Don’t cut corners with USB PD compliance

Article By : Mike Micheletti

Amazon's well publicised decision to ban non-certified USB Type-C cables has underscored the importance of compliance testing for USB products.

Spiderman once said "with great power comes great responsibility." The USB developer community working on PD (Power Delivery) technology couldn't agree more. The PD introduction has created a surge in development from OEMs eager to drive the next wave of USB innovation. Yet, a storm of bad publicity for USB Type-C and PD erupted after several reports of damage from poorly designed USB Type-C cable implementations. Amazon's well publicised decision to ban non-certified USB Type-C cables has underscored the importance of compliance testing for USB products. This has led to a new focus on certification for USB Type-C and PD.

In September 2014, the USB-IF (USB Implementers Forum) joined with some of the largest consumer electronics OEMs to release the new USB Type-C interconnect. Designed to enable slim industrial designs plus easy-to-use connectors and cables, the Type-C specification also supports PD 2.0 protocol. PD is an optional feature for Type-C connected peripherals and allows higher power and faster charging for USB devices. The addition of alternate modes that allow DisplayPort and other connections over Type-C cables has captured the imagination of the consumer electronics market.

The PD capability is one of the key features driving adoption of this new technology in the market. Battery powered devices—smart phones, tablets and laptops—are prime applications for the higher charging power offered in PD. Many of these PD capable devices will also possess DRP (dual-role power) functionality, which brings the ability to operate as both a power source and a power sink.

USB Type-C devices that support USB 2.0 or USB 3.1 will be subject to all the existing USB Compliance requirements including PHY, Link, and Framework. In addition, USB Type-C devices will also be required to meet the Type-C Functional Compliance Specification.

Currently considered late draft at this writing, the Type-C spec focuses primarily on the logical state transitions including proper entrance to the "attached" and "unattached" states required for upstream and downstream facing ports.

PD is an optional feature of USB Type-C, specifically for devices that require more than 3 A default current available on the new USB-C interconnect. Understandably, there has been a high level of interest in PD from platform vendors looking for faster charging (laptops, tablets and smart phones). Across the board, these OEMs are eager to launch products with "full-featured" PD capabilities including DRP and alternate modes. These "full-featured" PD and USB Type-C products are substantially more complex to test and certify than those with fewer features. Figure 1 shows the classes of devices that PD supports. Provider Consumer are devices that prefer to function as a Source and will favour negotiating the Source role at initial connect time. Consumer Provider devices are the opposite. They will favour negotiating the Sink role at connect time. Both types will potentially accept a role swap in the future. Typically, a tablet or handset (battery powered) will use the Consumer/Provider class because they have a limited ability to act as a source.

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*__Figure 1:__ Dual Role Power devices are considered “full-featured” PD implementations and are subject to the most comprehensive testing.*

USB PD compliance plan

In its ongoing efforts to promote and ensure successful USB adoption, the USB-IF finalized the Power Delivery Compliance Plan in June 2016. This new test specification defines over 400 assertions that address all the elements of the logical PD protocol, including message formatting and content. The majority of the tests focus on procedures such as discovery, negotiation, and role swaps. Additional tests are provided to measure negotiated power and voltage levels as these are also considered critical for safety. Only PD enabled devices will be subject to the additional tests defined in the plan.

The PD and USB Type-C compliance tests are supported by USB test platforms, which utilise native USB Type-C ports to connect directly to USB devices over the Type-C cable. Designed to emulate PD source and sink behaviours, these testers will programmatically link to DUTs and negotiate power contracts like full-featured PD devices. Mimicking real PD link conditions while capturing power and protocol messages, these systems have become the cornerstone for certifying and testing PD end-products.

Early in the PD Compliance development process, members of the Power Delivery Compliance Working group agreed that OEMs would be required to provide a text file that describes the different PD features the device supports. This VIF (Vendor Information File) is used in almost every test to devise the correct power requests and enable the test system to operate with minimal user intervention. The VIF is the vendor’s declaration that also serves as an official record that the USB-IF relies on to document certified feature sets. Mismatches and simple errors in the VIF file are one of the most common reasons for devices failing USB PD Compliance testing.

For example, DRPs are subject to the most stringent testing because they have to seamlessly operate as either a Sink or Source. These role-swap features allow both power and data roles to be dynamically reversed. On the other end of the spectrum, "Consumer-only" devices that support PD can skip the swap-and-power-source tests. These simpler devices are primarily tested to verify they negotiate and sink power within the defined limits of the PD protocol.

Next: Follow USB-IF's test procedures for compliance »

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