To USB or Not to USB


Relationships Between USB Specs

Welcome to Factual Friday.

This week I wanted to clarify, with accuracy, the relationships between USB Type-C, USB 3.1, Power Delivery and DisplayPort specifications. If you are designing the USB portion of your latest and greatest product then understanding these relationships is critical to ensure you are developing against the right set of specs. Of course if you are just integrating the DesignWare USB Solution IP into your design then you really don’t have to worry about this complexity as we’ve done the worrying for you, however I bet you still find this interesting. Thanks to Morten Christiansen, Synopsys’ USB technical expert, for the background data which enabled me to write this blog.

Trust me, this is the simple version, I might have been a little wordy… never me….

So to begin with, here is the simplified view of the relationship between the latest USB specifications and the linkages to the MIPI and VESA specs. Click the image for the larger view.

Relationships Between USB Specs

Starting in the middle you have the popular USB specifications with the classic USB 1.1 and the ubiquitous USB 2.0, aka Hi-Speed USB, at the bottom. There are related specifications supporting USB Charger detection, confusingly called Battery Charging but trust me it was part of the now defunct USB Power Delivery 1.0 specification. While USB 3.1, with blazingly fast transfer speeds, has been in the news more lately, USB 2.0 is far from its end of life. USB 2.0 is still used across products in multiple market segments and recently has become hot, hot, hot for use in the Internet of Things (IoT) products. Within IoT the benefit of USB 2.0 is that it supports the data transfer needs, internal connectivity, manufacturing and test, recovery modes, debug as well as of course power delivery for portable devices. The race to get the next killer IoT product to the market has driven the creation of a new breed of DesignWare USB IP (USB 2.0 and the classic USB 2.0 Full Speed aka USB 1.1), specifically customized to the need of the IoT product with a focused feature set and greater ease of use for rapid integration. This new solution is the lowest risk USB IP of choice delivering highest quality, reliability across Process, Voltage and Temperature variants with a support infrastructure you can trust.

Moving up the USB chain (in time) you encounter the USB 3.0 specification, aka SuperSpeed USB which introduced the 5 Gb/s data transfer rate. USB 3.0 SuperSpeed was designed to offer this higher data transfer speed to devices that needed it as well as maintaining backward compatibility with USB 2.0 to ensure interoperability and a smooth technology transition. Helping designers transition to USB 3.0 is the DesignWare USB IP which reduces the impact to the SoC by delivering up to 50% smaller area, lowest power consumption while maintaining the highest performance operation. Using the DesignWare USB 3.0 IP can save you three months or more of effort.

Now as we get to USB 3.1, aka SuperSpeed 10Gbps, (aka SuperSpeedPlus) things start to get a little more complex. The first is that there is a Gen1 and a Gen2 USB 3.1. USB 3.1 Gen1 defines the 5Gb/s data transfer rate, the same as the older USB 3.0 specification. The USB 3.1 Gen2 introduces the SuperSpeedPlus data transfer rate of 10 Gb/s, a 2x improvement. But USB 3.1 Gen 2 also introduces a slew of other capabilities such as improved data encoding for more efficient data transfer, higher through-put and improved I/O power efficiency. Again the USB-IF thought about backward compatibility and defined USB 3.1 to be backward compatible with USB 3.0 (USB 3.1 Gen1) and amazingly USB 2.0. Synopsys mirrored this in our DesignWare USB IP by making the programing interface compatible across DesignWare USB 2.0, USB 3.0 and USB 3.0 IP solutions meaning you can use the same software drivers. You can again trust Synopsys’ IP solution as proven by the fact that we, Synopsys, were the first to receive full USB-IF certification.

Next up is USB Type-C aka, USB-C. This is typically where confusion starts as it’s mixed up with USB 3.1 and I’ve blogged on this topic before. Summarizing, USB Type-C defines a connector standard while the USB 3.1 defines a technology standard. USB Type-C was designed to support USB 2.0, USB 3.0 and USB 3.1, which you can see represented in the above diagram. It’s the USB Type-C specification which intersects with MIPI and VESA.

The USB Type-C specification linkage with MIPI is based around efficient debug which has been driven by the MIPI Debug Working Group. The direct intersect is Gigabit Debug for USB which is built on top of the MIPI Trace wrapper and System Trace, SneakPeek specifications. Here is a great article on SneakPeek, Gigabit Trace Framework and Gigabit debug for USB. Look out for a White Paper from Synopsys soon. Formally, MIPI Debug does not require USB Type-C but there are additional MIPI specification developments and the recent USB Debug Accessory spec that makes Type-C the connector of choicefor debug.

The USB Type-C specification linkage to VESA is driven by what is commonly known as “DisplayPort alt mode” which is DisplayPort running over USB Type-C. The latest DisplayPort specification defines up to 8K at 60 Hz A/V performance which require DisplayPort PHY’s similar to USB 3.1 Gen2, 10 Gb/s performance. Synopsys already provides an IP solution supporting these latest specifications, USB 3.1 Type-C with DisplayPort.

Hopefully at this point you can see that there are individual specifications for USB, DisplayPort and debug and that it’s the USB Type-C specification where they intersect. Even with this USB-C can be used separately for USB 2.0, USB 3.0 and USB 3.1 or these can be used without the USB Type-C, but who in the world would do that…..

Finally sprouting out of the top of USB Type-C is the new USB Power Delivery 2.0 and the Port Controller Specifications. However searching for a Type-C Port Controller (HW) Specification gives no results. Nor is there a corresponding Type-C Port Manager (SW) specification. Only the HW/SW interface is specified since the HW specification is inherent from the Type-C spec. Some of the SW spec is inherent from Type-C, USB 3.1 and Power Delivery specification…….

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Seems simple doesn’t it. Just remember this is the simplified version and might have brushed over some of the details. I like to say “Don’t let thetechnical details get in the way of a good story”. I say this and others by the way, see here. In this case I’ve presented technical accuracy but have brushed over the multitudes of other spec’s that string along with USB.

I’ve been building a Van de Graaff Generator at home and I must admit I’m happy with the way it’s turned out. Check out the 4-6″ sparks I can generate. The 2nd picture is a little grainy but I love the plasma spark vs. the full on lightening effect.

Mick;s Van de Graaff Generator in full effect

Mick;s Home Made Van de Graaff Generator

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