4 ms·
Why would you design in reverse polarity protection? It adds cost, and the mechanical connector already makes it impossible for VDD to short to GND in USB type
by krapht 11y ago
Why would you design in reverse polarity protection? It adds cost, and the mechanical connector already makes it impossible for VDD to short to GND in USB type C because of its rotational symmetry.
- andrey-g 11y agoI constantly have Vdd shorts due to ground loops when measuring stuff. But a more frequent case is worn out cables (every lightning cable I've owned has eventually broke). Its not rare enough of a failure mode to ignore.
- krapht 11y agoYes, and that's why the USB spec puts the burden of protecting against electrical shorts on the controller. The device side shouldn't have to worry about trying to shunt 100 Watts of power somewhere. I mean this kind of protection is way beyond typical ESD protection due to the power involved. It would add a lot of cost to do this, maybe even a dollar or two in raw material.
- mikeash 11y agoIf you stop the current from flowing, you don't have to shunt any power anywhere. You'd have to open the circuit in the face of 5V of potential hooked up backwards, not really a tall order.
- krapht 11y agoI don't want to be a broken record, but you're right, not a tall order, but no it's unreasonable on account of cost. It's impossible for USB C to be hooked up backwards by its mechanical design, so why guard against it? The high-side transistor to prevent reverse polarity faults could be 3 to 4 cents in manufacturing cost! It's also not trivial to make sure that transistor trips fast enough to prevent all your low-voltage ICs from being destroyed.
- mikeash 11y agoSure, but the magnitude of that cost matters a lot. You were saying a dollar or two in raw materials to protect against 100W, now you're saying three or four cents. A few cents to protect hundreds of dollars in equipment from electrical faults seems worthwhile to me.
- teraflop 11y agoThat makes sense from the consumer's point of view. But imagine you're a manufacturer, producing USB device controller chips by the million. Would you really be willing to spend tens of thousands of dollars to protect against a failure mode that can only happen due to some other company's extreme negligence?
- mikeash 11y agoYes, but of course it doesn't matter what I think. It's not abnormal to add protection that isn't needed when everything works properly. As noted elsewhere, the USB spec already requires resettable overcurrent protection, for example.
- knughit 11y agoTens of thousands of dollars total is far less than it costs a company just to moot the question.
- deleted 11y ago[deleted]
- krapht 11y agoSorry I was not clear in my comments. The dollar solution is for some type of crowbar circuit. This is the most robust solution. The 3-4 cents is for specifically, reverse polarity protection when somehow, a -5V differential is applied to device power inputs. This cheaper solution only narrowly addresses this particular power fault, and no others.
- klodolph 11y agoShorts and polarity reversal aren't the same thing, and the protection circuitry is different.
- earlz 11y agounless the connector were cut, or some other catastrophic failure. I tend to think ruining a devices USB controller should be near impossible short of something like the USB killer (http://arstechnica.com/security/2015/10/usb-killer-flash-drive-can-fry-your-computers-innards-in-seconds/ http://arstechnica.com/security/2015/10/usb-killer-flash-dri...)
- lucaspiller 11y agoI was playing with a FTDI (a clone :D) and an Arduino and accidentally mixed up VCC and GND from an external power source. I only realised because Minicom was failing to connect to the device. Then I realised what I had done, I tried removing it from the USB port and putting it back in, but it wouldn't work. I then tried a mouse too, but that wasn't working either. Thinking I'd killed a USB port on my Mac (well there are two, so...) I then tried rebooting and that fortunately fixed it.
- earlz 11y agoYep, did almost exactly this, shorted a converted 3.3V from a different USB port (same machine) to ground on a different USB port and end result is my motherboard lost all USB connectivity until I rebooted
- nerfhammer 11y agothe usb ports on macs are supposed to have overcurrent protection. You actually get a dialog popup in the OS when it happens. I know because I've triggered it doing something with an arduino.
- marcosdumay 11y agoWell, it protects against some metal object getting inserted into the USB port.
- digi_owl 11y agoThis was a C-to-A adapter cable. what was wrong was that the Vbus on one end was hooked to the GND of the other end.
- deleted 11y ago[deleted]
- vacri 11y agoLook for a long response near the end of the article comments, from Mike Crawford. It goes into detail as to why that protection isn't designed in - it's not cost (diodes are cheap), but power drop, which has side effects.