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USB 4.0 Will Enhance USB3 Vision Standard to Push PCI Express to the Device Edge

POSTED 10/06/2020  | By: Winn Hardin, Contributing Editor

What if you could achieve the same optimized PC-host system performance without a frame grabber as you could with the frame grabber? That’s part of the appeal of a new enhancement to USB3 Vision standard under development today.

USB Everywhere

The USB3 Vision standard gained widespread acceptance from the machine vision industry because of the availability of USB ports on virtually every computational device, including PCs, laptops, and embedded systems. Additionally, each time the USB standard increases its capability and bandwidth, the machine vision industry USB3 Vision standard also automatically benefits, according to Bob McCurrach, AIA’s Director of Standards Development. For example, USB 3.0 initially offered 5 Gb/s, followed by 10 Gb/s for USB 3.1 and 20 Gb/s for USB 3.2, with the addition of multiple data lanes.

The enhancements that USB 4.0 enables for USB3 Vision represents a much more significant increase in capability, according to Eric Gross, Principal Engineer of Vision at NI and USB3 Vision chairman. “When USB 4.0 came out, not only did it increase speeds to 32-40 Gb/s, but also it introduced another technology — Thunderbolt 4 — to the standard. (Currently, Camera Link HS can achieve upward of 48 Gb/s, while CXP can achieve 25 Gb/s with plans to extend to 50 Gb/s.) Thunderbolt is even higher performance than USB by itself, encapsulating a new protocol that lets you utilize PCI Express bus under the hood.”

Direct PCI Express Channel Between Device, PC

Today, PC-host systems that use frame grabbers offer higher performance than USB3 Vision and GigE Vision because the frame grabber is optimized for machine vision purposes in ways that USB3 Vision and GigE Vision are not. The hope, Gross says, is that by leveraging Thunderbolt, USB3 Vision can achieve the identical efficiency of a frame grabber solution with all the conveniences of the USB-based camera.

In addition to extending the PCI Express bus out to the peripheral device, the USB 4.0/Thunderbolt standard also delivers faster memory access. While USB 3.0 leverages direct memory access (DMA), every data transaction still has to go through the USB 3.0 host controller, which adds some amount of overhead and latency. “Whereas with Thunderbolt, the overhead goes away because we can optimize the mechanism of how the PCI Express transfer works and match it to the needs of machine vision,” Gross explains.

USB4.0 offers several other benefits to device and system designers. For example, as USB devices take advantage of faster speeds on the newer versions of the standard, existing off-the-shelf ASICs for interfacing camera sensors and FPGAs to the USB bus are not readily available. Direct implementation with FPGAs becomes expensive due to the high-speed serial interfaces needed, not to mention the USB cores that must be integrated. With Thunderbolt technology, there are off-the-shelf chipsets supporting the full 40Gbit speed that can interface through PCI Express to FPGAs in a simple manner for implementors.

Thunderbolt 4 technology leverages the same cables as the previous USB 3.2 standard. While active cables are needed to get longer distances at the higher speeds, USB’s position as a mass-market interface should ensure the technologies for range-extended cables becomes available.

According to Gross and McCurrach, the eventual enhancements to the USB3 Vision will leverage the majority of the USB3 Vision standard as it is today. “USB3 Vision is layered on top of the USB 3.0 standard,” Gross says. “USB4.0 to the USB3 Vision standard will define a separate layer of data transfer just for PCI Express — that’s the only difference between USB3 Vision over Thunderbolt — so the vast majority of the standard is unchanged. From an efficiency standpoint, combining USB and Thunderbolt into one standard, potentially with one driver package that works for both, just makes sense.”

“Thanks to the latest chipsets available for PCs, from the host side, most newer hosts with Type C ports will natively support Thunderbolt as well as USB, so the user gets to choose between USB and higher capacity Thunderbolt,” McCurrach added. “From the machine vision camera or device side, the OEM will get to choose based on the performance they want to achieve.”

According to Gross and McCurrach, the AIA USB3 Vision Standards Committee, USB4 Working Group is finalizing a white paper that lays out the design of this addition to the standard. That paper is expected early in 2021. For more information about USB3 Vision Standards Committee, the USB4/Thunderbolt 4 standard, and related information, please contact Bob McCurrach.