Seshan's Blog

keyboard_backspaceBack to Posts
USB Power Delivery All The Things!
September 18, 2019

It’s 2019, and we’re seeing USB-C everywhere, and it’s awesome!

One of the things that really excites me is USB-PD, a technology that falls under the USB-C umbrella that allows devices to negotiate up to 100W of power.

Every since I got my new Macbook Pro, I’ve loved that fact that pretty much anything can charge my laptop. At the high end there is the official Apple 87 Watt charger, but I’ve even managed to trickle charge my Mac with a regular-old USB 2.0 wall adapter. It’s not actually that useful, but it does work!

I was recently helping someone fix their laptop (it was stuck in a Windows boot loop). One of the problems was he didn’t have his charger nearby, and without being able to actually boot Windows we had no idea if the laptop would last long enough (thankfully it did).


And so herein lies the problem: the laptop had a USB-C port, but didn’t support USB Power Delivery. And the worst part, there is no way to know this just by looking at the port. And so begins the long list of USB-C problems…

Even worse, there isn’t really a way to know the capabilities of a USB-C port visually at all. USB-C is a huge standard: USB-PD, ThunderBolt, Alternative Modes. Technically a USB-C Physical port can even just be a USB 2.0 host!

Even on a physical level there have been problems, back in 2016 there were reports of cheap USB-C cables causing ports to explode. Another example is the Nintendo Switch, which is advertised as USB-C compliant but actually deviates from the spec, both physically and electronically, causing many problems with third-party accessories.

Another problem is that USB-C isn’t that trivial to implement. As I’ve said, it’s a huge standard with lots of pieces. The USB-PD stack alone requires a fairly powerful CPU to be embedded in the USB Host controller. All of this makes a complete, standards compliant USB-C implementation fairly expensive, atleast in contrast to previous USB standards.

Thankfully, it doesn’t have to be this way and things are getting better! Compared to a few years ago, manufacturers are producing USB C ports with consistant support, and pretty much all the physical woes have been sorted out. Even better, USB 4 is around the corner. Gone will be the days of USB C 2x(3.1) Gen 2 x 2^2 / 10 Power Delivery with ThunderBolt 3 and Alternative Modes. One of the problems has been that Intel retained licenses rights over TunderBolt, meaning it’s implementation has been left out of a lot of devices (and as such it’s plethora of uses, such as eGPUs using PCIe over ThunderBolt).

So here is to a hopefully bright future where USB C will rule them all, and we can’t stop guessing at what the damn port actually can do.

ID: 226