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USB Type-C Cable and Adapter Tips and Tricks

This page provides reference information for manufacturers of USB-C parts. It addresses common misunderstandings and errors in building legacy cables (Type-A or microB to USB-C) and power adapters. For complete specifications, tolerances and application rules, see the latest version of the USB Type-C specification.

Legacy Cables

The following table describes the type of resistor and configuration (pull-up or pull-down) that is required for each type of legacy cable. All cables must be capable of supporting 3A, regardless of Rp/Rd.

Resistor Rp (pull-up between VBUS and CC) Resistor Rd (pull-down between CC and GND) ID Pin USB-C Spec Section
USB Type-C plug to USB 3.1 Type-A plug 56 kΩ Rp Open Section 3.5.1
USB-C plug to USB 2.0 Type-A plug 56 kΩ Rp Open Section 3.5.2
USB-C plug to Type-A receptacle Open 5.1 kΩ Rd Section 3.6.1
USB-C receptacle to microB plug Not allowed per the spec Section 2.2
USB-C plug to microB plug Open 5.1 kΩ Rd unconnected Section 3.5.7
USB-C plug to microB receptacle 56 kΩ Rp Open unconnected Section 3.6.2

Cable FAQ

  • My legacy cable conforms to the specification and is rated for 3A. Can I use the Rp 3A resistor? 
    • No. You must use the 56 kΩ Rp. The resistor indicates the capability of the power adapter, not the capability of the cable. It is not safe to use the 3A Rp: if the power adapter is not capable of providing 3A it could overheat.
  • Why does my cable need to be rated for 3A if only the standard Rp is used?
    • The standard Rp indicates that the power sink needs to use some other method of finding what current the power source can provide. This method could be BC1.2 or a proprietary discovery scheme (such as the voltage set on D+/D-). Using these methods, a sink could discover the source is capable of up to 3A. All cables must be capable of supporting 3A.

Power Adapters

For USB-C power adapters, the following table specifies the resistor type and configuration.

Pull-up on CC1 Pull-up on CC2 VBUS
5V 3A power adapter with USB-C receptacle 10 kΩ 10 kΩ Cold
5V 1.5A power adapter with USB-C receptacle 22 kΩ 22 kΩ Cold
5V 3A power adapter with captive USB-C cable 10 kΩ open Cold or Hot
5V 1.5A power adapter with captive USB-C cable 22 kΩ open Cold or Hot

VBUS Cold : When nothing is attached to the USB-C receptacle or plug, VBUS must be 0V or vSafe0V. 5V shall be applied to VBUS only when a UFP is detected by monitoring voltage on the CC pin. 5V should only be applied when voltage vRd on CC is 0.85V < vRd < 2.45V for a 3A power source. Please see Tables 4-23, 4-24, and 4-25 of the Type-C specification for the appropriate values of vRd minimum and maximum voltages for Default USB Power, 1.5A, and 3.0A levels.

VBUS Hot : When nothing is attached to the USB-C plug, 5V may be applied to VBUS.

Important Note : If your charger implements USB Power Delivery, regardless of connector type (Receptacle or captive cable), VBUS Cold is required.

USB Battery Charging v1.2

For chargers with a USB-C receptacle, it is highly recommended the port also support USB Battery Charging v1.2 in order to allow legacy devices using Type-A plugs or Micro-B receptacles to charge.

To implement a BC1.2 Dedicated Charging Port (DCP), D+ and D- lines in the receptacle must be shorted together.  Please see the USB Battery Charging v1.2 Spec for more details on how to implement DCP or CDP.

USB PD Power Rules

Power adapters with maximum power <= 15W may support USB Power Delivery. Power adapters with maximum power > 15W must support USB Power Delivery. When initially specifying the voltage and current capability of an adapter which supplies > 15W, pay close attention to Power Rules, in USB PD R2.0 V1.2 Section 10.

USB PD Revision 2.0 specifies normative voltage rails of 5V, 9V, 15V, and 20V. In order to support a particular voltage rail, all voltage rails lower must be supported up to 3A.

Power Adapter FAQ

  • In a 5V 3A or 5V 1.5A supply can I connect CC1 and CC2 and use a single shared resistor? 
    • No. This design fails with active cables, emarked cables or any device that requests Vconn. These cables have an Ra pulldown on one of the pins, preventing accurate detection of CC voltage if the pins are shorted at the adapter.
  • May a Type-C charger support a proprietary method of changing VBUS in addition to or instead of USB Power Delivery? (For example, Qualcomm QuickCharge, MediaTek PumpExpress, others)
    • No. Section 4.8.2 of the USB Type-C Specification explicitly forbids proprietary methods that change VBUS from the default voltage defined by USB 2.0 and USB 3.1 specifications (maximum 5.5V). This applies to both power sources and power sinks. If the power adapter incorporates a Type-C plug or a Type-C receptacle, that connector must not support any dynamic voltage method other than USB Power Delivery.