USB-C: much more than a reversible cable

We are all familiar with the ‘new’ format USB-C connector these days. The ‘micro’ connector is almost retired and I can’t remember when I last saw a ‘mini’ in the wild. But USB-C is about much more than the (admittedly convenient) reversible connector. It’s also about 80 Gbit/sec data transfer rates and 240W of power delivery!

USB Type-C, commonly referred to as USB-C, has become increasingly prevalent on consumer electronic devices for data transfer, power delivery and audio/video functions. Its adoption is about to be turbocharged owing to recent EU legislation mandating its use as the standard wired charging port for all new products across a broad range of categories.

Evolution of the USB-C ecosystem

The USB-C ecosystem is complex and presents numerous considerations for the system designer.

Like its predecessors, USB-C is an open standard developed by the USB Implementors Forum Inc. (USB-IF), a non-profit organisation consisting of a consortium of companies, including major technology manufacturers like Intel, Microsoft, Apple and many others. USB-C was originally introduced in 2014 and represents a significant advancement in USB connectivity due to its reversible and versatile design, support for higher data transfer rates, and provision for new features including power delivery and alternate modes. The connector was developed to address the evolving needs of modern electronic devices and simplify the connectivity of peripherals and accessories. As an open standard, USB-C can be implemented by manufacturers without the need for proprietary licenses, which has contributed to its rapid adoption.

The EC has recently introduced legislation to harmonise the use of the USB-C connector as a common charging port, to harmonise USB Power Delivery as a common fast-charging system, and to unbundle the sale of the charger from the sale of the electronic device. The new rules will apply to all new handheld mobile phones, tablets, digital cameras, handheld videogame consoles, headphones, headsets, portable speakers, e-readers, keyboards, mice, portable navigation systems and earbuds sold in its member countries from Autumn 2024. It will expand to include laptops from 2026. The EC’s rationale for introducing the new rules is to bring benefit to the consumer (reduced spend on standalone chargers, reduced problems due to non-compatible chargers) and the environment (reduction in e-waste from disposed and unused chargers). The new rules have been captured within a revision of the Radio Equipment Directive, which defines essential requirements that radio products must meet to gain the CE mark and to be marketed within member states.

The new rules are not a result of the EC’s first attempt at harmonising charging ports on smartphones. In 2009, the EC negotiated a Memorandum of Understanding (MoU) that was signed by 14 prominent smartphone producers (including Apple) to adopt the USB Micro-B connector for charging ports. This MoU offered a significant loophole however, as it permitted the use of adapters for devices that did not have a built-in Micro-B interface, which allowed Apple to produce iPhones using their proprietary Lightning connector (introduced in September 2012). At this point however, the Lightning connector was technically superior to the Micro-B – it offered greater durability, reversible fit, built-in security for authorised cables, and in specific cases faster charging and higher data transfer speeds – and from Apple’s perspective allowed collection of royalties through their MFi Program. The USB-C connector has now largely superseded the micro-B connector, and itself is arguably technically superior to the Lightning connector (it offers significantly faster data transfer rates and higher power delivery, albeit with a marginally larger size). Apple has pushed-back against the new legislation, stating “strict regulation mandating just one type of connector stifles innovation rather than encouraging it, which in turn will harm consumers in Europe and around the world.” However, it appears to have conceded with recently released iPhones and AirPods products transitioning to the USB-C ecosystem.

Power delivery

The power transfer capability of the USB interface has increased significantly over evolutions of the specification; a USB 1.0 or 2.0 host is able supply a maximum of 2.5W (5V at 500mA, perhaps typical for a desktop keyboard and mouse), whereas the latest USB Power Delivery (USB-PD) specification (v3.1, announced in 2021) can provide up to 240W (48V at 5A, sufficient to fast-charge a high-performance laptop). Prior to the introduction of USB-PD, the USB bus voltage was fixed at 5V, which could only support modest power levels (up to 15W, in specific cases) due to current limits of interconnects. USB-PD allows devices interconnected with certain USB-C to USB-C cables to handshake with each other and dynamically agree a ‘power contract’, which support standard fixed-voltage operation at 5V, 9V, 15V, 20V, extended operation at 28V, 36V or 48V, or operate in a Programmable Power Supply (PPS) mode, allowing the device being powered to request intermediate voltage. All USB-C cables must be able to carry a minimum 3A current; some cables can support up to 5A, but must contain an e-marker chip to identify the cable and its capability. In addition to providing vastly higher maximum power transfer capabilities, USB-PD offers additional features which may benefit the system designer:

• Power direction is not fixed – known as Dual Role Ports. A USB-C port on a tablet may be used to charge the device or to provide power to a connected accessory. One smartphone may be used to charge another. A port’s power role can be independent of its data role.
• Power direction can change quickly – known as Fast Role Swap. Consider a laptop being charged by mains adapter, connected via a USB-C hub, to the laptop; if the external power is interrupted, the USB-C port-pair between the hub and laptop may perform a Fast Role Swap in which the laptop changes from receiving power (charging) to providing power to the hub (thus presenting an uninterrupted supply to any connected peripherals).
• Optimisation of power at system-level – low power devices (e.g. headsets) may negotiate for only the power they require, thereby maximising the power available for other peripherals, or a system may employ dynamic power management across multiple peripherals (e.g. a peripheral may dynamically negotiate a higher power for a given application).
• PPS modes can simplify device-side battery charging – a device may request a supply voltage to match the state-of-charge of the internal battery (and therefore reduce device-side conversion losses).

Gbit/sec data transfer

USB data transfer capabilities have also advanced substantially. USB 1.0/1.1 (providing up to 12Mbps operation, ‘full speed’) and USB 2.0 (providing up to 480Mbps operation, ‘high speed’) communicate using a single differential pair between a host and device. USB 3.0 and above use additional signalling pairs for higher speed operation, while maintaining the USB 2.0 pair for backwards compatibility. The USB 3.x generation standards provide evolutionary steps to increase bandwidth (USB 3.0 supports up to 5Gbps, USB 3.1 up to 10Gbps and USB 3.2 up to 20Gbps using bonded 10Gbps channels). The various data configurations offered by the USB 3.x standards, in addition to supporting variants of Type-A, Type-B and the Type-C connector, result in a complex ecosystem and nomenclature. USB4 further increases bandwidth (up to 80Gbps, USB4 Gen 4), and reduces configuration options (USB4 mandates the use of both Type-C connectors and USB Power Delivery). Further, USB4 is aligned to the Thunderbolt specification (developed by Intel in collaboration with Apple and later donated to USB-IF).

Physical interfaces

The longevity of the USB-C connector remains to be seen – its predecessors (USB Micro-A/B, Apple Lightning, and to lesser extent Mini DisplayPort) are giving-way to the USB-C connector – and the data and power delivery performance of the interface is currently outpacing demands of devices. Interestingly, the EC’s common charger rules do not include wireless charging – which remains a developing technology – so perhaps Apple’s next move will be to develop a connector-less iPhone. Also notably omitted from scope are wearable devices, including smart watches and fitness trackers, which may continue to be supplied with proprietary charging solutions.

The long-term environmental benefits of moving to a common charger makes sense subjectively. However, the EC’s own statistics suggest that the proportional reduction in annual e-waste from disposed and unused chargers will be less than 10%. Further, there will inevitably be a short-term spike in e-waste, as consumers dispose of now-legacy chargers and cables.

Conclusion

USB-C doesn’t just describe a convenient connector format. It describes a sophisticated system for data connectivity and power delivery. Like its forebears, it can provide a 5V supply and a serial data interface for simple devices. But it can also distribute 100s of watts of power among multiple peripherals while delivering Gbit/sec data throughput. If you find yourself designing systems with USB-C, take care to clarify what’s being asked for when you read ‘USB-C’ in a specification.