I want to build an embedded device which is using a USB-C for debugging purposes and USB 2 functionality.

Now I am not sure whether or not I understood the USB-C Standard correctly.

  1. Is it correct, that if I connect the CC1 and CC2 to pin with a pulldown of 5.1 kOHM to GND that other devices detect the debug accessory mode and therefore is it save for a connecting device that none of the 14 pins are used?

  2. Is it still possible to use the D+/D- pins for USB 2 purposes?

The idea is, that I short-circuit the pairs Axx and Bxx and connect SWD-Signals to the Pins A11,A10,A8, A2,A3 and vice versa for B and connect USB 2 to the D+/D- Pins as well.

3.Is there a tool I am able to detect from a computer in which mode a device is?

Thx for your help


1 Answer 1


I think you read too quickly, this is a little trickier than this.

I'll use terminology of spec. DTS is the Debug and test system, TS is Target system (your device under test). DAM will refer to Debug Accessory Mode.

TS is meant to be a final product, so it should be a compliant final product before all. Detection of DTS presence is just added functionality. TS should have a "normal" Type-C Receptacle, with normal Rp/Rp (Source) or Rd/Rd (Sink) CC1/CC2 resistors (or a Type-C controller, maybe with DRP capabilities, and even with PD support).

DTS on the other hand is not meant to be a valid consumer product, it is meant to be a manufacturer-specific test fixture. That's why it relies on a Type-C Plug with either Rp/Rp (to connect to a Sink) or Rd/Rd (to connect to a Source).

TS can tell it was connected to a DTS when it sees both CC1 and CC2 pulled, rather than only one (remember cables must have one of CCs connected to ground through Ra on each side). DTS implements an invalid Type-C Cable configuration.

So now for the answers:

  1. It depends whether you are implementing a Source or Sink TS (and if your DTS is Sink or source). Rd/Rd is used on DTS Sinks. Rp/Rp used for DTS Sources and can also announce various current sourcing conditions, See table B-2 in Type-C spec release 2.0, p. 323.

  2. When DAM is detected by your TS, you fall in a manufacturer-specific port configuration where most pins (all pins except power and CC) can be repurposed. You can repurpose some pins to their non-DAM mode (i.e. not repurpose at all). This is vendor-specific anyway. Only requirement is to follow electrical requirements at all times. See B.2.1. p. 219.

  3. This does not really make sense. You must have a specific DTS that implements a Type-C plug to connect to your TS with DAM enabled. Host connection is not even part of this setup (the fact that your DTS fixture may actually connect to a host is outside of the scope of DAM).


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