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Exploring the design implications of USB Type-C

Posted: 26 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:USB? USB Type-C? configuration channel? biphase mark coding? downstream facing port?

Do you recall the old spiritual song "Toe bone connected to the foot bone; Foot bone connected to the heel bone," etc.? I hum that sometimes when I'm trying to figure out how to navigate the innumerable and multi-generational USB cables lying around the house.

With USB, there's always been a Type-A connector and a different Type-B connector and therefore different device slots to find and fit. But those "which way is up?" days are fast coming to close with the launch of USB Type-C, a universal bidirectional cable that's being designed in as we speak to devices around the world.

Max Maxfield does the history of USB connector technology his usual delightful justice. A sampling:

"I fear that the creation of Type-A and Type-B connectors is where the creators of USB started to get a tad over-enthusiastic and went a little awry. First of all, these connectors are polarized, which means you have to work out which side is the 'top' before plugging them in. I cannot tell you how many times I've tried to plug one in the wrong way over the years, which has resulted in much gnashing of teeth and rending of garb, let me tell you."

Plug and Play
These will become distant memories as Type-C connectors begin to flood the marketplace, promising to eliminate teeth gnashing and garb rending. USB Type-C is a single connector at both ends, non-polarized, that delivers speeds up to 20 Gb/second and power delivery of up to 100W for fast charging.

In addition to making users' lives easier, there are myriad implications, all the way from how (and how many) power supplies will be designed in the future to how device form factors will shrink thanks to Type-C's tiny (24 pins, 3mm high x 8mm wide) size. For electronics systems designers, it's time to suit up because USB Type-C starts a whole new ball game.

"Type-C strictly speaking is not just a connector standard. It's much more inclusive than the connector," said Charles Qi, systems solution architect with Cadence. "It's a very big deal for the USB community and a very big deal for IP developers as well."

New design considerations
Here are some issues to be aware of:

In order to detect the cable orientation properly, there is an analog detection circuit to be designed around the configuration channel (CC) pin. Based on the detection, the USB 3.x port needs to selectively drive one of two sets of pins defined for the Type-C connector. This is called bus routing selection. This can be achieved through analog switches or two sets of transceivers that can be independently activated.

USB Type-C supports two additional current grades, 1.5A and 3A, to be delivered through the new cable, in addition to the 900mA and 500mA limits supported respectively by USB 3.1 and USB 2.0. USB Type-C allows the downstream facing port (DFP) to advertise its current sourcing capability through the CC pin.

Additionally, the USB Type-C cable may support the USB Power Delivery standard for up to 5A and 20V over VBus. So you have to make sure the other side can't be damaged. This means there needs to be an automatic discovery mechanism through the cable to figure out what is the capability of the device that gets connected to the cable, Qi said.

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