Given the scenario in the USB Type-C Specification Figure A-2 "Example 3.5mm to USB Type-C Adapter Supporting 500mA Charge-Through".... Figure A-2 "Example 3.5mm to USB Type C Adapter Supporting 500mA Charge-Through

How does the USB Type-C Receptacle know that a USB Type-C plug with audio-jack is connected, and still get charge through the VBUS?

  1. According to the Figure 4-14 "Connection State Diagram: Sink with Accessory Support", a Sink will loop through the states Unattached.SNK and Unattached.Accessory until either a DFP Connection is detected or an Accessory Connection is detected. But what about the case where an Audio Accessory must be detected without losing power, this state machine doesn't seem to address such a scenario.

Figure 4-14 Connection State Diagram: Sink with Accessory Support

I am guessing that the Rd and the Ra would interfere with each other, getting an equivalent resistance smaller than Ra. In this case, how will VBUS still know that it needs to provide power to a sink and not an accessory? How will the receptacle enter the state to identify an accessory if its CC lines are being pulled up by Rp? Am I missing something?

Let me know if I can do anything to make this question clearer.


2 Answers 2


this may help you: A UFP is defined as having an Rd on its CC lines. If Rd is less than Ra (Ra = 1 kiloOhm ± 20 %), then the DFP would recognize this device as an audio accessory

Type-C receptacle <------->   [TypeC-Plug <=> Audio 3.5mm]
CC1               <------->   CC1 <--- 
CC2               <------->   CC2 <---

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I think the short answer is that you can't, at least not with the passive audio accessory mode. The usb-c spec doesn't suggest any way that charging would work with audio accessory mode (what you show above). [ You might be lucky and find your phone manufacturer will just take power from vbus is it's presented, but the tales of woe of people trying exactly this when trying to connect their android phones to cars suggest that doesn't work. ]

What you need is an adapter that implements either the digital "Audio Accessory mode" or the "Audio Device Class 3 specification" (which may be described as containing a DAC). Of course all of these adapters look the same and have no helpful descriptions - as a rough guide I believe all adaptors that work with the google pixel contain a DAC.

With these adapters the phone/device is driving the usb bus and finding a usb sound card in the adaptor which provides microphone and headset ports. This leaves the power and CC lines free to perform the usual USB-C PD negotiation and allow your phone to charge, or power the usb soundcard as appropriate.

Note that quite a lot of these adaptors use usb 2.0 signaling with the D+/D- wires, that means that you can't use non usb-c signalling [shorting D+ to D-], to indicate to the device that it should charge as that needs the same pins. However the USB-C mechanism will work fine (either using the static resistances you quote above for "powered cable with sink or vconn accessory attached", or using the proper PD signalling)


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