or consumer. It is possible to connect two power consumers, which would negotiate not to transfer power (and this is a valid use case for data transfer between portable devices). Power provider ports may be marked with a battery symbol, so in many cases users will know which ports on a dock or laptop are configured to provide power. In that case, the power-providing port must be capable of supplying at least 1.5A at 5V (7.5W, the same as BC1.2) to use a USB-IF approved charging logo. Higher power levels are not guaranteed even on a Type-C port labeled with a charging icon. Since the power contract could be any power level from 7.5W to 100W, the user will only know what's happening if one of the devices reports on the contract (indicating a power level or a fast-charging connection). This use case certainly has the potential to create unpredictable and frustrating user experiences, but with good reporting and user interfaces, it could also be a perfect user experience.
In conclusion
, the addition of USB4 will significantly increase bandwidth and capabilities for USB applications that can utilize it, but USB 2.0 and USB 3.x will continue to be used in applications that can handle lower data transfer rates. Power consumption will increase, but with smart application design, this doesn't necessarily have to lead to poor user experiences.
Author: Fionn Sheerin, Senior Product Marketing Engineer, Power and Analog Interfaces Division, Microchip Technology
