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Bench Talk for Design Engineers

Bench Talk

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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics


USB 3.2 Doubles Type-C Cable Speed, Adds To Naming Confusion Lynnette Reese

Scheduled for release any time now, the anticipated USB 3.2 spec will double the number of lanes for data transfer. The USB 3.0 Promoter’s Group, in a press release earlier this year to pre-announce USB 3.2, stated, “USB Type-C cables were designed to support multi-lane operation to ensure a path for scalable performance. New USB 3.2 hosts and devices can now be designed as multi-lane solutions, allowing for up to two lanes of 5Gbps or two lanes of 10Gbps operation…a USB 3.2 host connected to a USB 3.2 storage device will now be capable of realizing over 2GBps data transfer performance over an existing USB Type-C™ cable that is certified for SuperSpeed USB 10Gbps.”

The new spec takes advantage of full-featured USB 3.1 Type-C cables, which have a total of 24 wires, of which eight are for supporting SuperSpeed; they also support Alternate Mode. Alternate Mode (Alt Mode) expands the versatility of USB Type-C cables to support DisplayPort, HDMI, and Thunderbolt 3 (with more to come in PCI-e and Ethernet). USB 3.2 usurps Alternate Mode to use the extra wires in the Type-C cable, and this is how the speed is doubled with USB 3.2-compliant products. Note that doubling the speed has little to do with the Type-C cable, which only needs to support SuperSpeed and Alternate Mode in order to complete the upgrade to double the USB speed.

Note that “full-featured” means a USB cable that supports Alternate Mode. A USB Type-C cable that supports USB 3.1 Gen 1 (formerly USB 3.0) has a top speed of nearly 5 Gbps.  If a USB 3.1 Gen 1 Type-C cable also has enough wires to support alternate mode and connects two USB 3.2-compliant products, that cable will have a 10Gbps throughput. A full-featured USB Type-C cable that supports USB 3.1 Gen 2 has a top speed of 10Gbps, but if that same cable connects two USB 3.2-compliant products, the cable can pass 20Gbps.

One of the more confusing issues with the increasing complexity USB is the number of names and specifications concerning speed, cables, power delivery, and alternate technologies that USB Type-C cables support. It seems clear that Alternate Mode and USB 3.2 are mutually exclusive. There is almost a separate specification for each USB benefit, as outlined in Table 1.

Table 1: The USB specifications that can be involved in the making of a USB Type-C cable.

Specification Also Known As (a.k.a) Description
USB Type-C USB-C Versatile cable specification. While a definite understatement, there’s only so much one can put in a table.
USB 2.0

High Speed

USB specification for up to 480Mbps. Requires only four wires. Works with both legacy and Type- C cables.
USB 3.1 (Gen 1) SuperSpeed,
USB 3.0
USB specification that includes speeds up to 5Gbps. Eight wires are needed to support USB 3.1 Gen 1/SuperSpeed. Works with both legacy and Type- C cables.
USB 3.1 (Gen 2) SuperSpeed +,
SuperSpeed 10,
USB 3.1
USB specification that includes speed up to 10Gbps. Eight wires are needed to support SuperSpeed. Supported in Type-C cables.
USB-Power Delivery USB-PD USB specification for up to five levels of power delivery for up to 20V at 5A for 100W. Five profiles exist that deliver: 10W, 18W, 36W, 60W, and 100W. Supported only in Type-C cables.
USB-Alternate Mode   Allows USB Type-C cables to be used for other technologies like DisplayPort, HDMI, MHL, and Thunderbolt, with potential for others in the future. May require USB-PD’s initial negotiation capabilities. Supported only in Type-C cables.
USB 3.2   Doubles speeds of USB 3.1 Gen 1 & 2 Type-C cables by using multiple lanes that could alternatively be used for other modes. Supported only in Type-C cables.
USB Battery Charging 1.2 USB-BC Legacy battery charging specification. Co-exists with USB-PD.

 

Type-C cables, now more universal than USB, lend themselves to multiple possibilities for confusion for consumers who might conclude that any USB Type-C cable “does it all.” A legitimate Type-C USB may support only USB 2.0 speeds, which means that it would only have 4 wires. At the other end of the spectrum, another USB Type-C cable can support up to 20Gbps (with USB 3.2-compliant products on both ends) can alternatively be used as an HDMI, DisplayPort, or Thunderbolt cable, or be used to communicate at 10Gbps while simultaneously powering (in either direction) up to 100W.

What’s more, USB packaging doesn’t clearly identify the modes of power operation or USB specification. The USB Implementers Forum (USB-IF) has established different logos to help identify what maximum speed the cable provides. The logos are the high-speed trident, the SuperSpeed trident, and the SuperSpeed 10 trident (a.k.a. SuperSpeed+). As of this writing, I was not able to find a logo to identify power ratings. However, a logo for the Certified USB Charger 45W was posted in mid-2016 on the USB.org site (Figure 1).

Certified USB Charger Logo

Figure 1: The Certified USB Charger 45W logo represents compliant USB products that have passed certification, identifying the charging capability of the (Type-C compliant) product in watts. (Source: USB.org)

According to an STMicroelectronics data sheet (Figure 2), six types of C-to-C cables can be gleaned from the USB Type-C specification: One that does USB 2.0 only, one that does USB 3.1 Gen 1, and one that does USB 3.1 Gen 2. In addition, each separate cable can also have one of two power ratings for 3A (a regular Type-C cable) or 5A (cable/device/host that support USB-PD). Suffice it to say, USB Type-C connectors are not all automatically operational at USB 3.1 speeds, nor at 5A.

USB modes of operation

Figure 2: USB modes of operation for power delivery. A USB Type-C cable can deliver up to 3A. If USB-PD is employed, then the Type-C cable can deliver up to 5A, depending upon the highest USB-PD profile that is supported. (Source: STMicroelectronics)

But again, this information isn’t available on USB package labels. According to the USB Implementers Group Forum (USB-IF, the group behind USB technology) Type-C will be used by over 2 billion devices by 2019, compounding the potential for confusion even more. As quoted from the USB-IF Developer Days Hong Kong USB-PD presentation on Day 2, “USB technology has evolved into highly complex and challenging designs.” It’s time for the USB-IF to host classes, online videos, or an edx.org course on how these cables work, what to expect, and what to look for. Not all compatible products are compliant, which makes a “buyer beware” situation for consumers. We want more from a single cable, but ironing out the wrinkles with the debut of the USB Type-C cable brings an awkward pause to the celebration of the most universal connector.



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Lynnette Reese holds a B.S.E.E from Louisiana State University in Baton Rouge. Lynnette has worked at Mouser Electronics, Texas Instruments, Freescale (now NXP), and Cypress Semiconductor. Lynnette has three kids and occasionally runs benign experiments on them. She is currently saving for the kids’ college and eventual therapy once they find out that cauliflower isn’t a rare albino broccoli (and other white lies.)


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