This file explains the basics of what I learned about USB-C PD to help development engineers on their projects. I'm not an english mother-language speaker, so here I leave my apology for any mistakes.
Again, this docs relates what I learned. I'm not an expert on USB-C PD.
The USB-C standard came with far more features than the previous standards. The thing that most catches the attention of common users is that you don't need to be aware of cable orientation (the connector is symmetrical).
One of the other features that came along with USB-C is the PD protocol. When well configurated, if the supplier, cable and sink device support the protocol, we can reach up to 100W power (20V, 5A). Yes, USB-C PD can deliver up to 20V. If supplier can offer, the sink device can choose between 5V, 9V, 15V or 20V, and the maximum current can also be configured. All that thanks to PD protocol.
Let's start by overviewing the pinout of USB-C connectors (AN1953):
- Plug pins (front view)
- Receptacle pins (front view)
This doc does not focus on data transmission, or alternate mode, so we'll ignore some of these pins.
The important pins for Power Delivery implementation is:
- VBUS: power supply bus
- GND: ground reference
- CC1: channel configuration pin
- CC2: channel configuration pin
- VCONN: used for active or electronically marked cables
Note that the cable has only one CC wire, and one VCONN wire, while the receptacle has two CC pins. When plugged, VCONN wire will be connected to one of the CC pins of the receptacle.
Obs: There are cables without VCONN wires, and so other wires, having only the essential for data transfer and simple power supply. Those cables can not be used for PD-protocol, neither for alternate function. A cable that have all the wires is called Full-Featured cable. All full-featured cables is an active cable (e-marked).
- DFP (Downstream Facing Port): USB Host (power supplier)
- UFP (Upstream Facing Port): USB Device (sinking device)
- DRP (Dual-Role Power): USB port that can operate as either a Source or a Sink.
- USB-BC (USB-Battery Charging): for USB 2.0
- e-marked cable (electronically marked cable): active cable
- Rp: pull-up resistor on DFP
- Rd: pull-down resistor on UFP
- Ra: pull-down resistor on e-marked cables
CC1 and CC2 pins are used to establish and manage the Source-to-Sink connection. Functionally, the configuration channel is used to serve the following purposes (USB-C Specs section 4.5.1):
- Detect attach of USB ports, e.g., a Source to a Sink,
- Resolve cable orientation and twist connections to establish USB data bus routing,
- Establish data roles between two attached ports,
- Discover and configure VBUS: USB Type-C Current modes or USB Power Delivery,
- Configure VCONN, and
- Discover and configure optional Alternate and Accessory modes.
Non-active cables will not have Ra. When an e-marked cable is connected, the Source has to be able to differentiate between the presence of Rd and Ra to know whether there is a Sink attached and where to apply VCONN. The Source is not required to source VCONN unless Ra is detected.
Tables references: (USB-C Specs)
As stated on (AN1953):
VCONN is a [...] power supply used to power circuits within the plug that are needed to implement electronically marked cables and VCONN-powered accessories. The DFP is responsible for supplying VCONN by default.
VCONN is required for PD-enabled port and USB3 support. The VCONN power supply can be supplied in one of two ways:
a) If a valid Rp/Rd connection is detected on one of the CC pins, the VCONN supply can be blindly routed to the opposite CC pin
b) After a valid Rp/Rd connection is detected on one of the CC pins, the opposite CC pin can be monitored for a valid Rp/Ra connection before routing the VCONN supply to the pin.
Microchip AN1953 - Introduction to USB Type-C
Universal Serial Bus Type-C Cable and Connector Specification - Release 2.3 October 2023