WHY is it bad for USB-C to use a single resistor on both CC lines?
tl;dr: Because it will make your active/e-marked cables fail to charge. And your device think it is a headphone jack. You could also potentially blow your DAC or charging circuit.

(Note: corrections, comments, or clarifications are welcomed.)

Recently I posted a critical review of a charger that bridged CC pins and used only one Rp resistor. The manufacturer stated it was for "compatibility" with non-compliant devices. Benson explained this is bad, but people may not understand *HOW* serious this matter is. Cue this post.


(You can follow along with the pictures below using a basic understanding of circuits. Or memes.)

In the first image, I show how things "should" work. Two separate Rp resistors. By varying Rp you change the voltage on the CC line, vRd-USB. The charger and device sense this voltage to find each other. The device also learns how much current it is allowed to pull. (This is where Benson's crusade began!)

In the second image, I show what happens when you start cutting corners and "things go bad". Some manufacturers -- saving $0.001 by omitting a resistor ( http://goo.gl/t8C8zq ) -- use a single Rp and bridge the CC pins instead.

This is a violation of Section "Initially, a Source exposes independent Rp terminations on its CC1 and CC2 pins, and a Sink exposes independent Rd terminations on its CC1 and CC2 pins"

If using an e-marked/active cable (as will be mandatory soon),  there is an Ra pulldown on the second CC pin inside the cable. With an unsafe charger there are now two paths to ground. You now have something called a "voltage divider" merging both CC pins. The formulas get a bit messy, but you can simplify the circuit. The resulting vRd-USB makes no sense to the device. It may charge slowly, not at all, or behave erratically.

First-rate [e-marked] cables like +StarTech.com's Thunderbolt 3, or Belkin's USB3.1Gen2 line won't even work with this kind of spec-violation!

(Edit: I direct this to all reputable manufacturers. If you are reading this, *please* consider joining Benson Leung in calling out non-compliant USB-C products. Manufacturers violating spec will make *you* and your well-engineered cables look bad. I can guarantee in the future, someone will post "Why isn't my ___ charging at full speed with my $40 name-brand cable? It works fine with my cheap [non e-marked] cable.")

In the third image, I show how a non-compliant charger may even cause an active/e-marked cable to appear as a "Audio Adapter Accessory"...  in other words, a headphone jack! If the partner device is "DRP" capable, it may misinterpret the CC bridging as the presence of Ra on both pins. This is a unique alternate mode that repurposes the USB D+/D- pins to output analog audio signals from the phone's DAC. (There is already one phone on the market that lacks a 3.5mm audio jack, with more coming.)

You may have read about how Benson is loudly warning Qualcomm QC is not OK on USB-C. This is one reason why. QC 3.0 modulates the voltages on... you guessed it... the USB D+/D- lines to change the voltage delivered to the phone. QC had to do this on USB-A since there were only 4 wires, [4 pins, which limits max current]¹. But USB-C has [2 pins/1 wire]¹ dedicated to negotiating voltage, and [at minimum 5 wires/11 pins, which allows carrying extra current]¹. (Apple chargers have D+/D- signaling too, but is merely the charger telling the phone it MAY take extra current. Not instructing the charger to jack up the voltage.)

The possibility exists -- however remote -- that an [bad] QC-over-USB-C charger, with an [bad] bridged CC pin, with a [GOOD] active cable, and [GOOD] phone, will result in the phone being fried.

(Disclaimer: this is a worst-case-scenario, based on my best understanding. I may have erred in a calculation -- darnit Jim I'm a MechE not an EE -- but it is my humble attempt at explaining how this one flaw [bridged CC pins] can blow up bigtime. I'll explore other "bad" scenarios in other posts.)

¹: Thank you to Benson for pointing out the bare minimums! Please see "Table 3-11 USB 2.0 Type-C Standard Cable Assembly Wiring" for details.

²:  I've since learned the "LeTV Le Max 2" is the exact phone that is vulnerable to this "worst case scenario". Now that I have this in mind, I may do a post on how to trigger this behavior.

#USB   #TypeC   #USBC
[Plus] Analyses CC-Rp Bridging
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