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Exploring USB Type-C DRP to USB Type-C DFP connection using USB C-Thru

Exploring USB Type-C DRP to USB Type-C DFP connection using USB C-Thru
by Rajaram Regupathy on 02-17-2016 at 12:00 pm

In a USB Type-C environment configuration process between a DRP and a DFP is as follows:

  • DRP to DFP attach/detach detection
  • Plug orientation detection
  • Initial DRP to DFP and power relationship detection
  • USB Type-C VBus current detection and usage

The USB Type-C Pins that we will be monitoring here are the VBUS and CC. The following are their characteristics:

VBUS carries a nominal 5V supply, which may be used by a device for power. The DFP’s USB Type-C receptacle VBUS pin shall remain unpowered until a UFP is attached. The VBUS pin shall return to the unpowered state when the UFP is detached.

CC1 & CC2
These pins are used to detect connections and configure the interface across the USB Type-C cables and connectors.
The expected behavior of a DRP with regard to a DFP:

  • In the unattached state the DRP alternates between Rp (5.5V) and Rd (1.1V).
  • In the unattached state the DFP presents Rp on the CC lines.
  • DFP detects the DRP’s Rd on CC and enters the attached state.
  • DFP turns on VBUS.
  • DRP detects Rp on CC and it also detects VBUS and enters the attached state.
  • DFP monitors CC for detach and when detected, enters the unattached state.
  • DRP monitors VBUS for detach and when detected, enters the unattached state and resumes toggling between Rp and Rd.

DRP presenting its capability over the CC lines

In the above setup USB C-thru is connected to a mobile phone. The mobile phone is a DRP and should present Rp and Rd alternatively on the CC lines. USB C-thru has test points for CC1, CC2/VConn and GND. and these are connected to the oscilloscope. When connected we can observe the following on the oscilloscope.

You can see that both CC1 and CC2 are toggling. Therefore the mobile phone is presenting both Rp and Rd on the CC lines.

CC line configuration and the reversible connector.

Now a DFP has been connected to the USB C-thru using a USB Type-C cable. On connection we see the following on the oscilloscope software.

You can see, above, that CC2 (orange) shows connection and that CC1 (blue) is not in use. The DFP receives Rd on CC2 and the connection is established on CC2. The cable orientation is determined based on the CC pin on which the connection is established.

Now if we remove the plug, you will see, above, that the CC pins are toggling indicating disconnection and then on reconnection using the reversed plug, a connection is established on CC1 and CC2 is unused.

In the above setup VBus and CC lines are connected to the oscilloscope. The oscilloscope software displays the following for this setup.

You can see that the VBus (orange) delivers a power of 5 V. and the CC (blue) line is configured. Now we will disconnect the DFP and reconnect. You can see in the following picture that the VBus powers down smoothly and the CC line starts toggling. On reconnection VBus powers up at 5V and the CC line is configured.

Through this test setup we could explore and test the implementation using the USB C-thru.
 USB C-thru is a result of the need for affordable and robust tools to explore USB Type-C and test USB Type-C/PD implementations. USB C-thru aids in monitoring the USB Type-C signals. This has a USB Type-C plug and a USB Type-C receptacle, test point for CC1, CC2/VConn, VBus, GND, and SBU. The test points can be connected to the oscilloscope to monitor the USB Type-C signals.

USB C-thru is an Open platform based on easy-to-use hardware. Even new users can build a breadboard setup and use USB C-thru to understand USB Type-C/PD and in an affordable way. USB C-thru is a key tool to learn new things and test USB Type-C/PD implementations. Being Open, the platform empowers users to build independently and eventually adapt the board to their particular needs. The aim is to empower users to learn, experiment, test and build.

USB C-thru is available on Crowd Supply ( through a funding campaign. Crowd Supply is a crowd-funding platform that helps bring within reach a community of enthusiasts, hobbyists and developers not possible earlier. This inspired us to deliver an affordable solution that is within reach of one and all and that encourages users to learn, create and succeed like never before. If this product interests you and/or will be helpful in your endeavors, please do support us via

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