The Power Delivery 2.0 Protocol in Action

Figure 1: A high-level view of a PD 2.0 power
contract negotiation

The Power Delivery 2.0 protocol brings a boatload of intelligence to the process of establishing power relationships between partners in a USB Type-C link. In previous posts, we've examined the basics of the two standards, the workings of USB Type-C cable detection, how dual-role port devices are handled, and PD 2.0 messaging. Next, let's look at PD 2.0 in action by deconstructing the initialization of a Google Chromebook laptop, which was one of the first Power Delivery devices on the market.

USB Type-C and Dual-Role Port Devices

Figure 1: This protocol analyzer capture
shows how a DRP alternates between
voltage source and sink until it sees one
or the other at the far end of a link

At this juncture in our ongoing series of posts about USB Type-C and power delivery, it's time to acknowledge that some Type-C devices are not strictly a power source or sink. Some are called out in the Power Delivery 2.0 specification as dual-role port (DRP) devices that may act as either a source or sink for power. Note that unlike in the original OTG spec, this applies only to a port's power role, not to being a USB host or connected device.

USB Type-C Cable Detection

Figure 1: The four possible states of
connection for a USB Type-C cable

Having examined some of the basics regarding the USB Type-C connector and the Power Delivery 2.0 specification that complements the Type-C spec itself, we'll turn our attention to the topic of cable detection. The Type-C connector is reversible, which is to say there are four ways a cable can be connected between upstream- and downstream-facing ports (Figure 1). Not only that, but there are different types of Type-C cables. So how do the devices discern what's between them?