The confusion that is USB C Cable and Power Speeds
OK, so I’ve finally spent the hour to go through every USB C cable I have and try to figure out how fast they are and how much power they can deliver. Digital Trends does a pretty good job of explaining the speed “crisis” which is basically that the USB C connector has literally half a dozen different speeds depending on the cable you buy (and close to a dozen power standards as well from 12W all the way to 100W (and headed to 250W). And they can carry a variety of different video signals as well using DisplayPort signaling.
How does all this work? Because USB cables are five twisted pairs
As Big Mess of Wires explains that’s because a USB cable actually has five different data wire pairs and you can use them differently which is the good mess of confusion, so here are some of the modes used with these five cable pairs:
- One pair (called D+/- is just for the older USB2 480Mbps).
- Split USB/DP mode. In this mode, you are using two pairs of USB for data and two pairs of DisplayPort to send video. The confusing thing here is that depending on whether the cable is the normal orientation (you have to pick one), then USB sends signals on TX1 pair and gets data from RX1 pair (pins A2/A3 are TX1 and pins B11/B10 are RX1). And DisplayPort is on TX2/RX2. If the cable is “flipped” then these reversed.
- DP Only. If you do not need any USB on the connection, then the DisplayPort 1.3/1.4can take over the whole cable and get all four twisted pairs. And each lane is 8Gps. As an aside to convert this into what resolution and speed. 4 lanes is enough for 4K at 120Hz with 24 bit color or 8K at 30Hz with standard color.
- Thunderbolt. Or you can throw all that away and just carry a completely different set of data that is encodes with PCI Express. That is what Intel did in a proprietary way with Thunderbolt 3 (Thunderbolt 1 and 2 used their own connectors, but 3 and 4 use USB C)
- Power. You can use any of these pairs to carry power depending on how beefy they are.
The net is that when you get a new cable, it isn’t crazy to get your label maker out and label is “properly” so you can figure out what it can do with a name like, “USB3 5Gbps/100W/DP 1.4 lane”
USB A/B/C only tells you the shape of the connector
So as a reminder, here is a decoder ring for you is that the first letter is the type of connector and there are three:
- USB A. This is the very old now square adapter. You mainly find these on old printers these days
- USB B. This is the “normal” rectangular flat connectors
- USB C. This is the oval reversible one that is on most new devices
USB 1/2/3/4 tell you how fast serial data goes
Then there is the speed at which these things run. This is the version number thingy that you oftentimes see and the big ones are listed here along with the name that @richtong thinks about these things)
- USB 1.0 (USB1 12Mbps). You almost never see these around these days but it was 12Mbps I think.
- USB 2.0 (USB2 480Mbps). This up to 480Mbps and is the most common speed for things like mice etc.
- USB 3.0 (USB3 5Gbps). This is up to 5Gbps and is “normal name”, but in the infinite wisdom of the USBIF, the correct name is USB 3.2 Gen 1 (confusing right) and the horrible marketing name is SuperSpeed USB (which I never see).
- USB 3.1 (USB3.1 10Gbps). This is up to 10Gbps but has gotten renamed to USB 3.2 Gen 2, ok that is confusing. And another horrible name but at least the speed is in in it SuperSpeed USB 10Gbps
- USB 3.1 Gen 2 (USB3.1 20Gbps) . Up to 20Gbps I can now see why they did this rename as it is called USB 3.2 Gen 2×2 wow is that terrible, but because it is 2 channels I can sort of see why they did this.
- USB 4 Gen 2×2 (USB4 20Gbps). And of course, they really blew it up with USB 4 which uses now-free Thunderbolt 3 technology from Intel to go faster. And they continue this weird naming, although you will see the much more sensible tradename of base technology and then the speed
- USB 4 Gen 2×3 (USB4 40Gbps). The highest speed if you can get it and it actually compatible with Thunderbolt 3 (Intel basically gave up its patents) because it uses the same protocol, so we’ve at last converged Thunderbolt and USB as a data protocol. So the big deal is the end of static allocation of bandwidth
Some pairs can transport DisplayPort Alt Mode and pass HDMI over it
Then for video which uses DisplayPort, you have these choices and note how DisplayPort has to track HDMI performance since with an HDMI adapter, you take HDMI signals and push them over DisplayPort. The technical term by the way is you use DisplayPort Alt Mode to work with the USB C physical cable.
- DisplayPort 1.2. Over four lanes, this is 17Gbps in HDR2 mode (High Bit Rate v2) with 4Gbps per lane which is enough to support HDMI 2.0 which need 17.92 Gps enough for 4K at 60Hz with no chroma compression 4:4:4
- DisplayPort 1.3. This moves to HBR3 and gives you up to 32Gbps with 8Gbps per lane. So this is enough for 4K 120Hz with 24-bit color or 8K 30Hz with 24-bit color. Finally there is Multistream Transport (MST) so you can drive two 4Kp60 at 24-bit color. And for HDMI and DVI adapters, it support HDMI 2.0 and HDCP 2.2 content protection carried over DisplayPort. Confused yet?
- DisplayPort 1.4. It uses the same HBR3 at 8Gbps per lane but it adds Display Stream Compression 1.2 (DSC) and Rec.2020 color space for HDR on HDMI
- DisplayPort 2.0. A brand new standard just coming out with a bump to 10Gbps per lane so 40Gbps maximum and works with 8K@60Hz displays. As an aside, if you only have two lanes because you need two for USB, then you can see show 8K at 30Hz at 10 bit per color (which is 30 bpr HDR uncompressed 4:4:4 color)
HDMI Alt Mode to carry dedicated HDMI Signals
Most devices are using DisplayPort Alt Mode since you get USB support too, but there are few with native HDMI Alt Mode. So given all this, how much bandwidth do you need for a given resolution but you can also carry HDMI directly over these pairs as of 2016. As HDMI Organization, Prodigital, Belden and Benq explain it is complicated because what is actually happening is that you need to convert the many HDMI pins into the fewer number of USB C pairs. Basically, you have to reconfigure the four data pairs into clock and the three HdMI TMDS channels
you have to ask if the cable is support full color or doing what is called chroma subsampling, that is are you carry all the colors called 4:4:4 or are you compressing to 4:2:0 or 4:2:2. This works because we are less sensitive to some color changes than others so a quick chart that shows what each standard is (and also notes how DisplayPort can carry)
- HDMI 1.3. 10.2Gbps and no 4K support.
- HDMI 1.4. 10.2Gbps supporting 4K@30Hz SDR. This requires two DP 1.2 lanes
- HDMI 2.0. 18Gbps supporting 4K@60 HDR. This needs four DP1.3 lanes. 4K video at 8 bit color aka 24 bit color aka SDR at no compression of chroma 4:4:4 requires at 60fps. So this needs 17.82Gbps and this is the maximum supported by HDMI 2.0.
- HDMI 2.1. 48Gbps supporting 4K@120Hz HDR and 8K@60Hz HDR. And this is just coming out. Note that this standard actually overruns the DisplayPort 2.0 bandwidth of 40Gbps, so there is a lot of confusion about what will happen here.
USB Power Delivery configures charging over some
Finally, in addition to the two data pairs and the differential pair (just for USB 2), there is a VBUS and GND pin that can deliver power in addition to data with USB Power Delivery as All About Circuits explains which use the Channel configuration twisted pair (CC1 and CC2) so that the two devices can negotiation for things like power needed and what wires are running in USB vs Alternative mode. And this is the reverse priority
- USB 2.0. In the USB B days, the standard was 5V at 500mA or 2.5W which was basically for a mouse.
- USB 3.1. Folks moved up by increasing the amperage. The tradeoff, of course, is the wires have to be thicker so for instance 5V@900mA which is 4.5W, or the 5W charger that Apple has used for years.
- USB BC 1.2. This is the final boost to 5V at 1.5A or 7.5 watts
- USB Type-C Current @ 1.5A. This is the same 7.5w
- USB Type C Current @ 3A. So this is 15W
- USB Power Delivery 1.0. Finally, just increasing the current so you can configure voltage and a 5A maximum current. 5V x 2A = 10W, 12V x 1.5A = 18W, 12V x 3A = 36W, 20V x 3A = 60W, 12V x 5A, 20V x 2A = 60W and finally 20V x 5A. Not all cables support this as you need a beefy enough wires so it doesn’t burn out.
- USB Power Delivery 2.0 dropped fixed profiles and they are more flexible with different amperages, so for instance at 5V you can get to 0.1-3A, then 9V goes 1.67-3A, then 15V to 1.8-3A, and finally 20V to 2.25-3A. so 60W is pretty standard. You need a properly rated cable to go to 3-5A at 20V.
- USB Power Delivery 3.0 introduces Programmable Power Supply protocol so you can now configure voltages in 20mV increments but only a few devices can handle it.
Thunderbolt 3/4 on USB C as Thunderbolt Alt Mode
The thing that Thunderbolt does is that it changes the world by taking over all four of the data pairs and using it’s own protocol. Basically, this is Intel technology which puts the PCI Express protocol as the native communications scheme and puts 4 lanes of PCI Express. The system then overlays on this, the USB and display traffic. This means that there is no hard staring like USB/DP mode and you get flexible bandwidth.
- Thunderbolt 3. Up to 40Gbps OK, want to be really confused, the Intel Thunderbolt specification uses the same USB C connector handles it signaling differently. They actually put serial PCI Express on it running at 10Gbps on each lane, so that’s how you get 40Gbps and two DisplayPort 1.2 streams so can handle two 4K@60 HDR monitors at once over a single connector. It also supports PD 3.0 so up to 100W through the power side. As an aside, the traffic is flexible, there is a 40Gbps overall ceiling in any combination of 4 lanes of PCI Express 3.0 (8.1Gbps per lane) and 8 lanes of Display Port 1.2 HBR2 (34.56Gbps). So if you want those two streams as an example, then only 8Gbps is left over for PCI Express 3.0 data. It is compatible with USB4.
- Thunderbolt 4. Up to 40Gbps as well (and only codified in July 2020), but it is compatible with USB 4 and can handle DisplayPort 1.4 and needs at least 32Gbps for the PCIe Link. It also uses Thuderbolt Alternative Mode USB hubs so you don’t have to just datachain over it. Finally, since USB 4 support DisplayPort 2.0 alt mode, you get 8K at 60Hz at HDR10 color. Note that since DisplayPort is not uni-directional, it can actually run at 80Gbps by turning all four pairs in the same direction.
A note on dongles and using separate USB C or Thunderbolt for 4K at 60Hz
Most of the time, you will want to hook your monitor directly into the computer so you don’t have to share lanes, but most dongles with HDMI input and a bunch of USB ports going into USB3 only provide two lanes of DisplayPort (so this is why it is hard to find a dongle that supports 4Kp60 with USB as well) and the trick that these dongles use is that they only support the slow USB 2.0.
The only way out of this is to get a Thunderbolt 3 or 4 hub since it has 20 or 40Gbps of data, then the DisplayPort data is encoded into the Thunderbolt data stream. There is not dedicated twisted pair. Then you can 4K60SDR