Introduction

With AMD's Socket AM5 platform around the corner, there has been some renewed interest in USB4, as things had been pretty quiet up until the ASMedia ASM4242 controller announcement last month, just ahead of Computex. As such, we thought it was a good time to put together an extensive look at what's happening in the USB space, what USB version 4 offers, and how it compares to solutions like Thunderbolt. Technically, Intel was the first company to launch a USB4-compatible host controller with their "Maple Ridge" Thunderbolt 4 controller, also known as "JHL8340" (single-port) and "JHL8540" (dual-port). For some reason, Intel isn't marketing any of its Thunderbolt 4 products as "USB4 compatible" despite the fact that they appear to be certified.



That said, it was reported almost a year ago that the USB PD controllers from TI Intel has certified for use with its Thunderbolt 4 controllers aren't USB4 certified (Igorslab). This means the whole solution can't be marketed as "USB4 compliant" despite being "USB4 compatible." That's why Intel asked its partners not to certify their Thunderbolt 4 devices for USB4, as they wouldn't pass the certification. It's unclear when this will change.

Intel's discrete Thunderbolt 4 controllers aren't listed in the public USB Implementers Forum database of certified devices even though a pair of its mobile SoCs with native Thunderbolt 4 support are. The database contains around 300 certified USB4 items already, ranging from the aforementioned Intel CPUs to connectors and e-marker chips, a surprisingly large amount of cables, and a few other things. The ASMedia ASM4242 is not yet one of the certified devices, and although that was what was being communicated, it seems to have been misunderstood by the Taiwanese media that were the first to report on the matter. On the other hand, AMD's mobile Ryzen 6000-series CPUs have passed certification, but are not listed in the public database, either, so it might just be some time lag between database updates.

It's unlikely that we'll see anyone outside of Intel and ASMedia producing another discrete USB4 host controller, largely due to lack of demand, but also based on the engineering requirements. That said, at some point in time, it's likely that we'll see various ARM based processors implementing USB4 support. As for Apple, it's unclear if their Thunderbolt 4 implementation will get certified for USB4, but this might, as in the case of Intel, not mean much as Apple's controllers are compatible with USB4. As to AMD's implementation, we don't know much about it either, but more information should hopefully appear as notebooks based on the Ryzen 6000-series start to arrive in the retail market.

That's the current state of USB4 in the market; next we're going to take a closer look at some of the advancements and improvements USB4 brings to the table, and try to clear up some of the confusion that seems to be going around. Some of it is thanks to the rather poor communication by the USB-IF itself, at least when it comes to providing clear information to consumers, as the USB-IF seems to cater largely to its members.

What is USB4?

The best way to think about USB4 is as a subset of Thunderbolt 4, where PCI Express tunnelling is optional (on Thunderbolt 4 it is required). Despite the fact that Intel is trying to downplay the capabilities of USB4 versus Thunderbolt 4, actual implementations on the host side are unlikely to differ much. For example, ASMedia's ASM4242 will support all the key features of USB4: PCIe tunnelling, USB4 speeds of up to 40 Gbps, USB 3.2 speeds of 20 Gbps, and DP Alt Mode, something other USB4 host controllers might not.

The minimal requirements, or common denominators, are a lot more basic as a USB4 host controller, hubs, and devices only need to support data speeds of 20 Gbps. With backwards compatibility to USB 3.2, 10 Gbps is enough, or in other words USB 3.2 Gen 2. This also applies to Thunderbolt, which is why it became the common "fallback" speed. PCIe tunnelling and Thunderbolt 3 support are also optional, alongside charging support and several other more or less important features Intel claims make Thunderbolt 4 the superior technology. However, in its full implementation, there's little difference between USB4 and Thunderbolt 4.

USB4 Tunnelling and DP Alt Mode

One of the key advancements with moving from USB 3.2 to USB4 is something referred to as Tunnelling. The short version of what tunnelling allows is a means of sending multiple different protocols simultaneously over the USB4 fabric. The longer version is that USB4 allows for not just USB data to be sent, but also PCI-Express and DisplayPort signals. Until the signal reaches an endpoint, such as a DisplayPort monitor, the signal is sent as encapsulated data within the USB data stream, and the actual DisplayPort signal only gets extracted once it reaches its destination. This is similar to how Thunderbolt works.



Unlike DP Alt Mode, which physically takes over two wire pairs of the four pairs in a USB Type-C cable for exclusive use for DisplayPort signals, tunnelling allows the DP 1.4 signal to be transmitted simultaneously with USB4 or PCIe signals. At higher resolutions and refresh rates, DP Alt Mode can even use all four data pairs in a USB-C cable, which means USB data transfers are limited to USB 2.0 speeds over a separate set of low-speed wires inside the USB-C cable. When tunnelling is used for DisplayPort 1.4 signals, DP will be prioritised over USB and PCIe data, due to a display signal being isochronous in nature, which means that an interruption to the signal would prevent a picture from being displayed on the screen. Although tunnelling doesn't solve this for high-resolution/high refresh rate displays today, future versions of USB4 should have a solution for this that still allows for high-speed data transfers. Note that Display Stream Compression (DSC) can be used to save bandwidth for the DisplayPort signal when tunneling is used.