Adding USB-C to your Carrier Board Design - Part 1

Dienstag, 31. Januar 2023
This is the first part of a two-part series that will help you to understand and implement USB-C in your next circuit board design. You will learn about:
  • USB-C pins and their usage in different configurations
  • How USB-C cables can be plugged in either way – up or down
  • How the same device can act either as a host or client
  • Power delivery with USB-C
  • New terms like DRP, DRD, UFP, and DFP
  • The difference between an active and passive USB-C cable and how adapter cables work
Let’s start with some context.
USB Standards and Connectors

Designed to standardize the connection between peripherals and personal computers, the USB was first released in 1996 and has been through 4 major generations so far: USB 1.x, USB 2.0, USB 3.x, and USB4. Since then, the standard replaced different interfaces, such as serial and parallel ports. It ensures that the devices are self-configuring and power-delivery capable, among other useful features. Throughout the evolution of the USB standards, different connectors were used, as you can see in the table below:

USB Standard Connectors

USB Connectors vs USB Standards - different standards and when each connector was introduced

USB-C Connector

As you can see in the table, USB-C seems to be just another connector in the standard. However, notice it replaces all the other connectors from USB 3.2 upwards.

Note also that connectors before USB-C were applied distinctively on the host side (type A and its mini/micro variant connectors) and client-side, type B and its mini/micro variant connectors. Therefore, an interesting feature of the USB-C connector is that it can be used on both the client and host sides.

Until the USB 2.0 version, the connectors within the standard had four (4) pins: Vbus, D-, D+, and GND. The USB 3.0 version introduced the SuperSpeed connector, which had five (5) additional pins; SSRX-, SSRX+, SSTX-, SSRX+ and GND_DRAIN. The connectors were modified to accommodate the extra pins. While the type-A connector was easily modified to accommodate five (5) more pins, the type-B received a little bump. The micro-B connector was turned into a confusing and not user-friendly connector.

Types of USB

Image reference
Type - A
Type - B
Micro - B

The USB-C connector was introduced to solve the user experience issue and became the first rotationally symmetrical standard USB cable.

It has 24 pins:
USB-C 24 pins connector
  • Compared to the previous iteration - which only had 1 lane of superspeed signals - the USB-C has 2 lanes of superspeed signals.
  • Additional rotational symmetrical power pins.
  • A symmetrical pair of D+ and D- signals (redundant on the device sides - the cable only routes one D+ and one D-).
  • The new signals: 2 pins for configuration control and 2 side-band usage pins.
Pin Usage
  • The USB-C can still be used for the default Low/Full/High-Speed (2.0) connection - using the default D+ and D- pair for the data signal.
  • It can also be used for SuperSpeed (3.x) connection - using the high-speed lanes and configuration control pins.
  • There's also the possibility to use it as a Power Delivery interface (Charger), where VBUS, GND, and Configuration controls are used.
  • In Alternate Mode, it can be used as a Display Port, for example. Up to 4 data lines are achieved by reusing the superspeed lane as display port signals and using the SBU as an auxiliary channel for configuring the displays. In this configuration, USB 2.0 can still be used with D+ and D- pins.
  • It can also be used as an Audio Adapter Accessory, with support for stereo headphones and a microphone.
Audio Adapter Accessory
In the documentation on USB-C, notice the presence of new terms not used extensively in the previous generations of the standard. Take a look at them:
  • DFP: Downstream Facing Port​ - USB host port and power source​.
  • UFP: Upstream Facing Port ​- USB client and power sink​.
  • DRD: Dual Role Device​ - Can act as host or client port (replacing the OTG term.)​
  • DRP: Dual Role Power​ - Can operate as a power provider or power consumer​.
The DFP and UFP terms are simple to understand, while the not-obvious DRD and DRP were intentionally created because of the two independent roles classification in the USB-C standard:
  • Regarding data direction - host vs. client.
  • Regarding power direction - power sink vs. power source.

Hence, the same device can act simultaneously as a client (data direction) and a power source (power direction). An example is some docking stations - they are USB clients but are the power source when charging the laptop.

Configuration Channel
Among the new pins brought in the USB-C, the configuration pins, CC1 and CC2, are notably important since it has 3 fundamental purposes:
  • Cable orientation detection:
    Since the cable is rotationally symmetrical, you can plug it in either orientation. Therefore, it's vital to detect this so the devices can multiplex necessary pins for proper communication. Remember, not all the pins are symmetrical in the connector.
  • Role detection:
    The previous standards made it fundamentally easy to determine the roles in communication with different connectors for the host-side (type-A) and client-side (type-B). That meant that, with compliant cables, it was impossible to connect two hosts together, for example.
    With USB-C, the shape of the connector is no longer preventing connecting two hosts. The role-detection function makes sure no damage is possible in such cases. Role detection is also essential in combination with Dual Role Devices for defining which device is the host, and which is the client in the communication.
  • Power detection and negotiation:
    Different power configurations are available, and both the client and host need to agree on a mutual arrangement to configure the power distribution. That includes different VBus voltages and maximum current capabilities.

That is accomplished by having the setup shown in the picture below:

Cable orientation and role detection setup

Cable orientation and role detection setup

  • In the upstream devices, there are pull-down resistors in CC1 and CC2 pins.
  • In the downstream devices, there are pull-up resistors in CC1 and CC2 pins.
  • In passive cables, there is only one routed CC pin pair.
By detecting the voltage in both pins CC1 and CC2, either side can easily define all the parameters:
  • Role detection - is there a voltage change? That means one side is UFP and the other is DFP. If a UFP device does not sense a voltage change, it means there is no DFP in the connection. If a DFP does not sense a voltage change, it means that there is no UFP at the other end. If the other end is connected to the same type of device, no problems occur since this voltage sense stage is before switching the power bus, as you will learn later in this blog.
  • Cable orientation - what pin has a voltage change? Since the cable has a single connection between the CC pins, only one of the pins will change its voltage. That indicates the orientation of the cable at each end.
  • Power detection - what is the voltage level? By having different values of pull-up resistors, the DFP device can easily announce to the UFP device the power delivery capability based on the resultant voltage level at the CC pin. Detailed information will follow.

The configuration channel is used as a bidirectional communication bus for advanced power negotiations between the devices. Active cables feature marker chips that can communicate over the CC bus the capability of the cable, regarding power handling. Detailed information will follow.

The other CC pin can be used to power any existing chips inside the cables. These chips can be markers, signal repeaters, or adapters. These active cables have a pull-down Ra resistor and are powered with a VCONN voltage, fixed in 5V - 1W max. Both CC pins can be switched to VCONN depending on the cable orientation - as the other remains available for being used in the communication bus. The switching is done by a VCONN Control Signal, which connects either CC1 or CC2 to the power supply. This arrangement can be seen in the following diagram.


One crucial aspect of USB-C is that the VBUS is not always powered. Unlike the previous USB standards, which had VBUS always powered, the VBUS on the USB-C is switched only after the source side detects a connected sink by observing the voltage levels at the CC pins.

Possible configurations
With that being said about the configuration channel, you can see how it all works in some scenarios:
  • DFP to UFP
Source to Sink

In this setup, the voltage on CC1 drops when the two devices get connected. The device USB controller then switches the VBUS source, enabling the sink device to be powered. Also, the controller detects that the cable is not flipped, and the CC2 pin is connected to VCONN to provide power to active cables. After this switching, the sink device can ask to be enumerated, like in the previous generations.

  • DRP to UFP
DRP to Sink

A DRP can be configured to work as a DFP. In this case, the CC pins are connected to pull-up resistors. After detecting a change in the voltage at the CC1, the device USB controller can activate the VBUS source and VCONN at CC2.

  • DFP to DRP

A DRP can also be configured to work as a UFP, in which case the CC pins are connected to pull-down resistors. After a CC voltage change detection, the DRP device USB controller can switch the VBUS sink, enabling it to be powered by the DFP device.

  • DRP to DRP

In this configuration, one of the devices has the pull-down resistor enabled, while the other has the pull-up resistors enabled in the CC pins. The device acting as a source then activates the VBUS source, and the sink device activates the VBUS sink.

  • Wrong combinations - Since the USB-C connectors are identical on both ends of the cable, it's possible to inadvertently have sources and sink devices connected. In both cases, the communication will not work, but there is no risk of damaging the devices.
    • Source to source
Source to Source

The VBUS source is not activated since there is no voltage change at the CC pins.

    • Sink to sink
Sink to Sink

There is no VBUS source. No voltage source conflict is possible.

  • Legacy devices
    • Legacy power sink connected to USB-C source

To provide compatibility with UFP devices from previous generations, it's possible to have a simple adaptor. This adaptor requires a pull-down resistor on the CC signal so the USB-C source device can detect the adapter and enable the VBUS power.

    • Legacy power source connected to USB-C sink
Legacy power source connected to USB-C sink

The opposite situation is also valid: a USB-C sink device can be connected to a source device from previous generations with a type-A to type-C cable. It requires a pull-up resistor internally in the CC line so that the USB-C device can sense a voltage change at its CC pins.

Power Delivery

A wide range of power delivery configurations is available with USB-C. The first three levels are backward compatible with non-USB-C systems (for example, with a Type-A connector).

Backward Compatible Power Delivery​
Power Mode CC Usage​ Detection​ Voltage​ Maximum Current​ Maximum Power​
Default USB 2.0 56kΩ pull-up at DFP or in Type-A adapter cable​ USB enumeration is required over USB 2.0 signals. Without enumeration, only 100 mA is allowed (2.5mA in suspend)​ 5 V​ 0.5 A​ 2.5 W
Default USB 3.x​ 56kΩ pull-up at DFP or in Type-A adapter cable​ USB enumeration is required over USB signals. Without enumeration, only 150 mA is allowed​ 5 V​ 0.9 A​ 4.5 W​
USB BC 1.2 56kΩ pull-up at DFP or in Type-A adapter cable​ Presence detection of battery charger host by measuring the resistance between D+ and D- lines of USB 2.0 Signals​ 5 V 1.5 A​ 7.5 W​

There are backward-compatible power configurations available with type-A to type-C cables. Note that the USB enumeration process is required - except in the USB BC 1.2 case - for a battery charger, placing a chip just for enumerating would not make sense. In this situation, a short connection between D+ and D- at the wall power adapter announces the presence of a battery charger to the sink device. That allows to draw up to 1.5A without the need for USB enumeration.

Simple Power Delivery​
Power Mode CC Usage​ Detection​ Voltage​ Maximum Current​ Maximum Power​
USB-C 5V/1.5A 22kΩ pull-up at DFP UFP detects pull-up resistor value at CC pin​ 5 V​ 1.5 A​ 7.5 W
USB-C 5V/3A 10kΩ pull-up at DFP​ UFP detects pull-up resistor value at CC pin​ 5 V​ 3 A​ 15 W​

With USB-C, two simple power modes are introduced that only require different pull-up resistor values at the DFP side. These modes only work with USB-C connectors on both ends of the cable. It does not work with legacy type-A to type-C adapter cables.

Advanced Power Delivery​
Power Mode CC Usage​ Detection Voltage​ Maximum Current​ Maximum Power​
USB PD Bus​ Bus communication between DFP, UFP, and marker cable​​ 5-20 V​ 5 A​​ 100 W​
USB PD Extended Power Range​​ Bus​​ Bus communication between DFP, UFP, and marker cable​​ 5-48 V​ 5 A​ 240 W​​

Using the CC channel as a communication bus to negotiate voltage and current, it's possible to get even more power available. If the current is higher than 3A or the voltage is higher than 20V, the cable needs the marker chip to communicate and allow for these higher power modes. Passive cables are only supported for up to 60W (3A/20V).

We can summarize the tables with the following diagram and observations:

Current and Source Power Rating
  • Up to 15W (5V/3A) announcement can be made with resistor values​ - only with 5V voltage.
  • Higher voltages and power require negotiation over the configuration channel​ bus. Currents higher than 3A require a cable with a marker-IC inside.

You now understand the fundamentals, the role definition and power delivery with USB-C. Stay tuned for our next blog, where we will provide real-world examples and discuss data signals in USB-C.

Peter Lischer
, Senior Hardware Developer, Toradex

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